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#826 2020-10-22 01:03:06

ganesh
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Re: Miscellany

804) Lignite

Lignite, generally yellow to dark brown or rarely black coal that formed from peat at shallow depths and temperatures lower than 100 °C (212 °F). It is the first product of coalification and is intermediate between peat and subbituminous coal according to the coal classification used in the United States and Canada. In many countries lignite is considered to be a brown coal. Lignite contains about 60 to 70 percent carbon (on a dry, ash-free basis) and has a calorific value near 17 megajoules per kilogram (7,000 British thermal units per pound).

It has been estimated that nearly half of the world’s total proven coal reserves are made up of lignite and subbituminous coal, but lignite has not been exploited to any great extent, because it is inferior to higher-rank coals (e.g., bituminous coal) in calorific value, ease of handling, and storage stability. In areas where other fuels are scarce, the production of brown coal far exceeds that of bituminous coal.

Most lignites are geologically young, generally having formed during the Mesozoic and Cenozoic eras (approximately 251 million years ago to the present). Many lignite beds lie close to the surface and are of great thickness, sometimes greater than 30 m (about 100 feet); they are easily worked, and the cost of production is low. The utilization of lignite is difficult because of its high water content, which can be as great as 75 percent in some varieties. Upon exposure and weathering, some of this water is given up, and disintegration, or crumbling, of the material occurs, which reduces the value of lignite as a fuel. Lignite also tends to disintegrate during combustion, and hence the losses through a grate may be relatively high. It requires special care in storing, is uneconomical to transport over long distances, and is subject to spontaneous combustion. Schemes for increasing the use of lignite have received attention in Australia, New Zealand, Canada, the United States, and elsewhere. The fuel is used primarily by local utilities and industries and by domestic consumers close to the mine sites.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

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#827 2020-10-23 00:39:37

ganesh
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Posts: 31,668

Re: Miscellany

805) Peat

Peat, fuel consisting of spongy material formed by the partial decomposition of organic matter, primarily plant material, in wetlands such as swamps, muskegs, bogs, fens, and moors. The development of peat is favoured by warm moist climatic conditions; however, peat can develop even in cold regions such as Siberia, Canada, and Scandinavia.

Peat is only a minor contributor to the world energy supply, but large deposits occur in Canada, China, Indonesia, Russia, Scandinavia, and the United States. In the early 21st century the top four peat producers in the world were Finland, Ireland, Belarus, and Sweden, and most of the major users of peat were these and other northern European countries. Peat is sometimes considered a “slowly renewable energy” and is classified as a “solid fossil” rather than a biomass fuel by the Intergovernmental Panel on Climate Change (IPCC). Although peat is not strictly a fossil fuel, its greenhouse gas emissions are comparable to that of fossil fuels.

Peat Formation

The formation of peat is the first step in the formation of coal. With increasing depth of burial and increasing temperature, peat deposits are gradually changed to lignite. With increased time and higher temperatures, these low-rank coals are gradually converted to subbituminous and bituminous coal and under certain conditions to anthracite.

Peatification is influenced by several factors, including the nature of the plant material deposited, the availability of nutrients to support bacterial life, the availability of oxygen, the acidity of the peat, and temperature. Some wetlands result from high groundwater levels, whereas some elevated bogs are the result of heavy rainfall. Although the rate of plant growth in cold regions is very slow, the rate of decomposition of organic matter is also very slow. Plant material decomposes more rapidly in groundwater rich in nutrients than in elevated bogs with heavy rainfall. The presence of oxygen (aerobic conditions) is necessary for fungal and microbial activity that promotes decomposition, but peat is formed in waterlogged soils with little or no access to oxygen (anaerobic conditions), largely preventing the complete decomposition of organic material. The formation of abundant peat was not possible before land plants spread widely during and after the Devonian Period (beginning approximately 419.2 million years ago). Peat moss (Sphagnum) is one of the most common constituents of peat.

Types And Processing

Peats may be divided into several types, including fibric, coarse hemic, hemic, fine hemic, and sapric, based on their macroscopic, microscopic, and chemical characteristics. Peat may be distinguished from lower-ranked coals on the basis of four characteristics: peats generally contain free cellulose, more than 75 percent moisture, and less than 60 percent carbon, and they can be cut with a knife. The transition to brown coal takes place slowly and is usually reached at depths ranging from 100 to 400 metres (approximately 330 to 1,300 feet).

Peat is usually hand-cut, although progress has been made in the excavation and spreading of peat by mechanical methods. Peat may be cut by spade in the form of blocks, which are spread out to dry. When dry, the blocks weigh from 0.34 to 0.91 kg (0.75 to 2 pounds). In one mechanized method, a dredger or excavator digs the peat from the drained bog and delivers it to a macerator (a device that softens and separates a material into its component parts through soaking), which extrudes the peat pulp through a rectangular opening. The pulp is cut into blocks, which are spread to dry. Maceration tends to yield more uniform shrinkage and a denser and tougher fuel. Hydraulic excavating can also be used, particularly in bogs that contain roots and tree trunks. The peat is washed down by a high-pressure water jet, and the pulp runs to a sump. There, after slight maceration, it is pumped to a draining ground in a layer, which, after partial drying, is cut up and dried further.

Uses

Dried peat burns readily with a smoky flame and a characteristic odour. The ash is powdery and light, except for varieties that have a high content of inorganic matter. Peat is used for domestic heating purposes as an alternative to firewood and forms a fuel suitable for boiler firing in either briquetted or pulverized form. Peat is also used for household cooking in some places and has been used to produce small amounts of electricity.

In horticulture, peat is used to increase the moisture-holding capacity of sandy soils and to increase the water infiltration rate of clay soils. It is also added to potting mixes to meet the acidity requirements of certain potted plants. Given that peat harvesting is destructive to wetland habitats, its use as a soil amendment has been discouraged by conservationists.

Peat can be used in water filtration and is sometimes utilized for the treatment of urban runoff, wastewater, and septic tank effluent. It is also used to soften aquaria water and to mimic habitats for freshwater fish.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#828 2020-10-24 00:45:11

ganesh
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Re: Miscellany

806) Anthracite

Anthracite, also called hard coal, the most highly metamorphosed form of coal. It contains more fixed carbon (86 percent or greater on a dry, ash-free basis) than any other form of coal and the least amount of volatile matter (14 percent or less on a dry, ash-free basis), and it has calorific values near 35 megajoules per kilogram (approximately 15,000 British thermal units per pound), not much different from the calorific values for most bituminous coal. Anthracite is the least plentiful form of coal. It is found mostly in the eastern part of the United States and makes up less than 2 percent of all coal reserves in the country. Smaller amounts of anthracite occur in South Africa, Australia, western Canada, China, and other countries.

Anthracites are black to steel gray in colour and have a brilliant, almost metallic lustre. They can be polished and used for decorative purposes. Hard and brittle, anthracites break with conchoidal fracture into sharp fragments. Unlike many bituminous coals, they are clean to the touch. Although anthracites are difficult to ignite, they burn with a pale blue flame and require little attention to sustain combustion. In the past they were used for domestic heating because they produce little dust upon handling, burn slowly, and emit relatively little smoke. Anthracite is rarely used for this purpose today because of its limited abundance and relatively high cost and the ready availability of other sources of energy (e.g., natural gas and electricity) for heating purposes. Although anthracites usually occur in geologically deformed areas, such as in the intensely folded sedimentary rocks of the anthracite region of Pennsylvania, U.S., their origin is due to higher than normal heating caused by the presence of nearby igneous intrusions or high geothermal gradients. Both of these phenomena produce temperatures much higher than those reached at depth in most sedimentary basins. For instance, in Antarctica, large igneous sills intruded the coal measures and converted some of the existing bituminous coal to anthracite. Temperatures ranging from 170 to 250 °C (about 340 to 480 °F) are thought to be necessary for the formation of anthracite.

anthracite-coal-250x250.jpg


It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#829 2020-10-25 00:31:06

ganesh
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Re: Miscellany

807) London Underground

London Underground, also called the Tube, underground railway system that services the London metropolitan area.

The London Underground was proposed by Charles Pearson, a city solicitor, as part of a city improvement plan shortly after the opening of the Thames Tunnel in 1843. After 10 years of discussion, Parliament authorized the construction of 3.75 miles (6 km) of underground railway between Farringdon Street and Bishop’s Road, Paddington. Work on the Metropolitan Railway began in 1860 by cut-and-cover methods—that is, by making trenches along the streets, giving them brick sides, providing girders or a brick arch for the roof, and then restoring the roadway on top. On Jan. 10, 1863, the line was opened, using steam locomotives that burned coke and, later, coal. Despite sulfurous fumes, the line was a success from its opening, carrying 9.5 million passengers in the first year of its existence.

In 1866 the City of London and Southwark Subway Company (later the City and South London Railway) began work on the “tube” line, using a tunneling shield developed by J.H. Greathead. The tunnels were driven at a depth sufficient to avoid interference with building foundations or public utility works, and there was no disruption of street traffic. The original plan called for cable operation, but electric traction was substituted before the line was opened. Operation began on this first electric underground railway in 1890 with a uniform fare of twopence for any journey on the 3-mile (5-km) line. In 1900 Charles Tyson Yerkes, an American railway magnate, arrived in London, and he was subsequently responsible for the construction of more tube railways and for the electrification of the cut-and-cover lines. The London Underground name first appeared in 1908. Stations functioned as air-raid shelters during World Wars I and II, with the tunnels of the unused Aldwych spur line housing artifacts from the British Museum.

The London Underground was nationalized in 1948 under the auspices of the London Transport Executive. Over the next half century, new lines were constructed, steam locomotives were completely replaced by electric ones, and new safety measures were introduced (including an automated announcement warning passengers to “mind the gap” between the train and the platform). In 2003 management of the Underground passed to Transport for London, a public entity that provides the Underground with human resources, such as conductors and station personnel. As part of a partnership scheme with the private sector, outside companies maintain the physical infrastructure of the Underground, including the stations, tracks, and railcars.

By the beginning of the 21st century, the London Underground served more than one billion passengers per year, with approximately 250 miles (400 km) of track connecting some 270 stations. As part of its ongoing upgrading of its rolling stock, the Underground introduced its first air-conditioned cars in 2010.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#830 2020-10-26 00:29:52

ganesh
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Re: Miscellany

808) Hand sanitizer

Hand sanitizer, also called hand antiseptic, handrub, or hand rub, agent applied to the hands for the purpose of removing common pathogens (disease-causing organisms). Hand sanitizers typically come in foam, gel, or liquid form. Their use is recommended when soap and water are not available for hand washing or when repeated hand washing compromises the natural skin barrier (e.g., causing scaling or fissures to develop in the skin). Although the effectiveness of hand sanitizer is variable, it is employed as a simple means of infection control in a wide variety of settings, from day-care centres and schools to hospitals and health care clinics and from supermarkets to cruise ships.

Types Of Hand Sanitizers

Depending on the active ingredient used, hand sanitizers can be classified as one of two types: alcohol-based or alcohol-free. Alcohol-based products typically contain between 60 and 95 percent alcohol, usually in the form of ethanol, isopropanol, or n-propanol. At those concentrations, alcohol immediately denatures proteins, effectively neutralizing certain types of microorganisms. Alcohol-free products are generally based on disinfectants, such as benzalkonium chloride (BAC), or on antimicrobial agents, such as triclosan. The activity of disinfectants and antimicrobial agents is both immediate and persistent. Many hand sanitizers also contain emollients (e.g., glycerin) that soothe the skin, thickening agents, and fragrance.

Effectiveness

The effectiveness of hand sanitizer depends on multiple factors, including the manner in which the product is applied (e.g., quantity used, duration of exposure, frequency of use) and whether the specific infectious agents present on the person’s hands are susceptible to the active ingredient in the product. In general, alcohol-based hand sanitizers, if rubbed thoroughly over finger and hand surfaces for a period of 30 seconds, followed by complete air-drying, can effectively reduce populations of bacteria, fungi, and some enveloped viruses (e.g., influenza A viruses). Similar effects have been reported for certain alcohol-free formulations, such as SAB (surfactant, allantoin, and BAC) hand sanitizer. Most hand sanitizers, however, are relatively ineffective against bacterial spores, nonenveloped viruses (e.g., norovirus), and encysted parasites (e.g., Giardia). They also do not fully cleanse or sanitize the skin when hands are noticeably soiled prior to application.

Despite the variability in effectiveness, hand sanitizers can help control the transmission of infectious diseases, especially in settings where compliance with hand washing is poor. For example, among children in elementary schools, the incorporation of either an alcohol-based or an alcohol-free hand sanitizer into classroom hand-hygiene programs has been associated with reductions in absenteeism related to infectious illness. Likewise, in the workplace, the use of alcohol-based hand sanitizer has been associated with reductions in illness episodes and sick days. In hospitals and health care clinics, increased access to alcohol-based hand sanitizer has been linked to overall improvements in hand hygiene.

Safety Concerns

Agencies such as the World Health Organization and the U.S. Centers for Disease Control and Prevention promote the use of alcohol-based hand sanitizers over alcohol-free products. Indeed, the use of alcohol-free products has remained limited, in part because of WHO’s and CDC’s focus on alcohol-based products but also because of concerns about the safety of chemicals used in alcohol-free products. Research has indicated that certain antimicrobial compounds, such as triclosan, for example, may interfere with the function of the endocrine system. Environmental contamination from triclosan is another concern. Disinfectants and antimicrobials also can potentially contribute to the development of antimicrobial resistance. In 2014, mounting concerns over triclosan led authorities in the European Union (EU) to restrict the chemical’s use in various consumer products in the EU.

