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colour vision

eye colour vision

The colour vision - The ability to see and distinguish the different parts of the colour spectrum, which consists of electromagnetic radiation (energy waves) with a range of wavelengths between about 400 and 700 nanometres (millionths of a millimetre).Different wavelengths trigger nerve signals in the retina (the lightsensitive layer of cells at the back of the eye); these signals pass to the brain and are interpreted as violet, indigo, blue, green, yellow, orange, and red.

RETINAL FUNCTION:
As light falls on the retina, it strikes light-sensitive cells called rods and cones. The rods can detect all visible light, but only the cones can distinguish
colour. There are three types of cone: red-sensitive, blue-sensitive, and green-sensitive. Each of these types of cone responds more strongly to a particular part of the light spectrum. The cones are most concentrated in a central area of the retina called the fovea; for this reason, colour vision is most accurate for objects that are viewed directly. Colour vision is, therefore, poor at the periphery of vision.

When light hits a cone, it causes a structural change in the pigment within the cone, which in turn causes the cone to emit an electrical signal. This signal passes to the brain via the optic nerve. Colour perception requires a minimum level of light; below this level, everything is perceived only by the rods and is seen as various shades of grey. (See also colour vision deficiency; eye; perception; vision.)
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colostrum

The colostrum - A thick, yellowish fluid that is produced by the breasts during the first few days after childbirth, before being replaced by breast milk.

Colostrum contains less fat and sugar, but more minerals and protein, than breast milk. It also has a high content of lymphocytes (white blood cells) and immunoglobulins, which help to protect the baby against infections.
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colostomy

colostomy

The colostomy - An operation in which part of the colon is brought to the surface of the skin through an incision in the abdominal wall. The exposed part is formed into a stoma, an artificial opening through which faeces are discharged into a bag attached to the skin. A colostomy may be either temporary or permanent.

A temporary colostomy may be performed at the same time as a partial colectomy (removal of part of the colon) to allow the remainder of the colon to heal without faeces passing through and contaminating it.

The colostomy is closed when the rejoined colon has healed. A permanent colostomy is needed if the rectum or anus has been removed (for example, to treat colon cancer) and normal defaecation is therefore impossible.
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colorectal cancer

The colorectal cancer -  A general term referring to cancer of the colon (see colon, cancer of) and/or of the rectum (see rectum, cancer of).
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Colorado tick fever

The Colorado tick fever - A viral illness transmitted by the bites of infected ticks. It occurs in the mountain areas of the western United States, usually in early summer.

Symptoms appear 3–6 days after a tick bite; they include flulike chills and fever, severe headache, nausea, and sometimes a red, raised rash. The illness usually lasts for a few days, subsides, then returns for a further few days. The virus can, how-ever, remain in the blood for several months.

Treatment involves removal of the tick as soon as possible and taking analgesic drugs (painkillers) if necessary. Tick bites can be prevented by wearing clothing that covers the arms and legs and tucking trouser legs into socks. (See also ticks and disease.)
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colonoscopy

The colonoscopy - The viewing of the inside of the colon using a flexible endoscope (fibre-optic instrument) known as a colonoscope, which is introduced through the anus and guided along the colon.

Colonoscopy is used to investigate symptoms such as bleeding from the anus and to detect physical abnormalities such as inflammation (see colitis), growths (see polyps), and cancer. Instruments may be passed through the colonoscope to take biopsy specimens or remove polyps. (See also endoscopy.)
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colonization

The colonization -  The multiplication of foreign organisms, such as bacteria, in a host body. (This process is not necessarily harmful.) The term “colonization” is also used to refer to the development of cancer cells in an area to which they have spread that is separate from the primary tumour.
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cancer of colon

The cancer of colon - A malignant tumour of the colon (the major part of the large intestine). Cancers of the colon or of the rectum (the lower part of the colon), which are generally referred to as colorectal cancer, are among the most common forms of cancer. They most often occur in people over the age of 60.

