So far, however, there has been no confirmation of Preyer’s theory. Errera[93] has brought forward against it another theory according to which the cause of sleep is not acid products, but certain alkaline substances described by M. Armand Gautier under the name of _leucomaines_. Gautier laid down that these substances act on the nervous centres and produce fatigue and sleepiness. According to Errera they might very well be the cause of sleep, as that comes on at a time when there is the greatest accumulation of these leucomaines in the body. He thinks that their action in producing sleep is a direct intoxication of the nerve centres. During sleep they are removed, and the disturbance which was produced in the organism is arrested. 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.