By comparison, concerns over the use of alcohol-based hand sanitizer have centred primarily on product flammability and ingestion, both unintentional (e.g., by young children) and intentional (by individuals seeking to abuse alcohol). With proper storage and strategies that limit access to alcohol-containing sanitizer (e.g., issuing hand sanitizer to individuals), the risk of fire or poisoning from accidental or intentional ingestion of alcohol-based hand sanitizers is considered to be low.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#831 2020-10-27 00:45:31

ganesh
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Re: Miscellany

809) Vaccine

Vaccine, suspension of weakened, killed, or fragmented microorganisms or toxins or of antibodies or lymphocytes that is administered primarily to prevent disease.

A vaccine can confer active immunity against a specific harmful agent by stimulating the immune system to attack the agent. Once stimulated by a vaccine, the antibody-producing cells, called B cells (or B lymphocytes), remain sensitized and ready to respond to the agent should it ever gain entry to the body. A vaccine may also confer passive immunity by providing antibodies or lymphocytes already made by an animal or human donor. Vaccines are usually administered by injection (parenteral administration), but some are given orally or even nasally (in the case of flu vaccine). Vaccines applied to mucosal surfaces, such as those lining the gut or nasal passages, seem to stimulate a greater antibody response and may be the most effective route of administration.

The First Vaccines

The first vaccine was introduced by British physician Edward Jenner, who in 1796 used the cowpox virus (vaccinia) to confer protection against smallpox, a related virus, in humans. Prior to that use, however, the principle of vaccination was applied by Asian physicians who gave children dried crusts from the lesions of people suffering from smallpox to protect against the disease. While some developed immunity, others developed the disease. Jenner’s contribution was to use a substance similar to, but safer than, smallpox to confer immunity. He thus exploited the relatively rare situation in which immunity to one virus confers protection against another viral disease. In 1881 French microbiologist Louis Pasteur demonstrated immunization against anthrax by injecting sheep with a preparation containing attenuated forms of the bacillus that causes the disease. Four years later he developed a protective suspension against rabies.

Vaccine Effectiveness

After Pasteur’s time, a widespread and intensive search for new vaccines was conducted, and vaccines against both bacteria and viruses were produced, as well as vaccines against venoms and other toxins. Through vaccination, smallpox was eradicated worldwide by 1980, and polio cases declined by 99 percent. Other examples of diseases for which vaccines have been developed include mumps, measles, typhoid fever, cholera, plague, tuberculosis, tularemia, pneumococcal infection, tetanus, influenza, yellow fever, hepatitis A, hepatitis B, some types of encephalitis, and typhus—although some of those vaccines are less than 100 percent effective or are used only in populations at high risk. Vaccines against viruses provide especially important immune protection, since, unlike bacterial infections, viral infections do not respond to antibiotics.

Vaccine Types

The challenge in vaccine development consists in devising a vaccine strong enough to ward off infection without making the individual seriously ill. To that end, researchers have devised different types of vaccines. Weakened, or attenuated, vaccines consist of microorganisms that have lost the ability to cause serious illness but retain the ability to stimulate immunity. They may produce a mild or subclinical form of the disease. Attenuated vaccines include those for measles, mumps, polio (the Sabin vaccine), rubella, and tuberculosis. Inactivated vaccines are those that contain organisms that have been killed or inactivated with heat or chemicals. Inactivated vaccines elicit an immune response, but the response often is less complete than with attenuated vaccines. Because inactivated vaccines are not as effective at fighting infection as those made from attenuated microorganisms, greater quantities of inactivated vaccines are administered. Vaccines against rabies, polio (the Salk vaccine), some forms of influenza, and cholera are made from inactivated microorganisms. Another type of vaccine is a subunit vaccine, which is made from proteins found on the surface of infectious agents. Vaccines for influenza and hepatitis B are of that type. When toxins, the metabolic by-products of infectious organisms, are inactivated to form toxoids, they can be used to stimulate immunity against tetanus, diphtheria, and whooping cough (pertussis).

In the late 20th century, advances in laboratory techniques allowed approaches to vaccine development to be refined. Medical researchers could identify the genes of a pathogen (disease-causing microorganism) that encode the protein or proteins that stimulate the immune response to that organism. That allowed the immunity-stimulating proteins (called antigens) to be mass-produced and used in vaccines. It also made it possible to alter pathogens genetically and produce weakened strains of viruses. In that way, harmful proteins from pathogens can be deleted or modified, thus providing a safer and more-effective method by which to manufacture attenuated vaccines.

Recombinant DNA technology has also proven useful in developing vaccines to viruses that cannot be grown successfully or that are inherently dangerous. Genetic material that codes for a desired antigen is inserted into the attenuated form of a large virus, such as the vaccinia virus, which carries the foreign genes “piggyback.” The altered virus is injected into an individual to stimulate antibody production to the foreign proteins and thus confer immunity. The approach potentially enables the vaccinia virus to function as a live vaccine against several diseases, once it has received genes derived from the relevant disease-causing microorganisms. A similar procedure can be followed using a modified bacterium, such as Salmonella typhimurium, as the carrier of a foreign gene.

Vaccines against human papillomavirus (HPV) are made from viruslike particles (VLPs), which are prepared via recombinant technology. The vaccines do not contain live HPV biological or genetic material and therefore are incapable of causing infection. Two types of HPV vaccines have been developed, including a bivalent HPV vaccine, made using VLPs of HPV types 16 and 18, and a tetravalent vaccine, made with VLPs of HPV types 6, 11, 16, and 18.

Another approach, called naked DNA therapy, involves injecting DNA that encodes a foreign protein into muscle cells. The cells produce the foreign antigen, which stimulates an immune response.

Benefits Of Vaccination

In addition to the development of memory B cells, which are capable of triggering a secondary immune response upon exposure to the pathogen targeted by a vaccine, vaccination is also beneficial at the population level. When a sufficient number of individuals in a population are immune to a disease, as would occur if a large proportion of a population were vaccinated, herd immunity is achieved. That means that if there is random mixing of individuals within the population, then the pathogen cannot be spread throughout the population. Herd immunity acts by breaking the transmission of infection or by lessening the chances of susceptible individuals coming in contact with a person who is infectious. Herd immunity provides a measure of protection to individuals who are not personally immune to the disease—for instance, individuals who, because of their age or underlying medical conditions, cannot receive vaccines or individuals who received vaccines but remain susceptible. Herd immunity played an important role in the successful eradication of smallpox, and it is vital in preventing the spread of diseases such as polio and measles.

Adverse Reactions

Vaccination carries some risk of reaction, though adverse effects typically are very rare and very mild. The most common reactions to vaccines include redness and soreness around the vaccination site. More severe adverse reactions, such as vomiting, high fever, seizure, brain damage, or death, are possible for some vaccines. Such reactions are exceptionally rare, however—occurring in less than one in a million people for most vaccines. Severe reactions also tend to affect only certain populations, such as persons whose immune systems are compromised by preexisting disease (e.g., HIV/AIDS) or who are undergoing chemotherapy.

Claims have been made that vaccines are responsible for certain adverse health conditions, particularly autism, speech disorders, and inflammatory bowel disease. Some of those claims focused on thimerosal, a mercury-containing compound used as a preservative in vaccines. Some people believed that autism was a form of mercury poisoning, caused specifically by thimerosal in childhood vaccines. Those claims have been discredited. Still, misinformation and fear generated by false claims about associations between autism and vaccines had a significant impact on individuals’ perceptions about vaccine safety. In addition, most individuals in countries where vaccination is widespread have never personally experienced vaccine-preventable disease. Thus, the focus of concern for some people shifted from the negative effects of vaccine-preventable disease to the possible negative effects of the vaccines themselves.

Complacency about vaccine-preventable diseases, combined with concerns over the effects of vaccination, led to decreasing levels of vaccination coverage in some areas of the world. As a consequence, not only were individuals susceptible to vaccine-preventable diseases, but, at population levels, vaccination rates dropped low enough to cause losses of herd immunity, thereby allowing outbreaks of disease. Such outbreaks brought high costs to societies, especially in terms of health and medical care, disability and economic strain, and loss of life. In the 20th century in Japan, England, and Russia, for example, numbers of children vaccinated against whooping cough dropped sufficiently low so as to enable outbreaks of disease that involved thousands of children and resulted in hundreds of deaths.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

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#832 2020-10-28 01:06:02

ganesh
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Re: Miscellany

810) Antibiotic

Antibiotic, chemical substance produced by a living organism, generally a microorganism, that is detrimental to other microorganisms. Antibiotics commonly are produced by soil microorganisms and probably represent a means by which organisms in a complex environment, such as soil, control the growth of competing microorganisms. Microorganisms that produce antibiotics useful in preventing or treating disease include the bacteria and the fungi.

Antibiotics came into worldwide prominence with the introduction of penicillin in 1941. Since then they have revolutionized the treatment of bacterial infections in humans and other animals. They are, however, ineffective against viruses.

The First Antibiotics

In 1928 Scottish bacteriologist Alexander Fleming noticed that colonies of bacteria growing on a culture plate had been unfavourably affected by a mold, Penicillium notatum, which had contaminated the culture. A decade later British biochemist Ernst Chain, Australian pathologist Howard Florey, and others isolated the ingredient responsible, penicillin, and showed that it was highly effective against many serious bacterial infections. Toward the end of the 1950s scientists experimented with the addition of various chemical groups to the core of the penicillin molecule to generate semisynthetic versions. A range of penicillins thus became available to treat diseases caused by different types of bacteria, including staphylococci, streptococci, pneumococci, gonococci, and the spirochaetes of syphilis.

Conspicuously unaffected by penicillin was the tubercle bacillus (Mycobacterium tuberculosis). This organism, however, turned out to be highly sensitive to streptomycin, an antibiotic that was isolated from Streptomyces griseus in 1943. As well as being dramatically effective against tuberculosis, streptomycin demonstrated activity against many other kinds of bacteria, including the typhoid fever bacillus. Two other early discoveries were gramicidin and tyrocidin, which are produced by bacteria of the genus Bacillus. Discovered in 1939 by French-born American microbiologist René Dubos, they were valuable in treating superficial infections but were too toxic for internal use.

In the 1950s researchers discovered the cephalosporins, which are related to penicillins but are produced by the mold Cephalosporium acremonium. The following decade scientists discovered a class of antibiotics known as quinolones. Quinolones interrupt the replication of DNA—a crucial step in bacterial reproduction—and have proven useful in treating urinary tract infections, infectious diarrhea, and various other infections involving elements such as bones and white blood cells.

Use And Administration Of Antibiotics

The principle governing the use of antibiotics is to ensure that the patient receives one to which the target bacterium is sensitive, at a high enough concentration to be effective but not cause side effects, and for a sufficient length of time to ensure that the infection is totally eradicated. Antibiotics vary in their range of action. Some are highly specific. Others, such as the tetracyclines, act against a broad spectrum of different bacteria. These are particularly useful in combating mixed infections and in treating infections when there is no time to conduct sensitivity tests. While some antibiotics, such as the semisynthetic penicillins and the quinolones, can be taken orally, others must be given by intramuscular or intravenous injection.

Categories Of Antibiotics

Antibiotics can be categorized by their spectrum of activity—namely, whether they are narrow-, broad-, or extended-spectrum agents. Narrow-spectrum agents (e.g., penicillin G) affect primarily gram-positive bacteria. Broad-spectrum antibiotics, such as tetracyclines and chloramphenicol, affect both gram-positive and some gram-negative bacteria. An extended-spectrum antibiotic is one that, as a result of chemical modification, affects additional types of bacteria, usually those that are gram-negative. (The terms gram-positive and gram-negative are used to distinguish between bacteria that have cell walls consisting of a thick meshwork of peptidoglycan [a peptide-sugar polymer] and bacteria that have cell walls with only a thin peptidoglycan layer, respectively.)

Mechanisms Of Action

Antibiotics produce their effects through a variety of mechanisms of action. A large number work by inhibiting bacterial cell wall synthesis; these agents are referred to generally as β-lactam antibiotics. Production of the bacterial cell wall involves the partial assembly of wall components inside the cell, transport of these structures through the cell membrane to the growing wall, assembly into the wall, and finally cross-linking of the strands of wall material. Antibiotics that inhibit the synthesis of the cell wall have a specific effect on one or another phase. The result is an alteration in the cell wall and shape of the organism and eventually the death of the bacterium.

Other antibiotics, such as the aminoglycosides, chloramphenicol, erythromycin, and clindamycin, inhibit protein synthesis in bacteria. The basic process by which bacteria and animal cells synthesize proteins is similar, but the proteins involved are different. Those antibiotics that are selectively toxic utilize these differences to bind to or inhibit the function of the proteins of the bacterium, thereby preventing the synthesis of new proteins and new bacterial cells.

Antibiotics such as polymyxin B and polymyxin E (colistin) bind to phospholipids in the cell membrane of the bacterium and interfere with its function as a selective barrier; this allows essential macromolecules in the cell to leak out, resulting in the death of the cell. Because other cells, including human cells, have similar or identical phospholipids, these antibiotics are somewhat toxic.

Some antibiotics, such as the sulfonamides, are competitive inhibitors of the synthesis of folic acid (folate), which is an essential preliminary step in the synthesis of nucleic acids. Sulfonamides are able to inhibit folic acid synthesis because they are similar to an intermediate compound (para-aminobenzoic acid) that is converted by an enzyme to folic acid. The similarity in structure between these compounds results in competition between para-aminobenzoic acid and the sulfonamide for the enzyme responsible for converting the intermediate to folic acid. This reaction is reversible by removing the chemical, which results in the inhibition but not the death of the microorganisms. One antibiotic, rifampin, interferes with ribonucleic acid (RNA) synthesis in bacteria by binding to a subunit on the bacterial enzyme responsible for duplication of RNA. Since the affinity of rifampin is much stronger for the bacterial enzyme than for the human enzyme, the human cells are unaffected at therapeutic doses.

