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#901 2021-01-10 00:20:46

ganesh
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Re: Miscellany

879) Guinea pig

Guinea pig, (Cavia porcellus), a domesticated species of South American rodent belonging to the cavy family (Caviidae). It resembles other cavies in having a robust body with short limbs, large head and eyes, and short ears. The feet have hairless soles and short sharp claws. There are four toes on the forefeet and three on the hind feet. Several breeds of domesticated guinea pigs exist, which are sometimes grouped by coat texture and hair length. The term guinea pig is also used colloquially to refer to a person who serves as a test subject in an experiment.

Among rodents, domestic guinea pigs are fairly large, weighing 500 to 1,500 grams (roughly 1 to 3 pounds) and having a body 20 to 40 cm (8 to 16 inches) long. The tail is not visible externally. There is a crest of longer hairs at the neck, but length and texture of the fur vary from smooth (short or long) to coarse and short or long and silky. Coloration is extremely variable: the coat may be white, cream, tan, reddish or chocolate brown, black, or a combined pattern.

Guinea pigs eat vegetation and do not require water to drink if supplied with sufficiently moist food, but they must have water if fed dry commercial food. They breed all year in captivity. Females bear up to 13 young per litter (4 is average); gestation takes 68 days. Although the young can scamper about and eat solid food the day they are born, they are not fully weaned for about three weeks. Females mature in two months, males in three, and captive guinea pigs live up to eight years, although three to five is typical.

No natural population of this species exists in the wild. Guinea pigs were apparently domesticated more than 3,000 years ago in Peru, coinciding with humans’ transition from a nomadic to an agricultural lifestyle. The Incas kept guinea pigs, and the animals were bred during the same period by various people who lived along the Andes Mountains from northwestern Venezuela to central Chile. These rodents remain a sustainable food source for the native peoples of Ecuador, Peru, and Bolivia, who either keep them in their homes or allow them to scavenge freely both indoors and out. Guinea pigs were taken to Europe in the 16th century, and since the 1800s they have been popular as pets. They are also used internationally as laboratory animals for studies of anatomy, nutrition, genetics, toxicology, pathology, serum
development, and other research programs.

The origin of the colloquial name guinea pig is a subject of much debate. The first part of the name may have been derived from the price of the animal in 16th- and 17th-century England—that is, possibly one guinea—or it may have arisen from the animals’ being carried to European markets after first being transferred to ships in ports in Guinea. The moniker could also have originated with a mispronounced form of the word Guiana, the name of the region where some guinea pigs were collected. Another possible etymology is from the name of the class of ships—the Guineamen—that transported the animal. These were vessels that made port in West Africa as part of the transatlantic slave trade. The second part of the name also originated with Europeans, who compared the squealing sound the animal made (as well as the taste of its cooked flesh) to that of a pig.

There are five nondomesticated members of the genus Cavia that are also called guinea pigs: the Brazilian guinea pig (C. aperea) found from Colombia, Venezuela, and the Guianas south to northern Argentina; the shiny guinea pig (C. fulgida), inhabiting eastern Brazil; the montane guinea pig (C. tschudii), ranging from Peru to northern Chile and northwestern Argentina; the greater guinea pig (C. magna), occurring in southeastern Brazil and Uruguay; and the Moleques do Sul guinea pig (C. intermedia), which is limited to an island in the Moleques do Sul archipelago off the southern coast of Brazil. Breeding and molecular studies suggest that the domestic guinea pig was derived from one of the wild Brazilian, shiny, or montane species.

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#902 2021-01-11 00:16:44

ganesh
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Re: Miscellany

880) Sheep

Sheep, (Ovis aries), species of domesticated ruminant (cud-chewing) mammal, raised for its meat, milk, and wool. The sheep is usually stockier than its relative the goat (genus Capra); its horns, when present, are more divergent; it has scent glands in its face and hind feet; and the males lack the beards of goats. Sheep usually have short tails. In all wild species of sheep, the outer coat takes the form of hair, and beneath this lies a short undercoat of fine wool that has been developed into the fleece of domesticated sheep. Male sheep are called rams, the females ewes, and immature animals lambs. Mature sheep weigh from about 35 to as much as 180 kg (80 to 400 pounds).

