If it were possible to accept Errera’s theory, a kind of analogy could be established between sleep and natural death on the one hand, and the arrest of development and death of yeast grown in nitrogenous media on the other hand, because in the latter case the poisoning is produced by an alkaline salt of ammonia. It must be confessed, however, that the actual state of our knowledge does not allow of a definite view of the real mechanism of the sleep-producing intoxication. Our ideas regarding leucomaines in general are still incomplete, and, recently, one of them, adrenaline, the product of the supra-renal capsules, has been investigated. Adrenaline is an alkaloid[94] which is produced in the supra-renal bodies and is discharged into the blood. It has the power of contracting arteries strongly, and has been used to control blood-pressure. When it is given in large quantities or in frequent doses, it acts as a true poison, whilst, in small doses, it produces anæmia of the organs and has a special influence on the nervous centres. Dr. Zeigan[95] has shown that a milligramme of adrenaline, mixed with five grammes of normal salt solution injected into the brain of cats, produces a soporific action. “About a minute after the injection, the animal appears to be plunged into deep sleep which lasts from 30 to 50 minutes. During this time, the sensitiveness of the animal has completely ceased throughout the body, and for some time after that it is much decreased. When they awake the animals seem to have been drunk with sleep for some time.” Sleep is generally associated with anæmia of the brain, and as adrenaline can actually produce such anæmia, it might be supposed that this narcotic substance is the most important of the organic products which give rise to sleep. Against this hypothesis, however, some weight must be given to recent investigations on fatigue and its causes.
Each stage in the advance of knowledge has had its influence on the study of the interesting and complex problem of sleep. When it was thought that alkaloids (ptomaines) were of great importance in infectious diseases, it was attempted to explain sleep as due to the action of similar bodies. Now, when we believe that in such diseases the chief part is played by poisons of extremely complex chemical composition, the attempt is made to explain fatigue and sleep by similar bodies.
Weichardt[96] has recently made the best known investigations in this direction. This young man maintains with ardour the view that during the activity of organs there is an accumulation of special materials which are neither organic acids nor leucomaines, but which are much more like the toxic products of pathogenic bacteria.
Weichardt made animals in his laboratory go through fatiguing movements for hours and then killed them. The extract from muscles of such animals had a powerful toxic effect when it was injected into normal animals, producing lassitude and sometimes death within 20 to 40 hours. As all attempts to determine the exact chemical nature of this fatigue-producing substance were baffled, it is impossible to get an exact account of it. Amongst its properties there is one of great interest. When it has passed into the circulation of normal animals in quantities insufficient to produce death, it excites the formation of an anti-toxin in the same way as a poison of diphtheria stimulates the production of a diphtheria anti-toxin.
When Weichardt injected into animals a mixture of the poison which produces fatigue with small doses of the serum antidote, no results followed. The neutralising effect of the antidote was apparent even when it was introduced by the mouth. Towards the end of his investigations, Weichardt supposed that it would be possible to obtain a material that would prevent fatigue.
Although it is still impossible to specify exactly the nature of the substances which accumulate during the activity of organs and which produce fatigue and sleep, it is becoming more and more probable that such substances exist, and that sleep is really an auto-intoxication of the organism. So far, such a theory has not been shaken by any argument. Recently M. E. Claparède,[97] a psychologist of Geneva, has argued against the current theory of sleep. He thinks that it is contradicted by the fact that new-born infants sleep a great deal, whilst very old people sleep very little. This fact, however, can readily be explained by the greater sensibility of the nerve centres of infants, as shown with regard to many harmful agencies. The other objections of Claparède, such as the fact that sleepiness is induced by exercise in the open air, or that excess of sleep itself produces sleepiness, are not really incompatible with the theory of auto-intoxication. They are facts of secondary importance probably depending on some complication which the present state of our knowledge makes it difficult to indicate exactly. The insomnia of neurasthenia, which Claparède brings forward as another objection, can readily be explained as due to hyperæsthesia of the nervous tissues which lose part of their sensitiveness to poisons.
On the other hand, there are many well established facts in agreement with the theory of auto-intoxication. Leaving out of the question sleep induced by narcotics, I may mention in this connection the so-called “sleeping sickness.” It has been proved that this disease is caused by a microscopic parasite, the Trypanosoma gambiense of Dutton, which develops in the blood and spreads to the liquid of the membranes surrounding the central nervous system. One of the most typical symptoms of the advanced stages of this disease is continual drowsiness. “The drowsiness increases progressively, and the habitual attitude becomes characteristic; the head is bent on the breast; the eyelids are closed; in earlier stages the invalid can be aroused easily, but, after a time, incurable attacks of sleep overcome the patient in all circumstances, but especially after meals. These fits of sleepiness become longer and deeper, until they reach a comatose condition from which it is almost impossible to arouse the patient.”[98] The total result of medical knowledge of this disease is that it is impossible to doubt that the sleepiness is due to intoxication produced by the poison of the trypanosome.
Claparède has opposed what he calls an “instinctive” theory to the toxic theory of sleep. According to this theory, sleep is the manifestation of an instinct “the object of which is to arrest activity; we do not sleep because we are intoxicated or exhausted, but to prevent ourselves from falling into such a condition.” However, in order to bring this narcotic instinct into play, certain conditions are necessary, one of which certainly would be the intoxication of the nerve centres. M. Claparède supposes that sleep is an active phenomenon, induced when waste matter begins to accumulate in the organism. “To bring about sleep, the nerve centres must be influenced by waste matter, and this influence can readily be regarded as a kind of intoxication.”
Hunger is an instinctive sensation as much as sleepiness, but it does not appear until our tissues are in a condition of exhaustion, the exact nature of which cannot as yet be indicated. There is no real contradiction between the toxic and instinctive theories of sleep. The two theories represent different sides of a special condition of the organism.