Antibiotic Resistance

A problem that has plagued antibiotic therapy from the earliest days is the resistance that bacteria can develop to the drugs. An antibiotic may kill virtually all the bacteria causing a disease in a patient, but a few bacteria that are genetically less vulnerable to the effects of the drug may survive. These go on to reproduce or to transfer their resistance to others of their species through processes of gene exchange. With their more vulnerable competitors wiped out or reduced in numbers by antibiotics, these resistant strains proliferate. The end result is bacterial infections in humans that are untreatable by one or even several of the antibiotics customarily effective in such cases. The indiscriminate and inexact use of antibiotics encourages the spread of such bacterial resistance.

Researchers are continually working to discover new antibiotics as a means of overcoming antibiotic resistance. Some potentially effective compounds that have been discovered include certain bacterial toxins and antimicrobial peptides. Novel treatment strategies, such as combining synergistic antibiotics to boost the killing of bacteria, are also under investigation. It may be possible to introduce compounds into bacterial populations that effectively resensitize the bacteria to existing antibiotic drugs.

Major Antibiotics

Each type of antibiotic has a specific application in medicine and can serve as a useful model for exploring the various mechanisms by which antibiotics exert their effects. The following sections focus on the penicillins and cephalosporins, imipenem, the antituberculosis antibiotics, and the agents aztreonam, bacitracin, and vancomycin. These agents and groups of agents further illustrate the chemical and functional diversity found among the antibiotics.

Penicillins

The penicillins have a unique structure, a β-lactam ring, that is responsible for their antibacterial activity. The β-lactam ring interacts with proteins in the bacterial cell responsible for the final step in the assembly of the cell wall.

The penicillins can be divided into two groups: the naturally occurring penicillins (penicillin G, penicillin V, and benzathine penicillin) and the semisynthetic penicillins. The semisynthetic penicillins are produced by growing the mold Penicillium under conditions whereby only the basic molecule (6-aminopenicillanic acid) is produced. By adding certain chemical groups to this molecule, several different semisynthetic penicillins are produced that vary in resistance to degradation by β-lactamase (penicillinase), an enzyme that specifically breaks the β-lactam ring, thereby inactivating the antibiotic. In addition, the antibacterial spectrum of activity and pharmacological properties of the natural penicillins can be changed and improved by these chemical modifications. The addition of a β-lactamase inhibitor, such as clavulanic acid, to a penicillin dramatically improves the effectiveness of the antibiotic. Several naturally occurring inhibitors have been isolated, and others have been chemically synthesized.
The naturally occurring penicillins remain the drugs of choice for treating streptococcal sore throat, tonsillitis, endocarditis caused by some streptococci, syphilis, and meningococcal infections. Several bacteria, most notably Staphylococcus, developed resistance to the naturally occurring penicillins, which led to the production of the penicillinase-resistant penicillins (methicillin, oxacillin, nafcillin, cloxacillin, and dicloxacillin). The use of several of these agents, however, has been severely limited by resistance; methicillin is no longer used, because of the emergence of methicillin-resistant Staphylococcus aureus (MRSA).

To extend the usefulness of the penicillins to the treatment of infections caused by gram-negative rods, the broad-spectrum penicillins (ampicillin, amoxicillin, carbenicillin, and ticarcillin) were developed. These penicillins are sensitive to penicillinase, but they are useful in treating urinary tract infections caused by gram-negative rods as well as in treating typhoid and enteric fevers.

The extended-spectrum agents (mezlocillin and piperacillin) are unique in that they have greater activity against gram-negative bacteria, including Pseudomonas aeruginosa, a bacterium that often causes serious infection in people whose immune systems have been weakened. They have decreased activity, however, against penicillinase-producing Staphylococcus aureus, a common bacterial agent in food poisoning.

The penicillins are the safest of all antibiotics. The major adverse reaction associated with their use is hypersensitivity, with reactions ranging from a rash to bronchospasm and anaphylaxis. The more serious reactions are uncommon.

Cephalosporins

The cephalosporins have a mechanism of action identical to that of the penicillins. However, the basic chemical structure of the penicillins and cephalosporins differs in other respects, resulting in some difference in the spectrum of antibacterial activity. Modification of the basic molecule (7-aminocephalosporanic acid) produced by Cephalosporium acremonium resulted in four generations of cephalosporins.

The first-generation cephalosporins (cefazolin, cephalothin, and cephalexin) have a range of antibacterial activity similar to the broad-spectrum penicillins described above. For instance, they are effective against most staphylococci and streptococci as well as penicillin-resistant pneumococci.

The second-generation cephalosporins (cefamandole, cefaclor, cefotetan, cefoxitin, and cefuroxime) have an extended antibacterial spectrum that includes greater activity against additional species of gram-negative rods. Thus, these drugs are active against Escherichia coli and Klebsiella and Proteus species (though several strains of these organisms have developed resistance). Cefamandole is active against many strains of Haemophilus influenzae and Enterobacter, while cefoxitin is particularly active against most strains of Bacteroides fragilis. Second-generation cephalosporins have decreased activity, however, against gram-positive bacteria.

The third-generation cephalosporins (ceftriaxone, cefixime, and ceftazidime) have increased activity against the gram-negative organisms compared with the second-generation agents. Most Enterobacter species are susceptible to these drugs, as are H. influenzae and various species of Neisseria. The antibacterial spectrum of the fourth-generation compounds (cefepime) is similar to that of the third-generation drugs, but the fourth-generation drugs have more resistance to β-lactamases.

Like the penicillins, the cephalosporins are relatively nontoxic. Because the structure of the cephalosporins is similar to that of penicillin, hypersensitivity reactions can occur in penicillin-hypersensitive patients.

Imipenem

Imipenem is a β-lactam antibiotic that works by interfering with cell wall synthesis. It is highly resistant to hydrolysis by most β-lactamases. This drug must be given by intramuscular injection or intravenous infusion because it is not absorbed from the gastrointestinal tract. Imipenem is hydrolyzed by an enzyme present in the renal tubule; therefore, it is always administered with cilastatin, an inhibitor of this enzyme. Neurotoxicity and seizures have limited the use of imipenem.

Antituberculosis antibiotics

Isoniazid, ethambutol, pyrazinamide, and ethionamide are synthetic chemicals used in treating tuberculosis. Isoniazid, ethionamide, and pyrazinamide are similar in structure to nicotinamide adenine dinucleotide (NAD), a coenzyme essential for several physiological processes. Ethambutol prevents the synthesis of mycolic acid, a lipid found in the tubercule bacillus. All these drugs are absorbed from the gastrointestinal tract and penetrate tissues and cells. An isoniazid-induced hepatitis can occur, particularly in patients 35 years of age or older. Cycloserine, an antibiotic produced by Streptomyces orchidaceus, is also used in the treatment of tuberculosis. A structural analog of the amino acid D-alanine, it interferes with enzymes necessary for incorporation of D-alanine into the bacterial cell wall. It is rapidly absorbed from the gastrointestinal tract and penetrates most tissues quite well; high levels are found in urine. Rifampin, a semisynthetic agent, is absorbed from the gastrointestinal tract, penetrates tissue well (including the lung), and is used in the treatment of tuberculosis. Rifampin administration is associated with several side effects, mostly gastrointestinal in nature. The drug can turn urine, feces, saliva, sweat, and tears red-orange in colour.

Aztreonam, bacitracin, and vancomycin

Aztreonam is a synthetic antibiotic that works by inhibiting cell wall synthesis, and it is naturally resistant to some β-lactamases. Aztreonam has a low incidence of toxicity, but it must be administered parenterally.

Bacitracin is produced by a special strain of Bacillus subtilis. Because of its severe toxicity to kidney cells, its use is limited to the topical treatment of skin infections caused by Streptococcus and Staphylococcus and for eye and ear infections.

Vancomycin, an antibiotic produced by Streptomyces orientalis, is poorly absorbed from the gastrointestinal tract and is usually given by intravenous injection. It is used for the treatment of serious staphylococcal infections caused by strains resistant to the various penicillins. Its use against MRSA led to the emergence of vancomycin-resistant Staphylococcus aureus (VRSA).

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#833 2020-10-29 00:26:38

ganesh
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Re: Miscellany

811) Antimicrobial agent

Antimicrobial agent, any of a large variety of chemical compounds and physical agents that are used to destroy microorganisms or to prevent their development.

The production and use of the antibiotic penicillin in the early 1940s became the basis for the era of modern antimicrobial therapy. Streptomycin was discovered in 1944, and since then many other antibiotics and other types of antimicrobials have been found and put into use. A major discovery following the introduction of these agents into medicine was the finding that their basic structure could be modified chemically to improve their characteristics. Thus, antimicrobial agents that are used in the treatment of disease include synthetic chemicals as well as chemical substances or metabolic products made by microorganisms and chemical substances derived from plants.

Although the term antibiotic is popularly used to refer to drugs that combat microbes, strictly speaking, it refers only to drugs that kill or inhibit bacteria.

Antiseptics And Germicides

The term antiseptic refers to agents applied to the living tissues of humans, other animals, and plants in order to destroy (bactericidal) or inhibit the growth (bacteriostatic) of infectious microorganisms. Antiseptics are used in medical practice to prevent or combat bacterial infections of superficial tissues and to sterilize instruments and infected material. A distinction must be made between antiseptics and chemotherapeutic agents, such as antibiotics and sulfonamides, which are administered by mouth or by injection for the treatment of internal or generalized infections but may also be applied locally in the treatment or prevention of superficial infections.

Many chemical compounds can kill bacteria, but many of them also exhibit properties that limit or prohibit their use. Most antiseptics are general protoplasmic poisons and if used in sufficient concentration are harmful to the body’s cells and tissues as well as to bacteria. Thus, an antiseptic is most valuable in the disinfection of contaminated wounds or skin surfaces when there is a wide margin between its bactericidal and toxic concentrations. When, however, an antiseptic is to be used to disinfect contaminated instruments or other inanimate objects, its toxic properties are not important, and many compounds (called disinfectants) may be used that cannot be applied to living tissues. The term disinfectant thus refers to substances that are used to destroy microorganisms on inanimate surfaces; e.g., surgical instruments, floors, and walls. Antiseptics, disinfectants, and antibiotics are all germicides; i.e., they are all substances that kill microorganisms.

The efficiency of an antiseptic must be measured in relation to three main factors: concentration, time, and temperature. It is desirable to know the minimum concentration at which an antiseptic will be effective. Some antiseptics such as phenol lose their activity sharply beyond a certain dilution, whereas mercurial preparations still inhibit bacterial growth at very high dilutions. The time that an antiseptic takes to act depends to some extent on its concentration, but the speed at which different antiseptics kill bacteria varies considerably; thus, the halogens (e.g., iodine and chlorine salts) act quickly, whereas mercurials, compounds of heavy metals, and dyes act slowly. Most antiseptics act more quickly under increased temperatures; the activity of coal-tar derivatives, for instance, is doubled by a rise in temperature from that of a cool room to body heat. Many antiseptics destroy certain types of microorganisms and not others. Many others will kill bacteria but not their spores, which are walled, usually dormant, reproductive bodies.

Alcohols are among the most widely used antiseptics, especially ethyl and isopropyl alcohol, which are commonly used in a 70 percent concentration with water. They are also widely used in combination with other antiseptic agents. The phenols contain a large number of common antiseptics and disinfectants, among them phenol (carbolic acid) and creosote, while such bisphenols as hexyl resorcinol and hexachlorophene are widely used as antiseptic agents in soaps. Chlorine and iodine are both extremely effective agents and can be used in high dilution. Chlorine is widely used in the disinfection of drinking-water supplies, and among its derivatives, the hypochlorite solutions (e.g., Dakin’s solution) are used in surgical practice. Iodine is an effective disinfectant of wounds, particularly when used in an alcohol solution. The salts of most metals are generally too toxic to use on living tissues, but complex organic mercury salts (e.g., mercurochrome, merthiolate) in alcohol solution are highly bacteriostatic and make useful wound disinfectants. The quaternary ammonium compounds are more widely used as disinfectants than as antiseptics. Certain acridine dyes are used as antiseptics, as are some aromatic, or essential, oils. Most acids and alkalis either are too caustic to tissues or are relatively inefficient bactericides.

Sterilization

Sterilization, which is any process, physical or chemical, that destroys all forms of life, is used especially to destroy microorganisms, spores, and viruses. Precisely defined, sterilization is the complete destruction of all microorganisms by a suitable chemical agent or by heat, either wet steam under pressure at 120 °C (250 °F) or more for at least 15 minutes, or dry heat at 160 to 180 °C (320 to 360 °F) for three hours.

Sanitization

A sanitizer is an agent, usually chemical in nature, that is used to reduce the number of microorganisms to a level that has been officially approved as safe. Sanitizers are commonly used to control bacterial levels in equipment and utensils found in dairies, other food-processing plants, eating and drinking establishments, and other places in which no specific pathogenic microorganisms are known to be present and destruction of all microorganisms may not be necessary.

Other Antimicrobials

Preservatives, usually chemical agents, are added to certain foods and medicines to prevent the growth of microorganisms that may cause spoilage or disease. Prophylactics also are agents used to prevent infections and diseases. Vaccination is the administration of harmless amounts of disease-causing microorganisms into animals, including humans, to prevent diseases. (See vaccine.) Sterile filtration usually removes large microorganisms (e.g., bacteria, fungi, and their spores) from heat-sensitive solutions, but this physical method does not effectively remove small infectious microorganisms (e.g., filterable viruses and rickettsias).

Modes Of Action

Some antiseptics, such as alcohols and quaternary ammonium compounds, act directly on microbial cells to dissolve them. Others may penetrate the cells and cause the release of amino acids, nuclear material, and other important chemical constituents. Some compounds penetrate microbial cell walls and inactivate essential membrane transport systems so that the cells can no longer obtain the nutrients necessary for them to survive and to reproduce. Others coagulate certain vital materials in cells, thereby destroying the microorganisms. A few agents disrupt the metabolism of the cells so that they can no longer assimilate nutrients; as a result, the cells starve and die.