A sheep regurgitates its food and chews the cud, thus enabling its four separate stomach compartments to thoroughly digest the grasses and other herbage that it eats. The animals prefer grazing on grass or legume vegetation that is short and fine, though they will also consume high, coarse, or brushy plants as well. They graze plants closer to the root than do cattle, and so care must be taken that sheep do not overgraze a particular range. Sheep are basically timid animals who tend to graze in flocks and are almost totally lacking in protection from predators. They mature at about one year of age, and many breed when they reach the age of about one and a half years. Most births are single, although sheep do have twins on occasion. The lambs stop suckling and begin to graze at about four or five months of age.

Sheep were first domesticated from wild species of sheep at least 5000 BCE, and their remains have been found at numerous sites of early human habitation in the Middle East, Europe, and Central Asia. Domesticated sheep are raised for their fleece (wool), for milk, and for meat. The flesh of mature sheep is called mutton; that of immature animals is called lamb. There were estimated to be more than one billion sheep in the world in the early 21st century. The major national producers are Australia, New Zealand, China, India, the United States, South Africa, Argentina, and Turkey. Countries that have large areas of grassland are the major producers.

Domestic sheep differ from their wild progenitors and among themselves in conformation, quantity and quality of fleece, colour, size, milk production, and other characteristics. Most breeds of domesticated sheep produce wool, while a few produce only hair, and wild sheep grow a combination of wool and hair. Several hundred different breeds of sheep have been developed to meet environmental conditions influenced by latitudes and altitudes and to satisfy human needs for clothing and food. Breeds of sheep having fine wool are generally raised for wool production alone, while breeds with medium or long wool or with only hair are generally raised for meat production. Several crossbreeds have been developed that yield both wool and meat of high quality, however. Of the more than 200 breeds of sheep in the world, the majority are of limited interest except in local areas.

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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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#903 2021-01-12 00:44:16

ganesh
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Re: Miscellany

881) Marathon

Marathon, long-distance footrace first held at the revival of the Olympic Games in Athens in 1896. It commemorates the legendary feat of a Greek soldier who, in 490 BC, is supposed to have run from Marathon to Athens, a distance of about 40 km (25 miles), to bring news of the Athenian victory over the Persians and then expired. The story of this messenger from the Battle of Marathon was later conflated with the story of another Greek soldier, Pheidippides, who ran from Athens to Sparta in advance of the fighting. Appropriately, in 1896 the first modern marathon winner was a Greek, Spyridon Louis.

In 1924 the Olympic marathon distance was standardized at 42,195 metres (26 miles 385 yards). This was based on a decision of the British Olympic Committee to start the 1908 Olympic race from Windsor Castle and finish it in front of the royal box in the stadium at London. The marathon was added to the women’s Olympic program in 1984.

After the Olympic Games championship, one of the most coveted honours in marathon running is victory in the Boston Marathon, held annually since 1897. It draws athletes from all parts of the world and in 1972 became the first major marathon to officially allow women to compete. Other premiere marathons are held in London, Chicago, Berlin, New York City, Tokyo, and Amsterdam. Marathons are not held on the track but on roads, and, despite the fact that courses are not of equal difficulty, the International Association of Athletics Federations (IAAF) does list world records for the marathon and also for the half-marathon. World-record times in the marathon steadily declined over the course of the 20th century from slightly under three hours to slightly more than two hours.

It was long considered necessary for a runner to prepare for a marathon by training over that distance. At the 1952 Olympic Games, however, Czech Emil Zátopek set an Olympic record of 2 hours 23 minutes 3.2 seconds, even though he had never run the distance before. In the decades following, other first-time marathoners also won premiere events and set records at the distance. By the late 20th century, road racing, and marathon running in particular, had grown to become a recreational activity with broad appeal. Ultramarathons, which are neither Olympic nor IAAF events, are longer races based on a specific distance or an allotted time period for competition, such as a 12-hour race.

