SLEEP (AS. slurp, Goth. slCps, OHG. staf, Ger. Sehlaf, sleep, from AS. slmpan, Goth. slPj urn, 0I1G. skifan. Ger. schlafen, to sleep; connected with OChurch Slay. slabfi, lax, Lat. labi, to slide, fall). A condition of the body in which the normal activity of the nervont system is so far reduced that self-consciousness and consciousness of surroundings are entirely wanting, or at an extremely low ebb. (On the question of dream less sleep and the consciousness of conditions be tween waking and sleeping. see DUEAmIxo and SOMNAMBULISM.) It is. futithermore. a no•nmal and rhythmic process, and as such differentiated from stupor, unconsciousness under drugs. and other cases of abnormal loss of consciousness. Its most conspicuous physiological features are cerebral, or at least cortical, amemia; relaxation of muscular tone; slower and deeper breathing; slower and weaker pulse; and lessened arterial pressure.
There are three main types of sleep theory. the circulatory, the chemical, and the histological. The first circulatory theory w•as that of con gestion. Sleep was the result of pressure upon the brain due to venous congestion. The evidence for this view came from the analogy between the condition of sleep and that produced by apoplexy, opiates, and the lethargy canned by pressure on the brain in cases of fractured skull. The second theory turns to amemia, the exact opposite of congestion. A large number of well-attested facts prove the existence of a cortical ampulla during sleep. Pressure upon the carotid arteries will produce a dream-like state of consciousness. In several instances of fractured skull direct me chanical measurements have demonstrated the amemia of sleep.
The chemical theories are of two types. accord. ing as they are based on (1) combustion or (2) auto-intoxication. The combustion theories, all of which are concerned with the use of oxygen or carbonic-acid gas, may be represented by Pflii ger's idea that the stored up intra-molecular oxygen is exhausted by activity (vibration and explosion) of nerve cells, and each cell finally becomes saturated with carbonic acid. The ex plosions of the cells become less numerous, and the condition of relative cerebral inactivity, sleep, thus results. This theory is not buttressed by sufficient experimental evidence, nor does our recent knowledge of the function of oxygen in the body warrant us in attributing sleep to its lack. In the auto-intoxication theories it is as
serted that certain products of decomposition of living substance influence the continuance of cell activity; in the older form of this theory the products mentioned were chiefly lactic acid and creatine; in the recent theories the influence of modern bacteriology has led to the substitution of certain poisons, such as the ptomaines and the lencomaines, which are formed more rapidly than they can be oxidized during active labor of the day. During sleep these poisons are gradu ally oxidized and removed from the blood. An excessive quantity of these substances produces insomnia, which, as we all know, is often char acteristic of extreme fatigue.
The rapid advance in histological technique within the last few years has led to certain dis coveries concerning the nature of the nerve cell and its processes, or the neurone, which shed some light upon the conditions of sleep. Of special interest are the results of investigations upon the connection of neurone to neurone. We know that each nerve element is structurally inde pendent, but functionally interdependent. Mi croscopic examination has shown that the nerve cell possesses different chemical properties when in a waking or a sleeping or fatigued condition, and that the disposition of the 'contact gran ules' or which some authorities deem the structural means for the interconnection of neurones, while functioning, varies according to the condition of activity or rest in the nervous system. Upon these facts various theories have been advanced, which find the cause of sleep in dissociations of the neurones. These theories have taken three principal forms: Dissociation through amceboid movements of cell processes, dissociation through interposition of neuroglia (non-nervous) cells, and profuse connection through torpor of processes.
But no single theory, whether vasomotor, chemical, or histological, is adequate for a com plete explanation of sleep. Recent observations of the daily life of protozoa and other simple forms show that such organisms never sleep, and, of course, never exhibit phenomena of fatigue. Somewhere in the line of evolution the phenome na of fatigue and sleep must make their appear ance. It seems likely, therefore, that profitable work upon the problem of sleep is to be expected in the future from the side of comparative physi ology and psychology.