CAUSES:
A genetic basis has been found for some types of colon cancer. Up to one in three cases are associated with a family history of colon disease. In particular, an inherited disorder called familial adenomatous polyposis (in which large numbers of polyps develop in the colon) greatly increases the risk. In the majority of cases, however, the precise cause is not known. Contributory factors include diet: eating a lot of meat and fatty foods and not enough fibre may increase the risk. The disease also sometimes occurs in association with ulcerative colitis.

SYMPTOMS:
The first symptoms of colon cancer include an inexplicable change in bowel movements (either constipation or diarrhoea), blood mixed in with the faeces, and pain in the lower abdomen. Sometimes, however, there are no symptoms until the tumour has grown large enough to cause an obstruction in the intestine (see intestine, obstruction of) or perforate it (see perforation).

DIAGNOSIS AND TREATMENT:
Successful treatment of cancer of the colon depends, crucially, on an early diagnosis. Screening may be carried out to detect early signs. This procedure includes a test to detect blood hidden in the faeces (see faecal occult blood test). If the test is positive, doctors may perform sigmoidoscopy and colonoscopy to view the inside of the colon. A biopsy may be performed at the same time. In most cases, a partial colectomy is carried out. In this procedure, the diseased part of the colon is removed, together with a surrounding area of healthy tissue, then the cut ends of the colon are rejoined. Surgery may be combined with radiotherapy or possibly with chemotherapy. The chance of survival depends on how far the tumour has spread, but colon cancer in its early stages can be cured.
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colon

colon

The colon - The major part of the large intestine.

STRUCTURE:
The colon is a segmented tube, about 1.3 m long and 6.5 cm wide, that forms a large loop in the abdomen. It consists of four sections: the ascending, transverse, and descending colons, and the S-shaped sigmoid colon, which connects with the rectum.

The colon consists of four layers. It has a tough outer membrane that protects it from damage. The next layer comprises muscles that contract and relax rhythmically to move the intestinal contents along (see peristalsis). Inside the muscular layer is a submucous coat containing blood vessels and lymph vessels (see lymphatic system). The innermost layer produces mucus, which helps to lubricate the passage of waste material.

FUNCTIONS:
The main functions of the colon are to absorb water and mineral salts from food residue and to concentrate the remaining waste products. The material that remains after digestion enters the colon from the small intestine. As this substance passes through the colon, the water and salts that it contains are absorbed into the blood vessels in the submucous coat. The waste material becomes increasingly concentrated and is finally expelled from the rectum in the form of faeces. (See also digestive system; intestine, disorders of.)


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coloboma

The coloboma - A rare birth defect in which a gap exists in the tissues of the eye. The gap may be in the eyelid or in part of the eyeball, such as the iris, retina, or choroid.

A coloboma in the iris will be visible as a black notch, or as a gap stretching from the pupil to the edge of the iris. The condition results from incomplete development of the eyes while the baby is still an embryo; this problem may, in turn, be linked to certain chromosomal abnormalities.

Coloboma may range from minor to severe. In some cases, it may cause blurred vision or decreased visual acuity.
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Colloid

Colloid

The Colloid - A form of fluid that is similar to a suspension (a fluid consisting of insoluble particles of a substance suspended in a liquid). Particles in a suspension are large and heavy enough to be separated from the liquid in a centrifuge.

A colloid has smaller, lighter particles that can only be separated out of the liquid by spinning at a very high speed. In medicine, plasma proteins are separated from blood and sometimes used in colloid preparations to treat shock. The term “colloid” also refers to a material that contains protein and is found in the thyroid gland.
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Collodion

The Collodion - A syrupy mixture of ether, alcohol, and pyroxylin used in skin preparations for minor cuts and abrasions. Collodion acts by evaporating rapidly to leave a protective film over the area.
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Collet Sicard syndrome

The Collet Sicard syndrome - A condition resulting from damage to the cranial nerves that control muscles in the tongue and throat. It is usually due to a head injury or to compression of the nerves by a tumour. The syndrome causes paralysis of muscles in the tongue, palate, throat, larynx (voicebox), and neck on one side. There is no cure, but the cause is treated if possible.
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