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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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#904 2021-01-13 00:02:54

ganesh
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Re: Miscellany

882) Cornea

Cornea, dome-shaped transparent membrane about 12 mm (0.5 inch) in diameter that covers the front part of the eye. Except at its margins, the cornea contains no blood vessels, but it does contain many nerves and is very sensitive to pain or touch. It is nourished and provided with oxygen anteriorly by tears and is bathed posteriorly by aqueous humour. It protects the pupil, the iris, and the inside of the eye from penetration by foreign bodies and is the first and most powerful element in the eye’s focusing system. As light passes through the cornea, it is partially refracted before reaching the lens. The curvature of the cornea, which is spherical in infancy but changes with age, gives it its focusing power; when the curve becomes irregular, it causes a focusing defect called astigmatism, in which images appear elongated or distorted.

The cornea itself is composed of multiple layers, including a surface epithelium, a central, thicker stroma, and an inner endothelium. The epithelium (outer surface covering) of the cornea is an important barrier to infection. A corneal abrasion, or scratch, most often causes a sensation of something being on the eye and is accompanied by intense tearing, pain, and light sensitivity. Fortunately, the corneal epithelium is able to heal quickly in most situations.

The collagen fibres that make up the corneal stroma (middle layer) are arranged in a strictly regular, geometric fashion. This arrangement has been shown to be the essential factor resulting in the cornea’s transparency. When the cornea is damaged by infection or trauma, the collagen laid down in the repair processes is not regularly arranged, with the result that an opaque patch or scar may occur. If the clouded cornea is removed and replaced by a healthy one (i.e., by means of corneal transplant), usually taken from a deceased donor, normal vision can result.

The innermost layer of the cornea, the endothelium, plays a critical role in keeping the cornea from becoming swollen with excess fluid. As endothelial cells are lost, new cells are not produced; rather, existing cells expand to fill in the space left behind. Once loss of a critical number of endothelial cells has occurred, however, the cornea can swell, causing decreased vision and, in severe cases, surface changes and pain. Endothelial cell loss can be accelerated via mechanical trauma or abnormal age-related endothelial cell death (called Fuchs endothelial dystrophy). Treatment may ultimately require corneal transplant.

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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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#905 2021-01-14 00:34:12

ganesh
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Re: Miscellany

883) Iris

Iris, in anatomy, the pigmented muscular curtain near the front of the eye, between the cornea and the lens, that is perforated by an opening called the pupil. The iris is located in front of the lens and ciliary body and behind the cornea. It is bathed in front and behind by a fluid known as the aqueous humour. The iris consists of two sheets of smooth muscle with contrary actions: dilation (expansion) and contraction (constriction). These muscles control the size of the pupil and thus determine how much light reaches the sensory tissue of the retina. The sphincter muscle of the iris is a circular muscle that constricts the pupil in bright light, whereas the dilator muscle of the iris expands the opening when it contracts. The amount of pigment contained in the iris determines eye colour. When there is very little pigment, the eye appears blue. With increased pigment, the shade becomes deep brown to black. Inflammation of the iris is termed iritis or anterior uveitis, a condition that commonly has no determinable cause. As a result of inflammation, the iris sticks to the lens or the cornea, blocking the normal flow of fluid in the eye. Complications of iritis include secondary glaucoma and blindness; treatment usually involves topical steroid eyedrops.

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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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#906 2021-01-15 00:45:47

ganesh
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Re: Miscellany

884) Placenta

Placenta, in zoology, the vascular (supplied with blood vessels) organ in most mammals that unites the fetus to the uterus of the mother. It mediates the metabolic exchanges of the developing individual through an intimate association of embryonic tissues and of certain uterine tissues, serving the functions of nutrition, respiration, and excretion.

All of the fetal membranes function by adapting the developing fetus to the uterine environment. Lying in the chorionic cavity (a thin liquid-filled space) between two membranous envelopes (chorion and amnion) is a small balloon-like sac, yolk sac, or vitelline sac, attached by a delicate strand of tissue to the region where the umbilical cord (the structure connecting the fetus with the placenta) leaves the amnion. Two large arteries in the umbilical cord radiate from the attachment of the cord on the inner surface of the placenta and divide into small arteries that penetrate outward into the depths of the placenta through hundreds of branching and interlacing strands of tissue known as villi. The chorionic villi cause the mother’s blood vessels in their vicinity to rupture, and the villi become bathed directly in maternal blood. The constant circulation of fetal and maternal blood and the very thin tissue separation of fetal blood in the capillaries from maternal blood bathing the villi provide a mechanism for efficient interchange of blood constituents between the maternal and fetal bloodstreams without (normally) allowing any opportunity for the blood of one to pour across into the blood vessels of the other.

