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coma,in astronomy: see cometcomet
[Gr.,=longhaired], a small celestial body consisting mostly of dust and gases that moves in an elongated elliptical or nearly parabolic orbit around the sun or another star. Comets visible from the earth can be seen for periods ranging from a few days to several months.
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coma,in medicine, deep state of unconsciousness from which a person cannot be aroused even by painful stimuli. The patient cannot speak and does not respond to command. Coma is the result of damage to the brain stembrain stem,
lower part of the brain, adjoining and structurally continuous with the spinal cord. The upper segment of the human brain stem, the pons, contains nerve fibers that connect the two halves of the cerebellum.
..... Click the link for more information. and cerebrum that may be caused by severe head or brain injury, cardiac arrest, strokestroke,
destruction of brain tissue as a result of intracerebral hemorrhage or infarction caused by thrombosis (clotting) or embolus (obstruction in a blood vessel caused by clotted blood or other foreign matter circulating in the bloodstream); formerly called apoplexy.
..... Click the link for more information. , diabetesdiabetes
or diabetes mellitus
, chronic disorder of glucose (sugar) metabolism caused by inadequate production or use of insulin, a hormone produced in specialized cells (beta cells in the islets of Langerhans) in the pancreas that allows the body to use and store
..... Click the link for more information. , drug overdose, shockshock,
any condition in which the circulatory system is unable to provide adequate circulation to the body tissues, also called circulatory failure or circulatory collapse. Shock results in the slowing of vital functions and in severe cases, if untreated, in death.
..... Click the link for more information. , or hemorrhagehemorrhage
, escape of blood from the circulation (arteries, veins, capillaries) to the internal or external tissues. The term is usually applied to a loss of blood that is copious enough to threaten health or life.
..... Click the link for more information. . It occurs just before death in many diseases. There are various depths of coma; the nature of the injury determines the level of supportive treatment necessary (see artificial life supportartificial life support,
systems that use medical technology to aid, support, or replace a vital function of the body that has been seriously damaged. Such techniques include artificial pacemakers, internal defibrillators, dialysis machines (see kidney, artificial), and
..... Click the link for more information. ). Survival and prognosis depend upon the cause, extent of damage, and duration of the coma.
The term persistent vegetative state was coined in 1972 to describe an unconscious state in which sleep and wake cycles remain and eyes may open, but there is no thinking, feeling, or awareness of one's surroundings (although one may react reflexive to certain stimulations). The brain stem is usually relatively intact but the cerebral cortex is severely impaired. It is this state that sometimes results from resuscitation and life support of people who otherwise would have died; partial emergence from such a state sometimes occurs with a year or two, but not after that.
See also head.
a life-threatening condition characterized by loss of consciousness, severe decrease or absence of reaction to external stimuli, extinction of reflexes (to the point of complete disappearance), disturbance of the depth and frequency of respiration, change in vascular tonus, acceleration or deceleration of the pulse, and upset of temperature regulation.
Coma is a consequence of deep inhibition in the cerebral cortex extending to the subcortex and lower sections of the central nervous system, as a result of acute disruption of blood circulation in the brain, head injury, inflammation (encephalitis, meningitis, malaria), intoxication (barbiturates, carbon monoxide), diabetes mellitus, uremia (urémic coma), and hepatitis (hepatic coma). In addition, there is disruption of the acid-base equilibrium in nervous tissue, oxygen starvation, disturbances of ion exchange, and energy starvation in the nerve cells. A precomatose state, during which the symptoms indicated develop, precedes coma. The condition can be treated by eliminating the primary causes of the coma and taking measures aimed at restoring the acid-base balance, reversing collapse, restoring respiration, and combating oxygen starvation.
an aberration of optical systems in which each part of the system at a distance d (circular zone) from the axis gives the image of a luminous point as a ring with a radius that increases as d increases; the centers of the rings are not coincident, so that their superposition, that is, the image of the point produced by the system as a whole, takes the form of an asymmetrical, diffuse spot (see Figure 1).
The size of this spot is proportional to the square of the aperture angle of the system and the distance of the point-object from the system’s axis. The coma is very large with parabolic mirrors; it is, in fact, the principal factor limiting their visual field (see Figure 2). In complicated optical systems, coma is usually corrected together with spherical aberration by matching lenses. If one of the lens surfaces is off-center when a system is assembled, coma will also distort the images of points located on the system’s axis.
a tuft of simple or pinnate hairs at the apex or base of the seeds of certain plants, for example, the willow herb. The coma is an outgrowth of the seed coat, or testa. It promotes distribution of seeds and fruits by means of wind, water, and animals.