THE PHYSIOLOGY OF SLEEP. 415 



sleep. And so, to the extent that the acts of the brain and cord and 

 their nerves are mental, and the acts or motions of the voluntary mus- 

 cles are bodily acts, to that extent, in sleep, the intercourse between 

 the mind and the body is suspended. 



In sleep the condition of the involuntary muscles aud of the volun- 

 tary nervous system is, we must assume, in some manner modified, 

 since these organs are transformed from the active into the passive 

 state. Respecting the condition of the muscles in sleep, no study 

 of a systematic sort has been carried out, but in relation to the brain 

 there has been much thoughtful study, upon which many theories have 

 been founded. 



The older physiologists regarded sleep as due to the exhaustion of 

 the nervous fluid ; during sleep, they held, this fluid accumulates in 

 the brain ; and, when the brain and the other centres and nerves of 

 the cerebro-spinal system are, to employ a common expression, re- 

 charged, the muscles are stimulated and the body awakes ; the brain 

 prepared to receive external impressions and to animate the muscles, 

 and the muscles renovated and ready to be recalled into activity. 

 This theory held its ground for many years, and, perhaps, still there 

 are more believers in it than in any other. It fails to convince the 

 skeptical because of its incompleteness, for it tells nothing about the 

 nature of the presumed nervous fluid, and we know nothing as yet 

 about this fluid. The primary step of the speculation is consequently 

 itself purely hypothetical. 



Another theory, that has been promulgated, is that sleep depends 

 on the sinking or collapse of the laminae of the cerebellum or little 

 brain. This theory is based on the experiment that compression of 

 the cerebellum induces sleep ; but the argument is fallacious, because 

 pressure on the larger brain, or cerebrum, is followed by the same 

 result. The theory of pressure has been proposed again in a different 

 way ; it has been affirmed that the phenomenon of sleep is caused by 

 the accumulation of fluids in the cavity of the cranium, and by press- 

 ure, resulting from this accumulation, on the brain as a whole. "We 

 know well that pressure upon the brain does lead to an insensible con- 

 dition resembling sleep, and in some instances, in which the skull has 

 been injured and an artificial opening through it to the brain has been 

 formed, pressure upon the exposed surface has led to a comatose con- 

 dition. I once myself saw a case of this nature. But the evidence 

 against this explanation is strong, because the sleeping brain has been 

 observed to be pale and too free of blood to convey any idea of press- 

 ure. 



In opposition to the pressure theory, Blumenbach contended that 

 sleep is due to a diminished flow and impulse of blood upon the brain, 

 for he argued the phenomenon of sleep is induced by exhaustion, and 

 particularly by exhaustion following upon direct loss of blood. Recently 

 Mr. Arthur Durham, in a very able communication, has adduced a 



