17ii THK \i:i;vors SYSTKM AND MS <-MXSKK\ \ rmx 



i- a limed, we have :ui munim theory; the alternative is 

 :i theory of to.r< ///in. 



The nightly (experience of falling asleep doc- not very 

 distinctly suggest fainting. But when the drowsiness is 

 unusually compelling and unwelcome, as when one nod- 

 in church, the resemblance does not appear so remote. 

 The rapidity of the passage from clear consciousness to the 

 realm of dreams or to oblivion favors the belief that a 

 mechanical factor has operated to determine it. The 

 most natural assumption is that a vasomotor change 

 has taken place. He well 1 has pointed out that such a 

 change may be due to fatigue of the vasomotor center. 

 During our waking hours this part of the autonomic 

 system is required to respond to frequent and varying 

 demands. Some of its duties have been indicated in an 

 earlier chapter. 



If the center is really subject to fatigue, we may sup- 

 po-e that it eventually becomes difficult to stimulate it 

 sufficiently to maintain the normal arterial tone. If it 

 lapses from its average condition, the blood-pressure will 

 decline and the flow will proceed more slowly through 

 ihe widened vessels. In the brain, encased as it is in a 

 rigidly walled enclosure, the slow flow will be in vessels 

 which cannot materially widen. Hence, the brain supply 

 will be reduced and the maintenance of its activities 

 threatened. It remains |x>ssihle in any ordinary case to 

 spur the center to renewed action, to raise the general 

 pressure, and so to drive the blood once more with an 

 adequate velocity through the capillaries of the brain. 



Drowsiness is the subjective sign of the slowing cir- 

 culation in the cerebrum. In lighting it off one resorts 

 in-tinctively to the employment of external stimulation. 

 This is the object attained by taking a slightly uncom- 

 fortable position, by rai-ing one'- eyes to the light, and 

 by moving parts of the body. The exclusion of stimuli, 

 conversely, favor- the relaxation of the vascular system. 



1 "Tcxt-l>uok <>!' Physiology," Kim, Kdition. \\ . H. S:mmlrrs Co., 

 I'liihi.Mphia, 191::. 