Nutrients, oxygen, and antibodies (proteins formed in response to a foreign substance, or antigen), as well as other materials in the mother’s blood, diffuse into the fetal blood in the capillaries of the villi, and nitrogenous wastes and carbon dioxide diffuse out of these capillaries into the maternal blood circulation. The purified and enriched blood in the capillaries of the villi is collected into fetal veins, which carry it back to the inner surface of the placenta and collect at the attachment of the cord to form the umbilical vein. This vein enters the cord alongside the two arteries and carries the blood back to the fetus, thus completing the circuit to and from the placenta.

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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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#907 2021-01-16 00:25:37

ganesh
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Re: Miscellany

885) Plasma

Plasma, also called blood plasma, the liquid portion of blood. Plasma serves as a transport medium for delivering nutrients to the cells of the various organs of the body and for transporting waste products derived from cellular metabolism to the kidneys, liver, and lungs for excretion. It is also a transport system for blood cells, and it plays a critical role in maintaining normal blood pressure. Plasma helps to distribute heat throughout the body and to maintain homeostasis, or biological stability, including acid-base balance in the blood and body.

Plasma is derived when all the blood cells—red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes)—are separated from whole blood. The remaining straw-coloured fluid is 90–92 percent water, but it contains critical solutes necessary for sustaining health and life. Important constituents include electrolytes such as sodium, potassium, chloride, bicarbonate, magnesium, and calcium. In addition, there are trace amounts of other substances, including amino acids, vitamins, organic acids, pigments, and enzymes. Hormones such as insulin, corticosteroids, and thyroxine are secreted into the blood by the endocrine system. Plasma concentrations of hormones must be carefully regulated for good health. Nitrogenous wastes (e.g., urea and creatinine) transported to the kidney for excretion increase markedly with renal failure.

Plasma contains 6–8 percent proteins. One critical group is the coagulation proteins and their inhibitors, synthesized primarily in the liver. When blood clotting is activated, fibrinogen circulating in the blood is converted to fibrin, which in turn helps to form a stable blood clot at the site of vascular disruption. Coagulation inhibitor proteins help to prevent abnormal coagulation (hypercoagulability) and to resolve clots after they are formed. When plasma is allowed to clot, fibrinogen converts to fibrin, trapping the cellular elements of blood. The resulting liquid, devoid of cells and fibrinogen, is called serum. Biochemical testing of plasma and serum is an important part of modern clinical diagnosis and treatment monitoring. High or low concentrations of glucose in the plasma or serum help to confirm serious disorders such as diabetes mellitus and hypoglycemia. Substances secreted into the plasma by cancers may indicate an occult malignancy; for instance, an increased concentration of prostate-specific antigen (PSA) in a middle-aged asymptomatic man may indicate undiagnosed prostate cancer.

Serum albumin, another protein synthesized by the liver, constitutes approximately 60 percent of all of the plasma proteins. It is very important in maintaining osmotic pressure in the blood vessels; it is also an important carrier protein for a number of substances, including hormones. Other proteins called alpha and beta globulins transport lipids such as cholesterol as well as steroid hormones, sugar, and iron.

The gamma globulins, or immunoglobulins, are an important class of proteins that are secreted by B lymphocytes of the immune system. They include most of the body’s supply of protective antibodies produced in response to specific viral or bacterial antigens. Cytokines are proteins synthesized by cells of various organs and by cells found in the immune system and bone marrow in order to maintain normal blood cell formation (hematopoiesis) and regulate inflammation. For example, one cytokine called erythropoietin, synthesized by specialized kidney cells, stimulates bone marrow blood progenitor cells to produce red blood cells. Other cytokines stimulate the production of white blood cells and platelets. Another protein system in the plasma, called complement, is important in mediating appropriate immune and inflammatory responses to a variety of infectious agents.