Side Effects And Drug Resistance

A number of antimicrobial compounds produce significant toxic effects in humans, but they are used because they have a favourable chemotherapeutic index; that is, the amount required for a therapeutic effect is below the amount that causes a toxic effect. The levels of these drugs in the patient must be controlled carefully so as not to reach toxic levels. Persons with certain altered organ functions, such as occurs in liver or kidney disease, are often especially susceptible to drug toxicity. The agents, however, can be used safely if drug concentrations in the blood are measured, the dose adjusted to avoid toxic levels, and organ function or toxicity monitored closely.

Whether an antimicrobial agent affects a microorganism depends on several factors. The drug must be delivered to a sensitive site in the cell, such as an enzyme that is involved in the synthesis of a cell wall or a protein or enzyme responsible for the synthesis of proteins, nucleic acids, or the cell membrane. Whether the antimicrobial agent enters the cell depends on the ability of the drug to penetrate the outer membrane of the cell, on the presence or absence of transport systems for the antimicrobial, or on the availability of channels in the cell surface. In some cases the microorganism prevents the entry of the antimicrobial by producing an enzyme that destroys or modifies the antimicrobial by transferring a chemical group. If the antimicrobial agent does not penetrate the organism or is destroyed or modified or if the organism does not contain a sensitive site, then the microorganism will not be affected; in such a case it is said to be resistant.
All agents can have adverse effects ranging from relatively harmless to serious and life-threatening. Direct toxicities are expressed in a variety of ways, and many of these are associated with the gastrointestinal tract (nausea, vomiting, and diarrhea) and skin rashes. They are usually minor and do not limit the use of the agent. In more extreme cases, the toxicities can result in serious damage to organs such as the kidneys, the liver, and the ears and to the nervous system. Some antimicrobial agents affect normal red blood cells, which can result in anemia. Allergic or hypersensitivity reactions can range from minor effects such as skin rash and itching to more serious effects that include choking and difficulty in breathing. In some cases a sudden and severe form of allergic reaction (anaphylaxis) can result in death.
The use of antimicrobial agents, in particular the broad-spectrum agents, can result in an alteration in the number and type of microorganisms normally found on the skin and mucosal surfaces. This is due to the inhibitory activity of the antimicrobial agent on sensitive microorganisms found on these tissues. The eradication of some organisms relieves the inhibitory activity they have on each other and thereby allows the surviving organisms to multiply. In some cases organisms (such as yeast) that are generally resistant to antibiotics increase to numbers sufficient to invade and infect tissue.

Some microorganisms have become resistant to drugs, requiring a continuing search for different (and often more expensive) agents. This increase in resistance to drugs has resulted from their widespread and sometimes indiscriminate use. Bacteria undergo spontaneous mutations, and exposure to an antibiotic can eradicate those bacteria sensitive to it while the resistant ones survive and multiply; by such means populations become resistant to a particular drug and sometimes to related drugs. Bacteria sensitive to antibiotics also can become resistant by acquiring resistance genes from other organisms, either by mating (conjugating) with bacteria containing resistance genes or by transduction (a process by which a bacterial virus, or bacteriophage, with resistance genes infects and incorporates these genes into a bacterium, thus conferring resistance). Resistance to antimicrobial agents also results from (1) decreased permeability of the organism to the drug, (2) deactivation or modification of the drug by an enzyme, (3) modification of the drug receptor or binding site, (4) increased synthesis of an essential metabolite whose production is blocked by the antimicrobial agent, or (5) production of an enzyme that is altered so that it is not inhibited or affected by the drug. Resistant bacteria are common in hospitals (nosocomial infections), where patients whose immunity is decreased can be infected.

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#834 2020-10-30 00:33:50

ganesh
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Re: Miscellany

812) Flower

Flower, the characteristic reproductive structure of angiosperms. As popularly used, the term “flower” especially applies when part or all of the reproductive structure is distinctive in colour and form.

In their range of colour, size, form, and anatomical arrangement, flowers present a seemingly endless variety of combinations. They range in size from minute blossoms to giant blooms. In some plants, such as poppy, magnolia, tulip, and petunia, each flower is relatively large and showy and is produced singly, while in other plants, such as aster, snapdragon, and lilac, the individual flowers may be very small and are borne in a distinctive cluster known as an inflorescence. Regardless of their variety, all flowers have a uniform function, the reproduction of the species through the production of seed.

Form And Types

Basically, each flower consists of a floral axis upon which are borne the essential organs of reproduction (stamens and pistils) and usually accessory organs (sepals and petals); the latter may serve to both attract pollinating insects and protect the essential organs. The floral axis is a greatly modified stem; unlike vegetative stems, which bear leaves, it is usually contracted, so that the parts of the flower are crowded together on the stem tip, the receptacle. The flower parts are usually arrayed in whorls (or cycles) but may also be disposed spirally, especially if the axis is elongate. There are commonly four distinct whorls of flower parts: (1) an outer calyx consisting of sepals; within it lies (2) the corolla, consisting of petals; (3) the androecium, or group of stamens; and in the centre is (4) the gynoecium, consisting of the pistils.

The sepals and petals together make up the perianth, or floral envelope. The sepals are usually greenish and often resemble reduced leaves, while the petals are usually colourful and showy. Sepals and petals that are indistinguishable, as in lilies and tulips, are sometimes referred to as tepals. The androecium, or male parts of the flower, comprise the stamens, each of which consists of a supporting filament and an anther, in which pollen is produced. The gynoecium, or female parts of the flower, comprises one or more pistils, each of which consists of an ovary, with an upright extension, the style, on the top of which rests the stigma, the pollen-receptive surface. The ovary encloses the ovules, or potential seeds. A pistil may be simple, made up of a single carpel, or ovule-bearing modified leaf; or compound, formed from several carpels joined together.

A flower having sepals, petals, stamens, and pistils is complete; lacking one or more of such structures, it is said to be incomplete. Stamens and pistils are not present together in all flowers. When both are present the flower is said to be perfect, or bisexual, regardless of a lack of any other part that renders it incomplete. A flower that lacks stamens is pistillate, or female, while one that lacks pistils is said to be staminate, or male. When the same plant bears unisexual flowers of both males and females, it is said to be monoecious (e.g., tuberous begonia, hazel, oak, corn); when the male and female flowers are on different plants, the plant is dioecious (e.g., date, holly, cottonwood, willow); when there are male, female, and bisexual flowers on the same plant, the plant is termed polygamous.

A flower may be radially symmetrical,  as in roses and petunias, in which case it is termed regular or actinomorphic. A bilaterally symmetrical flower, as in orchids and snapdragons, is irregular or zygomorphic.

Pollination

The stamens and pistils are directly involved with the production of seed. The stamen bears microsporangia (spore cases) in which are developed numerous microspores (potential pollen grains); the pistil bears ovules, each enclosing an egg cell. When a microspore germinates, it is known as a pollen grain. When the pollen sacs in a stamen’s anther are ripe, the anther releases them and the pollen is shed. Fertilization can occur only if the pollen grains are transferred from the anther to the stigma of a pistil, a process known as pollination.

There are two chief kinds of pollination: (1) self-pollination, the pollination of a stigma by pollen from the same flower or another flower on the same plant; and (2) cross-pollination, the transfer of pollen from the anther of a flower of one plant to the stigma of the flower of another plant of the same species. Self-pollination occurs in many species, but in the others, perhaps the majority, it is prevented by such adaptations as the structure of the flower, self-incompatibility, and the maturation of stamens and pistils of the same flower or plant at different times. Cross-pollination may be brought about by a number of agents, chiefly insects and wind. Wind-pollinated flowers generally can be recognized by their lack of colour, odour, or nectar, while animal-pollinated flowers  are conspicuous by virtue of their structure, colour, or the production of scent or nectar.

After a pollen grain has reached the stigma, it germinates, and a pollen tube protrudes from it. This tube, containing two male gametes,  extends into the ovary and reaches the ovule, discharging its gametes so that one fertilizes the egg cell, which becomes an embryo, and the other joins with two polar nuclei to form the endosperm. (Normally many pollen grains fall on a stigma; they all may germinate, but only one pollen tube enters any one ovule.) Following fertilization, the embryo is on its way to becoming a seed, and at this time the ovary itself enlarges to form the fruit.

Cultural Significance

Flowers have been symbols of beauty in most civilizations of the world, and flower giving is still among the most popular of social amenities. As gifts, flowers serve as expressions of affection for spouses, other family members, and friends; as decorations at weddings and other ceremonies; as tokens of respect for the deceased; as cheering gifts to the bedridden; and as expressions of thanks or appreciation. Most flowers bought by the public are grown in commercial greenhouses or horticultural fields and then sold through wholesalers to retail florists.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

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#835 2020-10-31 00:22:43

ganesh
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Re: Miscellany

813) Dog

Dog, (Canis lupus familiaris), domestic mammal of the family Canidae (order Carnivora). It is a subspecies of the gray wolf (Canis lupus) and is related to foxes and jackals. The dog is one of the two most ubiquitous and most popular domestic animals in the world (the cat is the other). For more than 12,000 years it has lived with humans as a hunting companion, protector, object of scorn or adoration, and friend.

The dog evolved from the gray wolf into more than 400 distinct breeds. Human beings have played a major role in creating dogs that fulfill distinct societal needs. Through the most rudimentary form of genetic engineering, dogs were bred to accentuate instincts that were evident from their earliest encounters with humans. Although details about the evolution of dogs are uncertain, the first dogs were hunters with keen senses of sight and smell. Humans developed these instincts and created new breeds as need or desire arose.

Dogs are regarded differently in different parts of the world. Characteristics of loyalty, friendship, protectiveness, and affection have earned dogs an important position in Western society, and in the United States and Europe the care and feeding of dogs has become a multibillion-dollar business. Western civilization has given the relationship between human and dog great importance, but, in some of the developing nations and in many areas of Asia, dogs are not held in the same esteem. In some areas of the world, dogs are used as guards or beasts of burden or even for food, whereas in the United States and Europe dogs are protected and admired. In ancient Egypt during the days of the pharaohs, dogs were considered to be sacred.

Dogs have played an important role in the history of human civilization and were among the first domesticated animals. They were important in hunter-gatherer societies as hunting allies and bodyguards against predators. When livestock were domesticated about 7,000 to 9,000 years ago, dogs served as herders and guardians of sheep, goats, and cattle. Although many still serve in these capacities, dogs are increasingly used for social purposes and companionship. Today dogs are employed as guides for the blind and disabled or for police work. Dogs are even used in therapy in nursing homes and hospitals to encourage patients toward recovery. Humans have bred a wide range of different dogs adapted to serve a variety of functions. This has been enhanced by improvements in veterinary care and animal husbandry.

Origin And History Of Dogs

Ancestry

Paleontologists and archaeologists have determined that about 60 million years ago a small mammal, rather like a weasel, lived in the environs of what are now parts of Asia. It is called Miacis, the genus that became the ancestor of the animals known today as canids: dogs, jackals, wolves, and foxes. Miacis did not leave direct descendants, but doglike canids evolved from it. By about 30 to 40 million years ago Miacis had evolved into the first true dog—namely, Cynodictis. This was a medium-size animal, longer than it was tall, with a long tail and a fairly brushy coat. Over the millennia Cynodictis gave rise to two branches, one in Africa and the other in Eurasia. The Eurasian branch was called Tomarctus and is the progenitor of wolves, dogs, and foxes.

The timing and location of dog domestication is less clear and has been a matter of significant debate, but there is strong genetic evidence that the first domestication events occurred somewhere in Central Asia before 15,000 years ago. Some genetic studies have suggested that wolves were domesticated 16,300 years ago to serve as livestock in China. Other genetic studies, however, have suggested that dog domestication began as early as 18,800–32,100 years ago in Europe or that early dogs dating from about 12,000 to 14,000 years ago came from a small strain of gray wolf that inhabited what is now India. Thereafter this wolf—known as Canis lupus pallipes—was widely distributed throughout Europe, Asia, and North America. However, one genetic study that compared the DNA of dogs and wolves inhabiting areas thought to have been centres of dog domestication suggests that dogs and modern wolves belong to separate lineages that share a common ancestor. It is also possible that some of the dogs of today descended not from the wolf but rather from the jackal. These dogs, found in Africa, might have given rise to some of the present native African breeds. A genetic study examining the migration of dogs to the Americas revealed evidence that dogs did not accompany the first humans to the New World more than 15,000 years ago; the study suggested that dogs came to the Americas only 10,000 years ago.

No matter what their origins, all canids have certain common characteristics. They are mammals that bear live young. The females have mammary glands, and they suckle their offspring. The early breeds had erect ears and pointed or wedge-shaped muzzles, similar to the northern breeds common today. Most of the carnivores have similar dental structures, which is one way paleontologists have been able to identify them. They develop two sets of teeth, deciduous (“baby”) teeth and permanent teeth.

Canids walk on their toes, in contrast to an animal like the bear, which is flat-footed and walks on its heels. Dogs, like most mammals, have body hair and are homeothermic—that is to say, they have an internal thermostat that permits them to maintain their body temperature at a constant level despite the outside temperature.

Fossil remains suggest that five distinct types of dogs existed by the beginning of the Bronze Age (about 4500 BCE). They were the mastiffs, wolf-type dogs, sight hounds (such as the Saluki or greyhound), pointing dogs, and herding dogs.

How Did Dogs Get to Be Dogs?

The origin of man's best friend has been a source of wonder and heated debate for centuries.

Even Charles Darwin was unsure whether the dog's true ancestry could be determined, because dog breeds vary so greatly. In fact, the domestic dog is far more variable in size, shape and behavior than any other living mammal, according to James Serpell, professor at the University of Pennsylvania School of Veterinary Medicine and editor of "The Domestic Dog: Its Evolution, Behaviour, and Interactions With People" (Cambridge University Press, 1995).

