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.