The electrolytes and acid-base system found in the plasma are finely regulated. For example, potassium is normally present in plasma in a concentration of only 4 milliequivalents per litre. A slight rise in plasma potassium (to 6–7 milliequivalents per litre) can result in death. Likewise, sodium, chloride, bicarbonate, calcium, and magnesium levels in the plasma must be precisely maintained within a narrow range. Smaller molecules such as sodium, potassium, glucose, and calcium are primarily responsible for the concentration of dissolved particles in the plasma. However, it is the concentration of much larger proteins (especially albumin) on either side of semipermeable membranes such as the endothelial cells lining the capillaries that creates crucial pressure gradients necessary to maintain the correct amount of water within the intravascular compartment and, therefore, to regulate the volume of circulating blood. So, for example, patients who have kidney dysfunction or low plasma protein concentrations (especially low albumin) may develop a migration of water from the vascular space into the tissue spaces, causing edema (swelling) and congestion in the extremities and vital organs, including the lungs.

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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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#908 2021-01-17 00:33:29

ganesh
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Re: Miscellany

886) Eyeball

Eyeball, spheroidal structure containing sense receptors for vision, found in all vertebrates and constructed much like a simple camera. The eyeball houses the retina—an extremely metabolically active layer of nerve tissue made up of millions of light receptors (photoreceptors)—and all of the structures needed to focus light onto it. The sclera, the tough protective outer shell of the eyeball, is composed of dense fibrous tissue that covers four-fifths of the eyeball and provides attachments for the muscles that move the eye. The sclera is itself covered anteriorly by the conjunctiva, a transparent mucous membrane that prevents the eye from drying out. At the front of the eye, the tear film covers the transparent cornea, the “window” through which light passes into the eye. Working in concert with the aqueous humour behind it, the cornea provides the greatest focusing power of the eye. However, unlike the lens, the shape and focusing power of the cornea are not adjustable. Other important structures in the eyeball include the iris and the lens. Much of the eyeball is filled with a transparent gel-like material, called the vitreous humour, that helps to maintain the spheroidal shape.

Immediately beneath the sclera is an underlying vascular layer, called the uvea, that supplies nutrients to many parts of the eye. One component of the uvea is the ciliary body, a muscular structure located behind the iris that alters the shape of the lens during focusing and produces the aqueous humour that bathes the anterior chamber. The other components of the uvea are the iris and the choroid. The choroid is a highly vascular tissue layer that provides blood to the outer layers of the retina that lie over it.

The cornea, where the focusing process begins, is curved to a much greater extent than the rest of the eyeball. Defects in corneal curvature cause a distortion of vision known as astigmatism. Behind the cornea is the anterior chamber, which extends posteriorly to the plane of the iris and pupil. It is filled with a watery fluid called the aqueous humour. The iris is a doughnut-shaped, muscular curtain that opens and closes to regulate the amount of light entering the eye through the pupil, the opening at the iris’s centre. The aqueous humour flows through the pupil from the posterior chamber (a small space between the iris and the lens) to the anterior chamber and out of the eye through the trabecular meshwork and Schlemm’s canal, which encircles the peripheral iris. Some aqueous humour also exits the eye directly through the ciliary body. The ciliary muscle attachments and the lens separate the aqueous humour in front from the vitreous humour behind.

The shape of the lens is controlled by the action of the ciliary body, altering the focusing power of the lens as needed. The cornea and lens focus an image onto the retina at the back of the eye. If the image is projected too far in front of the retina, it causes the visual defect called myopia, or nearsightedness. If the image is theoretically focused “behind” the retina, the result is hyperopia, or farsightedness. If no deformation of the lens is present, the image is projected onto the fovea, a structure near the centre of the retina that contains a large number of cone photoreceptors and that provides the sharpest vision. When stimulated by light, retinal photoreceptor cells send signals to neighbouring cells in the retina that then relay the signals through the optic nerve to the visual centres of the brain.

eye-diagram-2.png


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

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#909 2021-01-18 00:23:13

ganesh
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Re: Miscellany

887) Blind spot

Blind spot, small portion of the visual field of each eye that corresponds to the position of the optic disk (also known as the optic nerve head) within the retina. There are no photoreceptors (i.e., rods or cones) in the optic disk, and, therefore, there is no image detection in this area. The blind spot of the right eye is located to the right of the centre of vision and vice versa in the left eye. With both eyes open, the blind spots are not perceived because the visual fields of the two eyes overlap. Indeed, even with one eye closed, the blind spot can be difficult to detect subjectively because of the ability of the brain to “fill in” or ignore the missing portion of the image.