There are many theories on how dogs evolved as a species, including the view that they are mixed descendants of two or more wild species, such as wolves, dingoes and jackals. But newer evidence hasn't supported that theory.

"Nowadays, based on a growing body of anatomical, genetic, and behavioral evidence, most experts believe that the dog originated exclusively from a single species: the gray wolf, Canis lupus," Serpell told Life's Little Mysteries.

The similarities between wolves and dogs are great. In the 1960s, ethologist John Paul Scott tried to untangle the behaviors of these two species, and created a catalog of 90 behaviors of dogs. All but 19 of them, however, were also observed in wolves, and the missing behaviors tended to be minor activities that probably had not been recorded at the time but do occur in wolves, Serpell said.

"Recent anatomical and molecular evidence has confirmed that wolves, dogs and dingoes are all more closely related to each other than they are to any other member of the family Canidae," Serpell said.

The oldest skeletal remains of probable domestic wolf-dogs were excavated from the Upper Paleolithic site of Eliseyevichi in western Russia, close to the Ukrainian border, and date as far back as 19,000 years. Two skulls resembled those of Siberian huskies in their general shape, according to Serpell.

Loyal companions

Bones of ancient domestic wolf-dogs also have been found in central Europe, the Near East and North America, where they appear to have been deliberately buried with their human companions or in separate graves.

The 14,000-year-old remains of a puppy and an elderly person were found buried together in Israel, Serpell said. The person's left hand was apparently positioned so that it rested on the dog's flank, which shows that the relationship between man and dog is one of the oldest and most durable of friendships, he said.

So what allows for dogs to get along with humans so well?

"Several biological and behavioral factors predisposed dogs to fit easily within human groups," said Leslie Irvine of the University of Colorado at Boulder. "They have a long primary socialization period during which they can become closely bonded with humans."

Dogs are active during the same hours as their owners, as opposed to nocturnal animals, said Irvine, author of "If You Tame Me: Understanding Our Connection With Animals" (Temple University Press, 2004). Their loyal and obedient behavior allows them to be house-trained and to be taught to behave in return for little more than a treat and a pat on the head.

In fact, a domestic dog considers its owner or owners to be its "pack," and the owners' home to be its territory, according to "Simon & Schuster's Guide to Dogs" (Fireside, 1980).

"If a reciprocal understanding and affection have grown up between man and dog, it is because the domestication of the dog took place through an agreement on work and the division of food and lodging," according to "Simon & Schuster's Guide to Dogs." "This resulted in an affectionate and intelligent cooperation and the integration of the dog into human society."

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#836 2020-11-01 00:36:27

ganesh
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Re: Miscellany

814) Scorpion

Scorpion, (order Scorpiones or Scorpionida), any of approximately 1,500 elongated arachnid species characterized by a segmented curved tail tipped with a venomous stinger at the rear of the body and a pair of grasping pincers at the front. Although scorpions are most common and diverse in deserts, they also live in many other habitats. Primarily nocturnal, scorpions often play the role of evildoers in fables and legends. Greek respect for scorpions prompted the naming of the constellation Scorpius, a sign of the zodiac.

General Features

Size range and diversity of structure

Scorpions are relatively large among terrestrial arthropods, with an average size of about 6 cm (2.5 inches). Scorpions exhibit few sexual differences, although males usually are more slender and have longer tails than females. Giants among scorpions include the black emperor scorpion (Pandinus imperator), an African species found in Guinea, which attains a body length of about 18 cm (7 inches) and a mass of 60 grams (more than 2 ounces). The longest scorpion in the world is the rock scorpion (Hadogenes troglodytes) of South Africa; females attain a length of 21 cm (8.3 inches). The length of the smallest scorpions, the Caribbean Microtityus fundorai, is 12 mm (0.5 inch). A few precursors of modern scorpions were comparative giants. Fossils of two species (Gigantoscorpio willsi and Brontoscorpio anglicus) measure from 35 cm (14 inches) to a metre (3.3 feet) or more, and an undescribed species is estimated to have been 90 cm (35.5 inches). Most species from deserts and other arid regions are yellowish or light brown in colour; those found in moist or mountain habitats, however, are brown or black.

Distribution and abundance

In addition to desert habitats, scorpions have adapted to temperate, subtropical, and tropical environments such as grasslands, savannas, and forests. They live on all major landmasses except Greenland and Antarctica. Their range extends from Canada and central Europe to the southern tips of South America (Tierra del Fuego) and Africa, and they have been accidently introduced into New Zealand and England. Scorpions have been found at elevations from sea level to 5,000 metres (more than 16,000 feet) in mountains of Europe and North and South America. A few species live as far north as southern Canada, southern Germany, and Russia.

Natural History

Reproduction and life cycle

Breeding is seasonal and generally occurs during the warm months, ranging from late spring through early fall. Males may travel hundreds of metres to find receptive females. It appears that males find females by localizing a pheromone that the female emits from the end of her abdomen. Mating in scorpions is preceded by a complicated and characteristic courtship initiated by the male. He first faces and grasps the female, using his pincers (pedipalps). Then the pair, directed by the male, moves sideways and backward in a dancelike motion called promenade à deux. These actions result from the efforts of the pair to find a smooth surface on which the male can extrude a glandular secretion that forms a stalk to which the spermatophore (male gamete-containing structure) is attached. He then maneuvers the female so that her genital opening contacts the spermatophore. Once she is positioned over the spermatophore, physical contact with it causes spermatozoa to be ejected into the genital opening (gonopore) of the female. Males that remain near females after mating are sometimes killed and eaten.

In general, females mate multiple times. In some species, mating must occur after each clutch of offspring is produced in order to fertilize another clutch of eggs. In others, multiple clutches of offspring can be produced from the sperm stored from a single mating. There are at least two species known that can produce offspring without mating at all (parthenogenesis).

The mother invests a great amount of time and energy in her offspring. Unlike most nonmammalian animals, scorpions are viviparous, giving birth to live young instead of laying eggs. Once fertilized, the eggs are retained in the female’s body, where the embryos are nourished in utero for periods varying from several months to a year. The birth process itself may last from several hours to several days. Temperate species usually give birth in spring and summer, whereas tropical species give birth throughout the year. Litter size averages 25, with a range of 1 to more than 100.

At birth a young scorpion is white and enveloped in a membrane, or chorion. After freeing itself, the immature scorpion crawls onto the mother’s back, where it remains for a period ranging from 1 to 50 days. During this time the young scorpions are defenseless and utilize food reserves in their bodies while receiving water transpired through the mother’s cuticle and taken up through their own. The young molt their soft embryonic cuticle for one that is fully functional when they assume independence. This early mother-young association is obligatory for newborns. Without it, they do not molt successfully and usually die. The young generally leave the mother soon after this first molt.

As in all arthropods, growth is accompanied by molting (ecdysis). Scorpions molt an average of five times (the range is four to nine) before reaching maturity. The number of molts in some species is variable. In some Centruroides, for example, small males mature after four molts and large males after five. There are no reports of molting’s occurring after reproductive maturity has been reached.

Food and feeding

Scorpions are opportunistic predators that eat any small animal they can capture. Common prey includes insects as well as spiders and other arachnids, including other scorpions. Less-common but regular prey includes pill bugs, snails, and small vertebrates such as lizards, snakes, and rodents. The only known specialist scorpion is the Australian spiral burrow, or spider-hunting, scorpion (Isometroides vescus), which feeds solely on burrowing spiders.

Most scorpions are sit-and-wait predators that remain motionless until a suitable victim has moved into an ambush zone. Scorpions can sense tiny ground vibrations, and some can detect airborne vibrations of flying insects. These behaviours are sophisticated to the extent that scorpions can determine the precise distance and direction of their prey. Once the prey has been detected, the scorpion turns, runs to the prey, and seizes it. The prey is stung if it is relatively large, aggressive, or active. Otherwise it is simply held by the pedipalps as it is eaten. Many of the thick-tailed scorpions (family Buthidae), however, actively search for prey. These species usually have long, slender bodies and pincers (chelae). Many have powerful venoms to compensate for their small pincers.

Scorpions lack conventional jaws, and their feeding habits are unusual. An additional pair of pincerlike appendages (chelicerae) are toothed, and, with these tools as well as the sharp edges of adjacent jawlike structures (maxillae and coxae), the scorpion chews the prey as quantities of digestive fluids secreted from the midgut pour over it. The victim’s soft parts are broken down, liquefied, and sucked into the scorpion’s stomach by a pumping action. The victim is gradually reduced to a ball of indigestible material, which is cast aside. Eating is a slow process, often taking many hours.

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#837 2020-11-02 00:17:53

ganesh
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Re: Miscellany

815) King cobra

King cobra, (Ophiophagus hannah), also called hamadryad, the world’s largest venomous snake, found predominantly in forests from India through Southeast Asia to the Philippines and Indonesia. The snake’s maximum confirmed length is 5.6 metres (18 feet), but most do not exceed 3.6 metres (12 feet). The king cobra is the sole member of its genus. It is classified as part of family Elapidae, the cobra family. Hamadryad, the alternative common name for the snake, is a Greek word meaning “wood nymph.”

Physical Features

The main feature that distinguishes the king cobra from other cobras is the possession of 11 large scales on the crown of its head. Adults may be yellow, green, brown, or black. The back is typically highlighted by yellowish or whitish crossbars or chevrons, and the underside may display a single colour that may or may not be ornamented with bars. The throat is light yellow or beige. Juvenile king cobras are smaller and black. Hatchlings are about 45–55 cm (18–22 inches) long and are black with yellow or white stripes.

Predators And Prey

The king cobra is an active hunter. It preys almost exclusively on other snakes, prowling in forests, fields, and villages in the daytime as well as at night. The king cobra’s greatest predators are humans, who harvest it and use certain body parts for food, medicine, and leather.

It is not normally aggressive to humans—bites are rare—but it is hostile and dangerous during the breeding season or when cornered or startled. When threatened, the king cobra lifts the front part of its body so it stands roughly perpendicular to the ground. Its threat display also includes the spreading of its hood, which is just below its head, and the utterance of a low-pitched hiss. In this posture it can move or lunge forward to strike its victim. The king cobra’s paralyzing venom is so copious and powerful that elephants have died within three hours of a bite to the toe or trunk. In captivity it is aggressive to strangers but recognizes its keeper and anticipates feeding time.

Breeding

The mating times of king cobras vary with region. Breeding behaviour is unique in that mated pairs remain together for the season, and the female builds a nest. Using a loop of her body as an arm, she pulls dead leaves, soil, and ground litter into a compact mound, in which she lays 20 to 50 eggs. She coils above or near the eggs for about two months. The female is known to fiercely defend the breeding ground, and, according to some wildlife officials and herpetologists, the male remains nearby and also guards the area.

Conservation Status

Despite the king cobra’s large geographic range, in 2010 the IUCN Red List of Threatened Species designated the snake as vulnerable. The IUCN’s decision was based on the fact that the king cobra population had declined by 30 percent between 1935 and 2010 and that the species faced ongoing threats of habitat loss and overharvesting.

King-Cobra-Snake-Cobra-Hamadryad-Free-Image-Reptil-6362.jpg


It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

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#838 2020-11-03 00:50:46

ganesh
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Re: Miscellany

816) Angina pectoris

Angina pectoris, pain or discomfort in the chest, usually caused by the inability of diseased coronary arteries to deliver sufficient oxygen-laden blood to the heart muscle. When insufficient blood reaches the heart, waste products accumulate in the heart muscle and irritate local nerve endings, causing a deep sensation of heaviness, squeezing, or burning that is most prominent behind or beneath the breastbone and over the heart and stomach. In some instances, the sensation may radiate into the shoulders, the neck, the jaw, or the arms on one or both sides of the body. A feeling of constriction or suffocation often accompanies the discomfort, though there is seldom actual difficulty in breathing. Symptoms usually subside within five minutes. In acute cases (e.g., unstable angina or acute coronary syndrome), the skin becomes pale and the pulse is weak. Although symptoms may be mild in some cases, the peculiar qualities of angina pectoris may induce anxiety.

Attacks of angina can be precipitated by walking or more strenuous exertion; by anger, fear, or other stressful emotional states; by exercise after a large meal; or by exposure to cold or wind. Attacks are apt to recur following less or no exertion as coronary heart disease worsens. Angina pectoris is rare in persons under middle age and tends to be more common in men than in women. Men and women sometimes experience different symptoms; women, for example, may experience nausea or vomiting, feel sharp pain rather than pressure in the chest, or have symptoms of increased duration. Differences in the characteristics of angina are attributed to differences in the underlying conditions that precipitate angina; for example, coronary artery disease frequently is associated with angina in men, whereas coronary microvascular disease is a common cause of angina in women.

An anginal attack can be relieved by rest or by taking nitroglycerin or other drugs that relax (and thus dilate) the blood vessels. The frequency of attacks can be lessened by the avoidance of emotional stress and by shifting to exercise that is less vigorous. In cases where the narrowing of the coronary arteries appears serious enough to cause a heart attack (myocardial infarction), methods must be used to widen the passages within the arteries or surgically replace the arteries with unblocked ones from another portion of the body.

Angina%20Pectoris.jpg


It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#839 2020-11-04 00:36:29

ganesh
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Re: Miscellany

817) Fever
   
Fever, also called pyrexia, abnormally high body temperature. Fever is a characteristic of many different diseases. For example, although most often associated with infection, fever is also observed in other pathologic states, such as cancer, coronary artery occlusion, and certain disorders of the blood. It also may result from physiological stresses, such as strenuous exercise or ovulation, or from environmentally induced heat exhaustion or heat stroke.

Under normal conditions, the temperature of deeper portions of the head and trunk does not vary by more than 1–2 °F in a day, and it does not exceed 99 °F (37.22 °C) in the mouth or 99.6 °F (37.55 °C) in the rectum. Fever can be defined as any elevation of body temperature above the normal level. Persons with fever may experience daily fluctuations of 5–9 °F above normal; peak levels tend to occur in the late afternoon. Mild or moderate states of fever (up to 105 °F [40.55 °C]) cause weakness or exhaustion but are not in themselves a serious threat to health. More serious fevers, in which body temperature rises to 108 °F (42.22 °C) or more, can result in convulsions and death.

