THE PHYSIOLOGY OF SLEEP. 249 



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 experi- 

 ments mentioned above have been given a similar interpretation, 

 and Tarchanoff s observations upon sleeping dogs, as well as direct 

 determinations upon man by Brush and Fayerweather, show that 

 the arterial pressure falls during sleep. Inasmuch as the lessened 

 pressure in the arteries is accpmpanied by a dilatation of the vessels 

 of the skin, as shown by the plethysmograph, 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 in- 

 directly by the circulation in other parts of the body. Constriction 

 of blood-vessels elsewhere increases 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 accompanied by a constriction of the blood-vessels 

 (of the skin) and a rise of arterial pressure, while, on the other 

 hand, mental inactivity and especially sleep are accompanied by 

 a dilatation of the blood-vessels of the body (skin vessels) and a 

 fall of arterial pressure. All of our facts, therefore, point to an 

 anemic condition of the brain during sleep, and some physiologists 

 have believed that this condition precedes and causes the state 

 of sleep, while others take the opposite view that it follows and 

 is merely one result of sleep. On the basis of the plethysmographic 

 experiments mentioned above the author* has proposed a theory 

 of sleep in which the diminished flow of blood to the brain is ex- 

 plained and is assumed to be the chief factor in bringing on sleep. 

 The theory assumes that the periodicity of sleep is dependent 

 mainly upon a rhythmical loss of tone in the vasomotor center in 

 the medulla in consequence of fatigue from continued activity 

 during the waking hours. That is, the vasomotor center is in 

 constant action during this period; the continued flow of sensory 

 stimuli and the constant activity of the brain act reflexly on this 

 center and through it cause a constriction of the blood-vessels of 

 the body, particularly of the skin, by means of which the blood- 

 flow through the brain is maintained with an adequate velocity. 

 In consequence of this varying but constant activity the center 

 undergoes fatigue; stronger and stronger stimulation is necessary 

 to maintain its normal tone, and eventually its .effect on the blood- 

 pressure becomes insufficient to maintain an adequate flow through 

 * Howell, "Journal of Experimental Medicine," 2, 313, 1897. 



