172 HUMAN PHYSIOLOGY 



is in a condition of reduced activity. Decerebrate 

 animals still appear to alternate sleep and waking, thus 

 showing that the change from one to the other occurs in 

 some degree in the brain-stem. From our human 

 standpoint we think of sleep as a suspension of con- 

 sciousness or at least a shutting out of impulses from the 

 receptors and a great diminution in those going to -the 

 muscles. The going and coming cannot entirely cease; 

 breathing goes on and many of the simpler reflexes can 

 be elicited. 



Aside from natural sleep there are two circumstances 

 which commonly suspend consciousness. One is the 

 temporary poisoning of the brain by drugs and the other 

 the type seen in fainting when the cerebral circulation 

 has partially failed. Is our sleep more like anesthesia 

 or like fainting? It is likely that it has points of re- 

 semblance to both. It is like anesthesia in that fatigue 

 substances gathering in and about the brain cells prob- 

 ably dispose to it, but it is like fainting also inasmuch as 

 the final relapsing from waking to sleep is believed 

 to depend on the lessening of blood flow through the 

 brain. A person can ordinarily be waked quickly from 

 sleep but not from anesthesia. The stimuli used to 

 overcome faintness would also serve to rouse a sleeper. 



The simplest way to picture sleep is perhaps to as- 

 sume that the central fact is a high resistance affecting 

 many paths. A blockade anywhere between the sense- 

 organs and the cerebral cortex would account for the 

 failure of sensation. A similar block on the path from 

 the motor areas to the muscles would result in relaxation 

 and quiescence. The interruption of association channels 

 would interfere with the synthetic processes of thought. 

 The dreaming consciousness, when we realize it at all, 

 has a character which suggests that the sensory currents 

 are much impeded and we know that the motor ex- 

 pression is generally limited. 



The first hour of the night's sleep is one of rapidly 

 deepening stupefaction. This has been proved by 



