100 



and Carroll in 39 " hypertonic " subjects with an average 

 systolic pressure of 204 mm. found a fall of 44 mm. after 

 two hours' sleep; at the moment of waking it rose 22 mm. 

 from the level present in sleep. More recently, in the last 

 year or two, C. Miiller in normal persons found the systolic 

 pressure to be down to 94 mm. in men and 88 mm. in women 

 during sleep, after a small dose of veronal. Blume, in 

 men and women with moderate day pressures, recorded 

 falls of 15 mm. and 21 mm. respectively, while in those 

 with high day pressures the falls averaged 31 mm. and 

 39 mm. Much importance has been attached by some 

 writers to this reduction of pressure; it has even been 

 regarded, though on very insufficient groimds, as the deter- 

 mining cause of sleep. 



It must be borne in mind that in the recumbent position 

 a fall of aortic pressure would be necessary to prevent the 

 pressure in the cerebral arteries from being higher in the 

 horizontal than in the erect position — from the influence 

 of gravity, the hydrostatic factor of the weight of the 

 column of blood between the levels of the heart and the brain. 

 Allowance for this factor in the recumbent posture cuts 

 down the observed lowering of arterial pressure to a com- 

 paratively small amount, probably much too small to play 

 the potent role that has been ascribed to it in the pro- 

 duction of sleep. Further, the crude analogy of uncon- 

 sciousness caused by an arrest or sudden great diminution 

 in the blood supply to the brain cannot be regarded as 

 valid when applied to the induction of the normal process 

 of sleep. 



Diminished vascularity of the brain substance during 

 sleep has been described by various writers (Durham and 

 others) on the ground of direct observations on exposed 

 portions of the brain surface. A similar change has been 

 inferred from the plethysmographic records obtained by 

 Mosso and his followers, who found evidence of an increased 

 volume of blood in the limbs (arm, etc.) during sleep, and 

 assumed this to be attended by lessened vascularity of the 

 brain, the converse conditions being present after awaking. 

 But this hypothesis has to be revised in view of Leonard 

 Hill's work establishing the practical accuracy of the old 

 Monro-Kellie doctrine that the amount of blood within the 

 skull is a constant quantity, whilst its distribution in 

 arteries, capillaries, and veins respectively varies in 

 different conditions. Weber's more recent work indicates 

 that when there is less blood in the limbs there is more in 

 the abdomen, not in the brain. 



Plethysmographic observations have clearly shown respon- 

 siveness to stimuli during sleep, inducing alterations in the 

 volume of a limb and showing certain changes in the dis- 

 tribution of the blood in the vascular system. But such 

 observations give no information as to the state of the 

 aortic blood pressure, upon .which the pressure in the 

 cerebral arteries depends. 