During fever the blood and urine volumes become reduced as a result of loss of water through increased perspiration. Body protein is rapidly broken down, leading to increased excretion of nitrogenous products in the urine. When the body temperature is rising rapidly, the affected person may feel chilly or even have a shaking chill; conversely, when the temperature is declining rapidly, the person may feel warm and have a flushed moist skin.

In treating fever, it is important to determine the underlying cause of the condition. In general, in the case of infection, low-grade fevers may be best left untreated in order to allow the body to fight off infectious microorganisms on its own. However, higher fevers may be treated with acetaminophen or ibuprofen, which exerts its effect on the temperature-regulating areas of the brain.

The mechanism of fever appears to be a defensive reaction by the body against infectious disease. When bacteria or viruses invade the body and cause tissue injury, one of the immune system’s responses is to produce pyrogens. These chemicals are carried by the blood to the brain, where they disturb the functioning of the hypothalamus, the part of the brain that regulates body temperature. The pyrogens inhibit heat-sensing neurons and excite cold-sensing ones, and the altering of these temperature sensors deceives the hypothalamus into thinking the body is cooler than it actually is. In response, the hypothalamus raises the body’s temperature above the normal range, thereby causing a fever. The above-normal temperatures are thought to help defend against microbial invasion because they stimulate the motion, activity, and multiplication of white blood cells and increase the production of antibodies. At the same time, elevated heat levels may directly kill or inhibit the growth of some bacteria and viruses that can tolerate only a narrow temperature range.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

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#840 2020-11-05 00:42:47

ganesh
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Re: Miscellany

818) Rheumatic fever

Rheumatic fever, inflammatory disease of the heart, joints, central nervous system, and subcutaneous tissues that develops after a throat infection with group A beta-hemolytic Streptococcus bacteria, including untreated scarlet fever or strep throat. Prevention is possible with penicillin, but specific treatment is not available. Rheumatic fever is particularly important because of the heart disease, which includes vascular damage, that may ensue. The illness occurs chiefly in children and young adults, with a peak incidence between ages 5 and 15.

When a streptococcal throat infection goes untreated, most patients recover without complications. Approximately 1 percent, however, develop rheumatic fever. The onset of the disease is most often characterized by the sudden occurrence of fever and joint pain and inflammation several days to six weeks after the streptococcal infection. Signs of cardiac involvement include heart murmurs, increased heartbeat rate, and heart enlargement. Inflammation of the heart muscle and supporting structures may lead to a permanent scarring and contracture of the heart valves and a marked decrease in life expectancy. Other symptoms of rheumatic fever include nodules beneath the skin and skin rashes, the most typical of which is erythema marginatum; Sydenham chorea, a nervous system manifestation marked by emotional instability and purposeless, involuntary movements of the arms and legs; abdominal pain; nosebleeds; weakness; and loss of appetite and body weight. Generally the clinical symptoms, severity, and aftereffects of an attack of rheumatic fever are highly variable, ranging from a condition so mild as to go unnoticed to a severe acute attack associated with cardiac failure and death.

During the course of rheumatic fever, the streptococcal organism may no longer be demonstrable in cultures of the throat or other infected body areas, but blood titres of antibodies against the streptococcus, such as antistreptolysin O, are high. All the numerous types of group A beta-hemolytic Streptococcus appear capable of inducing rheumatic fever in susceptible individuals; infection with one type confers no immunity against the others, and individuals who have experienced one attack of rheumatic fever are especially prone to subsequent attacks. Both the initial and recurrent attacks can be effectively prevented with penicillin. Symptomatic treatment of the condition includes the use of salicylates such as aspirin or one of the steroid hormones. Surgery may be advocated to relieve the narrowing of the openings of the heart valves. Rheumatic fever patients must receive antibiotics on a regular basis for the remainder of their lives because their damaged heart valves predispose them to the development of bacterial endocarditis.

The exact cause of rheumatic fever is not clear, although most authorities favour the theory that the disease results from an autoimmune reaction, involving the production of antibodies that attack the body’s own tissues. The autoimmune reaction is believed to be triggered by components of the streptococci (antigens) whose structure resembles that of molecules found in human tissue (“self antigens”). Because of this resemblance, the antibodies that recognize streptococcal antigens may mistakenly react with similarly shaped antigens of certain cells of the body—such as those of the heart. By binding to these self antigens, the antibodies cause the tissue damage characteristic of rheumatic fever.

Since the mid-20th century the incidence and severity of rheumatic fever and other streptococcal infections, such as scarlet fever, have declined precipitously in the developed countries. This decline has occurred independently of the use of penicillin and other drugs and may simply signal the gradual dying out of the disease. However, in many other parts of the world rheumatic fever remains a serious and prevalent disease.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#841 2020-11-06 00:18:47

ganesh
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Re: Miscellany

819) Laparoscopy

Laparoscopy, also called peritoneoscopy, procedure that permits visual examination of the abdominal cavity with an optical instrument called a laparoscope, which is inserted through a small incision made in the abdominal wall. The term comes from the Greek words laparo, meaning “flank,” and skopein, meaning “to examine.”

The laparoscope is a type of endoscope—i.e., a device similar to a small telescope that is equipped with a light source. Laparoscopy came into use early in the 20th century. It was first used as a means of diagnosing abdominal pain. By the 1960s gynecologists were using the laparoscope in operations such as tubal ligations. Modern laparoscopes have been fitted with fibre-optic lights and small video cameras that allow a surgical team to view the abdominal tissues and organs on a monitor in the operating room. These improvements have expanded the applications of laparoscopy. Today the technique is not only used to obtain diagnostic information but employed in a variety of surgeries, including removal of the gallbladder (cholecystectomy), appendectomy, hysterectomy, repair of hernias, and removal of cancerous tumours.

Laparoscopy is a minimally invasive surgical procedure because it requires a much smaller incision than traditional surgery does, causing less damage to nerves, muscles, and skin. It can be performed with only local anesthesia and a mild sedative. To begin the procedure, carbon dioxide is pumped into the abdomen, thereby expanding the abdominal cavity to provide the physician with space to maneuver instruments. Next a small incision is made for the laparoscope. Additional tiny cuts can be made if surgical instruments such as forceps and scissors are needed in the procedure. Valuable diagnostic information can be obtained by examining a biopsy specimen of the liver or abdominal lesions. The benefits of laparoscopic surgery include a reduction in postoperative pain, brief recovery times, and shortened hospital stays.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

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#842 2020-11-07 00:47:02

ganesh
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Re: Miscellany

820) Black panther

Black panther, colloquial term used to refer to large felines classified in the genus Panthera that are characterized by a coat of black fur or large concentrations of black spots set against a dark background. The term black panther is most frequently applied to black-coated leopards (Panthera pardus) of Africa and Asia and jaguars (P. onca) of Central and South America; black-furred variants of these species are also called black leopards and black jaguars, respectively. In addition, the term is sometimes used to describe dark-coloured bobcats, lynx, jaguarundis, tigers, and pumas (cougars), even though reports of black-coloured representatives of some species, such as the puma, have not been confirmed.

Black coat coloration is attributed to the expression of recessive alleles in leopards and dominant alleles in jaguars. In each species, a certain combination of alleles stimulates the production of large amounts of the dark pigment melanin in the animal’s fur and skin. Although melanin concentrations often vary between members of the same litter, individuals displaying completely black coats are rare.

The appearance of a black coat may be influenced by other factors, such as the angle of incident light and the animal’s life stage. For example, some melanistic leopards and jaguars display totally black coats, because the finer details of their fur may be masked by diffuse light. In full sunlight, however, the faint spotted pattern of the coat may emerge. Also, blackened or near-blackened coats may result from the retention of black spots from the juvenile stage, which may complement other concentrations of dark-coloured fur, into adulthood. In other species, such as lynx, the appearance of black or near-black fur might also be explained by seasonal colour changes.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

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#843 2020-11-08 00:40:43

ganesh
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Re: Miscellany

821) Feline

Feline, (family Felidae), any of 37 cat species that among others include the cheetah, puma, jaguar, leopard, lion, lynx, tiger, and domestic cat. Cats are native to almost every region on Earth, with the exception of Australia and Antarctica. They are carnivorous mammals that live in a wide variety of habitats, but they are typically woodland animals.

Most cats are patterned with spots, stripes, or rosettes, but some, such as the puma (Puma concolor), jaguarundi (Herpailurus yaguarondi), and lion (Panthera leo), are uniform in colour. Black or nearly black coats occur in individuals of several species. Although lynx (genus Lynx) have a stubby tail, most cats have a long tail that makes up about a third of the animal’s total length. The head is characterized by a short nose and round face, usually with short ears. The only cat with a well-developed mane is the male African lion. Cat feet have sharp claws that are retractile except in the cheetah. In most felids the male is larger than the female.

Cats are noted for purring when content and for snarling, howling, or spitting when in conflict with another of their kind. The so-called “big cats” (genus Panthera), especially the lion, often roar, growl, or shriek. Usually, however, cats are silent. Many cats use “clawing trees,” upon which they leave the marks of their claws as they stand and drag their front feet downward with the claws extended. Whether such behaviour is for the purpose of cleaning or sharpening the claws or simply to stretch is debatable, but the behaviour is innate; kittens raised in isolation soon begin to claw objects.

The larger cats are strong, fierce, and extremely dangerous when hungry. Because of their large size, they occasionally attack humans. Although tigers and leopards are most noted as man-eaters, lions and jaguars can also be dangerous. In North America the puma, also known as the cougar or mountain lion, tends to avoid contact with humans, but a few attacks occur annually, especially in areas where development encroaches on areas of high puma density, such as the western United States. Similarly, attacks on livestock often necessitate removal of problem animals.

The fur of some cats is sometimes in great demand, especially fur with contrasting colours and patterns such as spots or stripes. The demand is such that some rare cats are hunted and trapped illegally and are in danger of becoming extinct. In North America none of the cats used by the fur trade is endangered. Strict regulations allow a sustainable take of animals from healthy populations of Canada lynx (Lynx canadensis), bobcat (Lynx rufus), and puma.

Natural History

The lion, tiger (Panthera tigris), and cheetah (Acinonyx jubatus) are mainly terrestrial, but they are agile climbers; the leopard (Panthera pardus), jaguar (P. onca), ocelot (Leopardus pardalis), and other cats are very much at home in trees. The larger cats range over large areas, often roving alone or with a companion. Occasionally one may become a member of a family group. Only lions are gregarious, with prides consisting of as many as 30 individuals.

Almost all cats feed on small mammals and birds or on large herbivores such as deer and various types of antelope. The fishing cat (Prionailurus viverrinus) feeds largely on fishes and clams or snails and thus fits into a slightly different niche than that of most cats. The flat-headed cat (Prionailurus planiceps) is the only species known to feed to any extent on vegetation such as fruit and sweet potatoes. Food caching occurs in larger cats, and some may drag their kill into a tree or place it under a bush after the initial gorging. Cats live on a feast-or-famine routine, gorging themselves when a kill is made and then fasting for several days.

Most hunting is done using vision and hearing. Typically solitary while hunting, a cat steals up to its prey on padded feet. Long, sensitive whiskers on the face aid the cat during the stalking of the prey by brushing against obstacles and enabling the cat to avoid making excessive noise. When close to its prey, the cat overwhelms it in a short, quick rush or leap. Cats can move very fast in a short dash but are not built for sustained speed. The cheetah, which usually hunts during the day, is credited with being the speediest of mammals, capable of speeds of more than 100 km per hour (62 mph). Cats rely on superior speed and reflexes to overtake their dodging prey, which often has greater endurance. If overtaken, the prey is thrown down and dispatched with a deep bite, usually to the neck.

The gestation period of most smaller cats is approximately two months, and that of the larger cats is closer to four months. One to six kittens make up the usual litter. Female cats may have from four to eight nipples. The breeding season usually is in the late winter or early spring. Some cats (lions, tigers, and leopards) are capable of breeding at any time during the year, and many species are induced ovulators, ovulation being induced by hormones released during copulation. The size of the animal does not seem to determine the litter size, number of litters, or time of the breeding season. In the larger cats, however, the initial breeding age is older; the females may be three or four years of age and males as old as five or six. Smaller cats may breed when less than a year old. Most litters are born in places seldom disturbed, such as in a rocky cavern, under a fallen tree, or in a dense thicket. The serval (Leptailurus serval) uses an old porcupine or aardvark burrow. In most species the male does not aid in the care and raising of the young, and in fact the female may have to guard against his attacks on the kittens.

Form And Function

The agility of a cat is evident in its anatomy. The clavicle, or collarbone, is much reduced in size. It does not connect with other bones but is buried in the muscles of the shoulder region. This allows the animal to spring on its prey without danger of breaking the bone. The hind legs are well developed, with powerful muscles that propel the cat in its spring toward or onto prey. In addition to the power of the hind legs, the animal uses strong back muscles to straighten the spinal column and provide extra force in springing and running.

Cats are generally nocturnal in habit. Their large eyes are especially adapted for seeing at night. The retina has a layer of guanine called the tapetum lucidum, which reflects light and causes the eyes to shine at night when illuminated. Cats have good senses of sight and hearing, but their sense of smell is not as developed as that of the canids, a fact suggested by the cat’s short snout.

A predisposition to cleanliness is well established among cats. They groom themselves with their rasping tongue, preening at length after a meal. Feces and urine are covered as a matter of habit. Cats differ in their reaction to water; most species are reluctant to enter it but will swim readily when necessary. Nervous tail wagging is common to all cats, from the lion to the house cat. Kittens learn it from the mother; the behaviour is associated with play, which is a prelude to predation as an adult.

