30 Journal of Comparative Neurology. 



the basis the association of somatic nervous changes with the 

 feehngs. 



It is needless to spend time with the earher attempts to ex 

 plain sleep. It has been ascribed to a sort of dropsical condi- 

 tion and to a dessication of the brain, to a plethora and poverty 

 of blood, to a change in the spleen and a collapse of the vetri- 

 cles, to an accumulation of carbon dioxide and simple fatigue, 

 to heat and electricity ; sleep has even been ascribed to a 

 peculiar explosive action in the brain. 



All modern authors agree that the active and quiescent 

 states ot the brain are constantly associated with changes in 

 blood pressure and composition, but there is httle unanimity as 

 to the exact chemical and physical conditions of sleep. It can- 

 not be doubted that oxygen is a very important factor in the 

 normal activity of the nerve cell as well as all other elements. 

 The well known facts that in a trepaned subject the brain be- 

 comes pale and shrunken during sleep and that ligaturing of the 

 carotid arteries causes unconsciousness, are explained by Preyer 

 on the assumption that the diminished quantity of oxygen is the 

 direct cause of the depressed function. He says, " Except the 

 liver there is no other tissue in the body which deprives the 

 blood corpuscles of their oxygen so rapidly as the brain." The 

 same effect as that produced by the ligaturing of the carotids 

 may be produced by diminishing the oxygen content of the at- 

 mosphere. However, it is impossible to demonstrate a suffi- 

 cient difference in blood supply to account for the peculiar phe- 

 nomena of sleep. It is necessary to assume that the oxygen is 

 otherwise employed in sleep than during waking. During act- 

 ive waking hours the muscle and nerve cells form an accumula- 

 tion of easily oxidizable substance (lactic acid) which during 

 sleep is oxidized, thus abstracting from the blood a large portion 

 of its oxygen. The experimental attempts to demonstrate the 

 availability of lactic acid and its compounds have been variously 

 interpreted and the result must at present be considered nega- 

 tive. 



Quite recently we have acquired more definite evidence of 

 ereat histological changes in nerve cells as a result of fatigue. It 