The optic disk can be seen in the back of the eye with an ophthalmoscope. It is located on the nasal side of the macula lutea, is oval in shape, and is approximately 1.5 mm (0.06 inch) in diameter. It is also the entry point into the eye for major blood vessels that serve the retina. The optic disk represents the beginning of the optic nerve (second cranial nerve) and the point where axons from over one million retinal ganglion cells coalesce. Clinical evaluation of the optic nerve head is critical in the diagnosis and monitoring of glaucoma and other optic neuropathies that may lead to vision loss.

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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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#910 Yesterday 00:03:28

ganesh
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Re: Miscellany

888) Retina

Retina, layer of nervous tissue that covers the inside of the back two-thirds of the eyeball, in which stimulation by light occurs, initiating the sensation of vision. The retina is actually an extension of the brain, formed embryonically from neural tissue and connected to the brain proper by the optic nerve.

The retina is a complex transparent tissue consisting of several layers, only one of which contains light-sensitive photoreceptor cells. Light must pass through the overlying layers to reach the photoreceptor cells, which are of two types, rods and cones, that are differentiated structurally by their distinctive shapes and functionally by their sensitivity to different kinds of light. Rods predominate in nocturnal animals and are most sensitive to reduced light intensities; in humans they provide night vision and aid in visual orientation. Cones are more prominent in humans and those animals that are active during the day and provide detailed vision (as for reading) and colour perception. In general, the more cones per unit area of retina, the finer the detail that can be discriminated by that area. Rods are fairly well distributed over the entire retina, but cones tend to concentrate at two sites: the fovea centralis, a pit at the rear of the retina, which contains no rods and has the densest concentration of cones in the eye, and the surrounding macula lutea, a circular patch of yellow-pigmented tissue about 5 to 6 mm (0.2 to 0.24 inch) in diameter.

When light enters the eye, it passes through the cornea and the lens and is refracted, focusing an image onto the retina. Light-sensitive molecules in the rods and cones react to specific wavelengths of light and trigger nerve impulses. Complex interconnections (synapses) between and within retinal cell layers assemble these impulses into a coherent pattern, which in turn is carried through the optic nerve to the visual centres of the brain, where they are further organized and interpreted.

eye-anatomy-retina.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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#911 Today 00:11:05

ganesh
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Re: Miscellany

889) Embryo

Human And Animal

Embryo, the early developmental stage of an animal while it is in the egg or within the uterus of the mother. In humans the term is applied to the unborn child until the end of the seventh week following conception; from the eighth week the unborn child is called a fetus.

A brief treatment of embryonic development follows.

In organisms that reproduce mating, the union of a male gamete and female gamete results in a zygote, or fertilized egg, which undergoes a series of divisions called cleavages as it passes down the fallopian tube. After several cleavages have taken place, the cells form a hollow ball called a blastula. In most mammals the blastula attaches itself to the uterine lining, thus stimulating the formation of a placenta, which will transfer nutrients from the mother to the growing embryo. In lower animals the embryo is nourished by the yolk.

By the process of gastrulation, the embryo differentiates into three types of tissue: the ectoderm, producing the skin and nervous system; the mesoderm, from which develop connective tissues, the circulatory system, muscles, and bones; and the endoderm, which forms the digestive system, lungs, and urinary system. Mesodermal cells migrate from the surface of the embryo to fill the space between the other two tissues through an elongated depression known as the primitive streak. As the embryo develops, the cell layers fold over so that the endoderm forms a long tube surrounded by mesoderm, with an ectodermal layer around the whole.

Nutrients pass from the placenta through the umbilical cord, and the amnion, a fluid-filled membrane, surrounds and protects the embryo. The division of the body into head and trunk becomes apparent, and the brain, spinal cord, and internal organs begin to develop. All of these changes are completed early in embryonic development, by about the fourth week, in humans.

Between the head and the heart, a series of branchial arches, cartilaginous structures that support the gills of fishes and larval amphibians, begin to form. In higher vertebrates these structures form part of the jaw and ear. Limb buds also appear, and by the end of the embryonic stage, the embryo is distinguishable as a representative of its species.

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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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