Cats are the most highly specialized of the terrestrial flesh-eating mammals. They are powerfully built, with a large brain and strong teeth. The teeth are adapted to three functions: stabbing (canines), anchoring (canines), and cutting (carnassial molars). Cats have no flat-crowned crushing teeth and thus do not chew or grind their food but instead cut it. All cats are adapted to be strict flesh eaters, an assumption made primarily on the basis of their digestive tract and dentition. In keeping with a carnivorous habit, the cat has a simple gut; the small intestine is only about three times the length of the body. The tongue in all cats has a patch of sharp, backward-directed spines near the tip, which has the appearance and feel of a coarse file; these spines help the cat lap up liquids and groom itself. There are five padded toes on the front foot and four on the rear. The first toe and its pad on the front foot are raised so that only four toes register in a track.

Cats have a reduced number of premolar and molar teeth; the typical dental formula includes only 30 teeth. The incisors are small and chisel-like, the canines long and pointed. The premolars are sharp, and occasionally an upper premolar may be lacking. The lower molar is elongate and sharp, the upper molar rudimentary. Because of the reduction in the number and size of the cheek teeth, a space remains between the canines and premolars in all cats except the cheetah. Felids form the most strictly carnivorous group in the order Carnivora, and the highly developed carnassial teeth reflect this specialized food habit. There is little if any specialization in the teeth for grinding or chewing. The strong masseter muscles, which raise the lower jaw, restrict lateral movement. The jaw primarily moves vertically for holding the prey in a viselike grip and for slicing off pieces of meat with the carnassials. Meat is thus cut off and swallowed in relatively unchewed chunks that are broken down by strong enzymes and acids in the digestive tract.

Evolution And Classification

The history of the cat family can be traced through the fossil record to the Late Eocene Epoch (about 37 million years ago). The “cat pattern” seems to have been established very early in the evolution of mammals, for the early cats were already typical cats at a time when the ancestors of most other modern mammalian species were scarcely recognizable. Cats of the subfamily Felinae appeared in western Eurasia about 10 million years ago and have continued almost unchanged into modern times. Genetic studies examining living and fossil pantherines—cats of the subfamily Pantherinae—suggest that the pantherine lineage emerged in central Asia some 16 million years ago and subsequently spread to other continents.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

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#844 2020-11-09 00:45:27

ganesh
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Re: Miscellany

822) Pyorrhea

Pyorrhea, more popularly known as periodontitis, is one of the most widespread dental conditions for humans. This type of dental sickness occurs when the periodontium tissues that surround and support your teeth become compromised. When left untreated, pyorrhea can lead to you lose teeth or outright ending up losing teeth when push comes to shove. Tooth loss happens because you start losing the alveolar bone surrounding the teeth as your periodontium deteriorates.

It’s also important to note that before you get full-blown pyorrhea, you first end up with gingivitis and it gets worse from there. Once you have symptoms of this gum condition, you should treat it to avoid it worsening to periodontitis or some other form of periodontal disease.

Signs and Symptoms of Pyorrhea

There are no symptoms to pyorrhea in its initial stages. However, keep in mind that you’re already at risk if you have bad dental hygiene habits or tend to neglect your teeth cleanliness.

As the disease progresses from gingivitis, it starts showing major signs and symptoms you should pay attention to, such as the following.

•    Bad breath
•    Loose teeth
•    Painful chewing
•    Swollen or puffy gums
•    Gums that bleed easily
•    Bad breath or halitosis
•    Recurrent gum swelling
•    Pus between your teeth and gums
•    Bright red, dusky red or purplish gums
•    New spaces developing between your teeth
•    Gums that feel tender or sensitive when touched
•    A change in the way your teeth fit together when you bite
•    Tooth or gum redness or bleeding when flossing or brushing
•    Blood when you bite into hard fruits like pears, apples, or guava
•    Deep pockets or spaces formed between the gums and the teeth
•    Loose teeth that move in place when you touch it with your finger or tongue
•    Gums that pull away from your teeth (recede), making your teeth look longer than normal

If these symptoms were to persist then you should consult your dentist or doctor ASAP. The more you ignore pyorrhea and allow it to progress the likelier you’ll get its most notable symptom, with is outright tooth loss.

Causes of Pyorrhea

Pyorrhea typically happens when you lack dental hygiene. It can also be hereditary in the case of aggressive pyorrhea or caused being immunocompromised, as in the case of cancer or HIV patients. Letting bacteria stick to your teeth in the form of filmy plaque or tartar will deteriorate your teeth from its enamel to its periodontium tissues.

At any rate, here are the most common causes of pyorrhea:

•    Poor Dental Hygiene: Bacteria tend to spread if you don’t kill them with mouthwash that kills 99.9 percent of them or brush your teeth clean off all plaque and food bits. Flossing also helps a lot in scraping the plaque and the remnants of rotting food in between the spaces of your teeth. If you don’t brush and floss twice daily and mouthwash occasionally, you’re in for a rude awakening.
•    Complacency and Neglect: Complacency is a slow but sure killer in and of itself. When you’re complacent with your dental habits to the point where the coating of plaque on your teeth has become tartar or seemingly enamel (it’s not, by the way), then that leads to dental destruction of the highest order. You don’t realize how neglect can affect your teeth and gums until it’s usually too late.
•    Plaque and Tartar: When bad oral bacteria are allowed to run rampant in your mouth, they tend to form off-white or yellow film of protein and leftover food that’s teeming with germs called plaque. When this plaque hardens due to calcium deposits being put in the mix, it turns into calculus or tartar. The longer plaque stays on tooth enamel, the worse off your teeth and periodontium will become down the line.
•    Periodontal Destruction: The disease is caused by the destruction of the periodontium tissue or the supportive tissue that keeps the teeth anchored to the gums and jawbone. Improper dental hygiene is what kills this tissue since that allows bacteria and their acidic waste products to proliferate. Bacterial populations should be kept low by ridding your mouth of leftover food, starch, and sugar and washing it with antiseptic mouthwash.
•    Gingivitis or Mild Periodontal Disease: Great things start from small beginnings.  Bad things too. Plaque left on your teeth can cause gingivitis, which is the mildest form of periodontal disease. This condition is defined as inflammation and irritation of the gum around the base of your teeth, which is also known as the gingiva. Gingivitis is reversible with good home oral care and professional dental treatment, as with periodontitis.
•    Periodontitis or Severe Periodontal Disease: By failing to reverse gingivitis, it can worsen and develop all the way into pyorrhea or periodontitis. In other words, gingivitis can directly result into periodontitis, which causes pockets to develop between teeth and gums as your periodontium tissues die out. These pockets where the periodontium and gingiva used to be will then be filled with plaque, tartar, and (of course) bacteria.
•    Diabetes and Pyorrhea: Certain conditions can cause other conditions to surface in your body. This is the case with diabetes and pyorrhea. If you have diabetes, your chances of also developing periodontitis also increase. Diabetics are often asked to observe dental hygiene to save themselves from further complications like dental disease.
•    Smoking: Smoking tobacco is bad for you in many ways, including its impact on your dental health. Yes, there are links between smoking cigarettes and pyorrhea development. Continuing to smoke while your periodontitis is being treated also interferes with the treatment so you better get a nicotine patch and wean yourself from smoking altogether.

Types of Pyorrhea

Pyorrhea or periodontitis come in several variations or types. The thing they have in common is the fact that having them usually leads to bacteria eating away at your periodontium to the point where your teeth loses its anchors to your jaw, making it easier for them to loosen and fall off.
The most common types of pyorrhea include the following.

•    Chronic Pyorrhea: The most common type of pyorrhea is the chronic kind. It affects mostly adults but it’s not unusual for children to suffer from this either. This condition is caused by plaque buildup that includes slow periodontal, gum, and bone deterioration that might get worse or improve over time depending on what the patient does about it.
•    Aggressive Pyorrhea: Aggressive periodontitis typically starts at early adulthood or childhood and affects only a select number of the population. It’s a hereditary type of periodontal disease that affects families for the most part. If left untreated, this destructive pyorrhea type will cause rapid progression of tooth and bone loss when push comes to shove. It should be dealt with ASAP.
•    Necrotizing Pyorrhea: Necrotizing periodontitis or periodontal disease is the worst type of pyorrhea that involves the death of supporting bone, tooth ligaments, and gum tissue caused by necrosis or lack of blood supply.  This then results in severe infection. This condition usually occurs in people who have a suppressed immune system such as from cancer treatment, HIV infection, and so forth. It’s also caused by malnutrition.

Treatment of Pyorrhea

Periodontal disease or pyorrhea is curable but there is a point of no return you should be aware of with this disease. You might not be able to save some or all of your teeth if your pyorrhea is particularly bad, despite it being curable or reversible.

Long story short, you should brush and floss daily as well as gargle with mouthwash in order to keep bacterial populations low and your pyorrhea risk even lower. Here are important things to remember:
•    Brushing: You should brush twice a day or even three times a day to avoid developing pyorrhea or any other kind of oral disease. You should also brush properly and don’t scrape too hard with your brush. Move the brush in a circular pattern and do it thoroughly but gently instead of roughly and shoddily. Brush at least 2-3 minutes daily and don’t forget to regularly change your toothbrush as well every 3-4 months.
•    Flossing: Kids nowadays know flossing as a dance but these Millennials and Gen Z children should also be aware of how to properly floss in the dental sense. Many simply slip the floss between the teeth then pull it out in mere seconds, thinking they’re done. What they’re supposed to do is use a piece of floss on each individual tooth and then scrape not only the stuck bits of food but also the plaque on the tooth surface until all the teeth have been properly flossed.
•    Mouthwash: Mouthwash is at least a bit more self-explanatory than flossing or brushing. Just gargle with the mouthwash and then spit it out, right? In certain situations, like when you have a bacterial infection or swelling infected gums, you might need to let that mouthwash stay in your mouth for at least 30 seconds before spitting it out. Ideally, you should use antiseptic mouthwash and use it after brushing and flossing. It also helps get rid of bad breath.
•    Home-Made Remedies: You can also make use of home-made remedies to help fight or prevent pyorrhea. For example, you can chew an onion in order to kill germs that assist in periodontitis development. Onion also helps treat the bleeding gums symptom of both gingivitis and periodontal disease. You even have the option to eat fruits enriched with Vitamin C such as lemon and guava. Lemon prevents gum inflammation as well.
•    Dentist Visit: You should regularly visit your dentist. This is important because he’s the one who’ll keep tabs on the healthiness of your gums. He can also do thorough cleaning of your teeth and gums that are even more effective than everyday brushing and flossing known as prophylaxis for at least every 6 months. Your dentist knows best and can provide the right treatment for your issues.
•    Deep Cleaning: If you’re a patient with advanced pyorrhea and deep periodontal pockets where there used to be gum tissue, bone, and ligaments, you should avail of this treatment. It’s a package deal to treat chronic gum disease using the services of scaling and root planning.
•    Scaling: This procedure involves the dentist removing tartar and plaque on your teeth, gums, and periodontal pockets. This buildup of yellowish and brownish material is directly causing your pyorrhea. Special debridement tools are needed to access these areas because they’re beyond the reach of flossing, tooth brushing, or prophylaxis treatments.
•    Root Planing: The infected root is then smoothened out using dental instruments like a drill or a laser. The bacteria usually collects at the root and the pockets caused by your receding gumline, thus necessitating cleanup and removal. What’s more, infected tooth roots need to be planed in order to begin their healing.
•    Aftercare: Your dentist will likely prescribe an over-the-counter (OTC) painkiller and antibiotic for your infection. He might also recommend follow-up visits to determine whether or not your deep-cleaned teeth require more scaling and root planning. These additional appointments also help him check the progress of your gingival and dental healing.

In Conclusion

Periodontal disease, like dental caries or tooth decay, is caused by neglect and not maintaining proper dental hygiene for the most part. Your food intake as well as whether you smoke and drink are also contributing factors. Therefore, observe proper dental hygiene and visit your dentist regularly. Keep him updated with your dental condition and what treatments you might need to prevent this sickness from happening or progressing.

If left untreated, pyorrhea can cause a lot of bone and tissue damage, to the point where you’ll outright lose teeth. You might end up having no choice but to wear dentures or get an expensive dental implant operation. Your immune system can also get quite strained when dealing with a chronic type of inflammation or infection, leading to extra complications if you’re not careful.

It’s also common sense to observe dental hygiene on top of getting periodontitis treatment from your dental professional because as always, an ounce of prevention will always trump a pound of cure.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

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#845 2020-11-10 00:31:00

ganesh
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Posts: 31,668

Re: Miscellany

823) Mamba

Mamba, (genus Dendroaspis), any of four species of large, arboreal, venomous snakes that live throughout sub-Saharan Africa in tropical rainforests and savannas. Mambas are slender, agile, and quick and are active during the day. They have smooth scales, flat-sided (coffin-shaped) heads, long front fangs, and a powerful neurotoxic venom.

The “black,” or black-mouthed, mamba (Dendroaspis polylepis), averaging 2–2.5 metres (6.6–8.2 feet) in length (maximum 3.5 metres), ranges in colour from gray to dark brown but is never actually black. Its name derives from the inside of the mouth, which is black, in contrast to the white mouths of green mambas and other snakes. The black mamba inhabits rocky savanna and can often be encountered on the ground, where it seems to be fond of termite mounds. It lays 6 to 20 eggs in termite mounds or tree hollows. Prey consists primarily of small mammals and birds.

The black mamba is one of Africa’s most dangerous snakes, because of its large size, quickness, and extremely potent venom. It has an aggressive reputation. Though unprovoked attacks on humans have not been proved, the snake will defend itself if threatened or molested. It has a nervous disposition, and, if disturbed, it may rear up and threaten with an open mouth and slightly expanded or flattened neck (or hood) before striking. Even though most bites are fatal, it is responsible for only a small number of deaths annually. In captivity, black mambas have lived more than 20 years.

