Coma (Gr. Kŵμa, lethargy), a condition resembling profound sleep, in which the activity of the sensory ganglia is more or less completely suspended. The sensorium consists of the ganglionic masses lying along the basis of the skull in man, and partly included in the medulla oblongata, described in the article Brain, as the tubercula quadrigemina, olfactory lobes, corpora striata, and optic thalami, in which the nerves of special sense and of common sensation have their central terminations. In complete coma the activity of these ganglia is suspended, so that the individual is neither conscious of impressions derived from the organs of sense, nor has any perception of self-existence from the recognition of cerebral changes; shut off from the external world, and from internal sensation, his existence is to all intents and purposes a nonentity, a state of psychical annihilation. In the simpler forms of coma there is only a suspension, not a perversion, of the cerebral functions; but in the graver cases the accompanying delirium shows an affection of the hemispheres.

Coma may be produced by congestion or haemorrhage in the brain, by any abnormal pressure on this organ, by the agency of narcotic poisons and alcohol, by exhaustion from loss of blood, by concussion of the brain, and by action on the blood of various morbid products generated within the system. Slight coma differs but little from profound sleep; the heavy sleep of the drunkard, or that after severe and long mental or physical exertion, is almost comatose, the person being quite insensible to ordinary external stimuli; this condition cannot be regarded as disease, but as the rest required for the regeneration of the body by the slow and unobstructed performance of the nutritive processes; so in the coma from concussion or deficient supply of blood to the brain, the person cannot be aroused from his deep sleep without danger of violent and perhaps fatal reaction. Medical writers describe two varieties or stages of coma: coma vigil, in which the patient opens his eyes when spoken to, instantly shutting them again, with delirium, muttering, and agitation, as in unnatural wakefulness; and coma somnolentum, in which, after momentary revival, the patient sinks immediately into an apparently profound sleep; they are simply two different degrees of the same affection.