THE PHYSIOLOGY OF SLEEP. 267 



5. Inhibitory Theories. Some authors regard the condition 

 of sleep as caused by a process of inhibition (Pawlow), although 

 the mechanism by which this inhibition is exerted and the cause 

 of its periodic recurrence are not explained. Shepard * defines 

 sleep as a dominance in attention of a group of fatigue sensa- 

 tions which inhibit the motor processes and the sensations aroused 

 through the usual avenues of sense. 



6. Anemia Theories of Sleep. It is known that experimental 

 interference with the supply of blood to the brain brings on uncon- 

 sciousness practically immediately. Quite naturally, therefore, 

 it has been suggested that the alternation between sleeping and 

 waking may be connected with a rhythmical variation in the 

 blood-flow through the cortex. Numerous facts in physiology 

 have made it probable that during sleep there is a diminished flow 

 of blood through the brain, a condition of cerebral anemia. In 

 animals with the brain exposed or with a glass window in the 

 skull it has been observed directly that the flow of blood to the 

 cortex is diminished during sleep. Mosso's plethysmographic 

 experiments mentioned above have been given a similar inter- 

 pretation, and Tarchanoff's observations upon sleeping dogs, as 

 well as direct determinations upon man by Brush and Fayer- 

 weather, show that the arterial pressure falls during sleep. 

 Hooker f has shown that the venous pressure also falls during 

 sleep, and that the cutaneous veins of the hand may exhibit a 

 pulse, indicating a dilatation of the cutaneous arteries. Inas- 

 much as the lessened pressure in the arteries is accompanied by a 

 dilatation of the vessels of the skin, as shown by the plethysmo- 

 graph, it is probable, when the facts previously mentioned are 

 taken into consideration, that the diminished pressure in the 

 arteries forces less blood through the brain and more through the 

 dilated vessels of the skin. In fact, as is explained in the section 

 on circulation, it is probable that the blood-flow through the 

 brain is normally regulated indirectly by the circulation in other 

 parts of the body. Constriction of blood-vessels elsewhere in- 

 creases arterial pressure and shunts more blood through the 

 brain, and vice versa. This general view is in accord with the fact 

 that sensory stimuli and increased mental activity are accom- 

 panied by a constriction of the blood-vessels (of the skin) and a 

 rise of arterial pressure, while, on the other hand, mental inac- 

 tivity and especially sleep are accompanied by a dilatation of the 

 blood-vessels of the body (skin vessels) and a fall of arterial 

 pressure. Many facts, therefore, point to an anemic condi- 



* Shepard, "The Circulation and Sleep" ("Univ. of Michigan Studies," 

 Scientific Series, vol. i, 1914). 



t Hooker, "American Journal of Physiology," 35, 73, 1914. 