The three green mamba species are smaller (1.5–2 metres, maximum 2.7 metres) and are usually found in trees. The East African green mamba (D. angusticeps) of East and South Africa, Jameson’s mamba (D. jamesoni) of Central Africa, and the West African green mamba (D. viridis) are all more timid than the black mamba and have not been reported to attack humans. Like the black mamba, they will flatten their necks into a narrow hood as a defensive posture. Green mambas prey upon birds, small mammals, and lizards and lay from 5 to 17 eggs. Two of the three green mambas have record longevities of more than 18 years in captivity. Mambas are members of the cobra family (Elapidae).

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#846 2020-11-11 00:29:31

ganesh
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Re: Miscellany

824) Viper

Viper, (family Viperidae), any of more than 200 species of venomous snakes belonging to two groups: pit vipers (subfamily Crotalinae) and Old World vipers (subfamily Viperinae), which are considered separate families by some authorities. They eat small animals and hunt by striking and envenomating their prey. Vipers are characterized by a pair of long, hollow, venom-injecting fangs attached to movable bones of the upper jaw (the maxillaries) that are folded back in the mouth when not in use. Their eyes have vertical pupils, and their scales are keeled. Vipers range in length from less than 25 cm (10 inches) in the Namaqua dwarf viper (Bitis schneideri) of southern Africa to more than 3 metres (10 feet) in the bushmaster (Lachesis muta) of the Amazon basin and Central America.

The pit vipers are found from desert to rainforest, primarily in the New World. This group includes copperheads, rattlesnakes, and fer-de-lances (genera Bothrops and Trimeresurus), among others. They may be terrestrial or arboreal. Some, such as the moccasins (genus Agkistrodon), are aquatic. Except for the egg-laying bushmaster, all pit vipers are live-bearers (viviparous).

Pit vipers are distinguished by a temperature-sensitive pit organ located on each side of the head midway between each nostril and eye. This structure is sensitive to infrared radiation, which enables the snake to “see” heat images of warm-blooded prey. As a pair, they provide a form of binocular vision that helps the snake accurately aim its strike at warm-blooded prey. At least some Old World vipers have infrared receptors in the same area as the pit organs, although there is no external evidence of them. Some boas and pythons have similar infrared organs located in pits between the lip scales.

Old World vipers live in desert to forest habitats of Europe, Asia, and Africa. They are typically slow, stocky, and broad-headed. Many, such as the European viper, or common adder (Vipera berus), and the Gaboon viper (Bitis gabonica), are terrestrial. In contrast, tree vipers (genus Atheris), such as Matilda’s horned viper (A. matildae) of Tanzania, are slender, prehensile-tailed, and arboreal. Some species lay eggs; others produce live young.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#847 2020-11-12 00:35:49

ganesh
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Re: Miscellany

825) Nail

Nail, in construction and carpentry, a slender metal shaft that is pointed at one end and flattened at the other end and is used for fastening one or more objects to each other. Nails are most commonly used to fasten pieces of wood together, but they are also used with plastic, drywall, masonry, and concrete. Nails are usually made of steel but can also be made of stainless steel, iron, copper, aluminum, or bronze. The pointed end of a nail is called the point, the shaft is called the shank, and the flattened part is called the head.

There are many different types of nails, the types depending on the material that they are driven into and the degree of holding power that they must have. Two basic classes of nails are common nails and finishing nails.  The most widely used of all nails, the common nail has a large, flat head that is driven in so that it is flush with the material’s surface. A finishing nail has a smaller, narrower head that is driven in below the material’s surface with a special tool called a nail set, or punch; the small depression remaining is filled in with putty. Because of their neater appearance, finishing nails are used mostly for interior paneling and cabinetwork. A box nail is similar to a common nail but has a slimmer shank and is used on lighter pieces of wood and on boxes. A casing nail is similar to a finishing nail but has a slightly thicker shaft and a cone-shaped head. Nails smaller than one inch long are called wire nails if they have a head and brads if they have a very small head or none at all. Extremely thick nails are called spikes.

Nails can be given specially worked shanks to give them greater holding power once they have been driven in; the ring nail has annular rings on its shaft, while the spiral shank nail has a groove running up it in a tight spiral, like that of a screw. Roofing nails have large, flat heads that can better hold down materials such as roofing felt and fibreboard. Certain other nails are specially hardened so that they can be driven into masonry or concrete, usually in the act of attaching wooden members to these materials.

Nails are made by feeding a thick, continuous steel wire into a machine where the wire is gripped between two dies and is cut to the desired length. Sufficient metal to form the head is allowed to protrude from the dies at one end and is then flattened into a head by a blow from a mechanized hammer. The other end of the piece of wire is cut into a point, after which the nail is ejected from the machine and may be smoothed (to remove the rough edges), polished, or plated. Wire-nail presses can produce nails at speeds of up to 800 per minute.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#848 2020-11-13 00:34:11

ganesh
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Re: Miscellany

826) Materials processing

Materials processing, the series of operations that transforms industrial materials from a raw-material state into finished parts or products. Industrial materials are defined as those used in the manufacture of “hard” goods, such as more or less durable machines and equipment produced for industry and consumers, as contrasted with disposable “soft” goods, such as chemicals, foodstuffs, pharmaceuticals, and apparel.

Materials processing by hand is as old as civilization; mechanization began with the Industrial Revolution of the 18th century, and in the early 19th century the basic machines for forming, shaping, and cutting were developed, principally in England. Since then, materials-processing methods, techniques, and machinery have grown in variety and number.

The cycle of manufacturing processes that converts materials into parts and products starts immediately after the raw materials are either extracted from minerals or produced from basic chemicals or natural substances. Metallic raw materials are usually produced in two steps. First, the crude ore is processed to increase the concentration of the desired metal; this is called beneficiation. Typical beneficiation processes include crushing, roasting, magnetic separation, flotation, and leaching. Second, additional processes such as smelting and alloying are used to produce the metal that is to be fabricated into parts that are eventually assembled into a product.

In the case of ceramic materials, natural clay is mixed and blended with various silicates to produce the raw material. Plastic resins are produced by chemical methods in powder, pellet, putty, or liquid form. Synthetic rubber is also made by chemical techniques, being produced, as is natural rubber, in such forms as slabs, sheeting, crepe, and foam for fabricating into finished parts.

The processes used to convert raw materials into finished products perform one or both of two major functions: first, they form the material into the desired shape; second, they alter or improve the properties of the material.

Forming and shaping processes may be classified into two broad types—those performed on the material in a liquid state and those performed on the material in a solid or plastic condition. The processing of materials in liquid form is commonly known as casting when it involves metals, glass, and ceramics; it is called molding when applied to plastics and some other nonmetallic materials. Most casting and molding processes involve four major steps: (1) making an accurate pattern of the part, (2) making a mold from the pattern, (3) introducing the liquid into the mold, and (4) removing the hardened part from the mold. A finishing operation is sometimes needed.

Materials in their solid state are formed into desired shapes by the application of a force or pressure. The material to be processed can be in a relatively hard and stable condition and in such forms as bar, sheet, pellet, or powder, or it can be in a soft, plastic, or puttylike form. Solid materials can be shaped either hot or cold. Processing of metals in the solid state can be divided into two major stages: first, the raw material in the form of large ingots or billets is hot-worked, usually by rolling, forging, or extrusion, into smaller shapes and sizes; second, these shapes are processed into final parts and products by one or more smaller scale hot or cold forming processes.

After the material is formed, it is usually further altered. In materials processing, a “removal” process is one that eliminates portions of a piece or body of material to achieve a desired shape. Although removal processes are applied to most types of materials, they are most widely used on metallic materials. Material can be removed from a workpiece by either mechanical or nonmechanical means.

There are a number of metal-cutting processes. In almost all of them, machining involves the forcing of a cutting tool against the material to be shaped. The tool, which is harder than the material to be cut, removes the unwanted material in the form of chips. Thus, the elements of machining are a cutting device, a means for holding and positioning the workpiece, and usually a lubricant (or cutting oil). There are four basic noncutting removal processes: (1) in chemical milling the metal is removed by the etching reaction of chemical solutions on the metal; although usually applied to metals, it can also be used on plastics and glass, (2) electrochemical machining uses the principle of metal plating in reverse, as the workpiece, instead of being built up by the plating process, is eaten away in a controlled manner by the action of the electrical current, (3) electrodischarge machining and grinding erodes or cuts the metal by high-energy sparks or electrical discharges, (4) laser machining cuts metallic or refractory materials with an intense beam of light from a laser.

Another further alteration may be “joining,” the process of permanently, sometimes only temporarily, bonding or attaching materials to each other. The term as used here includes welding, brazing, soldering, and adhesive and chemical bonding. In most joining processes, a bond between two pieces of material is produced by application of one or a combination of three kinds of energy: thermal, chemical, or mechanical. A bonding or filler material, the same as or different from the materials being joined, may or may not be used.

The properties of materials can be further altered by hot or cold treatments, by mechanical operations, and by exposure to some forms of radiation. The property modification is usually brought about by a change in the microscopic structure of the material. Both heat-treating, involving temperatures above room temperature, and cold-treating, involving temperatures below room temperature, are included in this category. Thermal treatment is a process in which the temperature of the material is raised or lowered to alter the properties of the original material. Most thermal-treating processes are based on time-temperature cycles that include three steps: heating, holding at temperature, and cooling. Although some thermal treatments are applicable to most families of materials, they are most widely used on metals.

Finally, “finishing” processes may be employed to modify the surfaces of materials in order to protect the material against deterioration by corrosion, oxidation, mechanical wear, or deformation; to provide special surface characteristics such as reflectivity, electrical conductivity or insulation, or bearing properties; or to give the material special decorative effects. There are two broad groups of finishing processes, those in which a coating, usually of a different material, is applied to the surface and those in which the surface of the material is changed by chemical action, heat, or mechanical force. The first group includes metallic coating, such as electroplating; organic finishing, such as painting; and porcelain enameling.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

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#849 2020-11-14 00:19:11

ganesh
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Re: Miscellany

827) Bleach

Bleach, solid or liquid chemical used to whiten or remove the natural colour of fibres, yarns, other textiles, and paper. In textile finishing, the bleaching process is used to produce white cloth, to prepare fabrics for other finishes, or to remove discoloration that has occurred in other processes. Bleach is also used as a disinfectant because of its microbicidal properties. Chlorine, sodium hypochlorite, calcium hypochlorite, and hydrogen peroxide are commonly used as bleaches.

Sunlight was the chief bleaching agent up to the discovery of chlorine in 1774 by Swedish chemist Karl Wilhelm Scheele and the demonstration of its bleaching properties in 1785 by French chemist Claude Berthollet. Bleaching powder, a solid combination of chlorine and slaked lime, introduced in 1799 by Scottish chemist Charles Tennant, was thereafter produced in large quantity to bleach cloth and paper. It had the same effect as chlorine and could be more easily handled and shipped, but it was unstable and contained a large proportion of inert material. It remained the standard bleaching agent until the 1920s, but then it was gradually replaced by liquefied chlorine and solutions of sodium hypochlorite.

In the production of bleaching powder, slaked lime spread on the floors of large rectangular chambers of lead or concrete is exposed to chlorine gas; or lime is propelled through horizontal tubes that are fed with the gas.

Hydrogen peroxide is commonly used to bleach cotton cloth, with sodium chlorite and sodium hypochlorite as alternatives. Wood and animal fibres are bleached by acidic reducing agents such as sulfur dioxide. In the pulp and paper industry chlorine dioxide, hydrogen peroxide, sodium peroxide, sulfur dioxide, sodium bisulfite, and sodium hydrosulfite are commonly used. Synthetic fibres are bleached with either oxidizing or reducing agents, depending on their chemical composition. Optical bleaches, fluorescent white chemicals giving off a bluish-white light, are not true bleaches.

Various bleach solutions are used as disinfectants. For example, dilute (about 5 percent) sodium hypochlorite solutions are used to sanitize food-processing equipment. Stronger solutions, ranging from about 10 to 20 percent bleach, may be used in hospitals, laboratories, and other settings where sanitization against potentially infectious microorganisms is critical.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#850 2020-11-15 00:27:04

ganesh
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Re: Miscellany

828) Donkey

Donkey, (Equus asinus), also called burro, domestic animal belonging to the horse family, Equidae, and descended from the African wild donkey (Equus africanus). It is known to have been used as a beast of burden since 4000 BCE. The average donkey stands 101.6 cm (40 inches) at the shoulder, but different breeds vary greatly. The Sicilian donkey reaches only about 61 cm (24 inches), while the large donkey of Majorca stands at about 157.5 cm (62 inches), and the American donkey has been measured to 167.6 cm (66 inches). In colour the donkey ranges from white to gray or black and usually has a dark stripe from mane to tail and a crosswise stripe on the shoulders. The mane is short and upright and the tail, with long hairs only at the end, is more cowlike than horselike. The very long ears are dark at the base and tip. Although slower than horses, donkeys are surefooted and can carry heavy loads over rough terrain. The mule is a hybrid, the offspring of the mating of a male donkey and a female horse. In some parts of the world where horses cannot easily survive or where extreme poverty prevents locals from owning horses, donkeys are the main beasts of burden and source of transportation.

Although the names donkey and burro are interchangeable, the term burro is widely used in the southwestern United States to describe small donkeys, burro being the word for donkey in the Spanish language. Feral donkeys, found in various parts of the world, are descendants of escaped or abandoned domestic animals. In the western United States, many authorities consider that the large population of feral burros is driving the desert bighorn sheep to extinction by competing for the limited resources of its very arid habitat. On tropical islands where plants evolved in the absence of large mammalian herbivores, feral donkeys pose a real threat of extinction for native plants. Efforts to remove donkeys from habitats where they are not native has generated a great deal of controversy, pitting animal rights groups against biologists and other conservation groups who see donkeys as an alien species and a threat to biodiversity conservation.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

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