^iz THE POPULAR SCIENCE MONTHLY. 



older men called it " stimulus " from without, is also undergoing rest 

 and repair, so that when it comes again into work it may receive bet- 

 ter the impressions it may have to gather up, and influence more effec- 

 tively the muscles it may be called upon to animate, direct, control. 



Thirdly, although in the organism during sleep there is suspension 

 of muscular and neiwous power, there is not universal suspension ; a 

 narrow, but at the same time safe, line of distinction separates the 

 sleep of life from the sleep of death. The heart is a muscle, but it 

 does not sleep, and the lungs are worked by muscles, and these do not 

 sleep ; and the viscera which triturate and digest food are moved by 

 muscles, and these do not sleep ; and the glands have an arrangement 

 for the constant separation of fluids, and the glands do not sleep ; and 

 all these parts have certain nerves which do not sleep. These all rest, 

 but they do not cease their functions. Why is it so ? 



The reason is, that the body is divided into two systems as regards 

 motion. For every act of the body we have a system of organs under 

 the influence of the will, the voluntary, and another system indepen- 

 dent of the will, the involuntary. The muscles which propel the body, 

 and are concerned in all acts we essay to perform, are voluntary ; the 

 muscles, such as the heart and the stomach, which we cannot control, 

 are involuntary. Added to these are muscles which, though commonly 

 acting involuntarily, are capable of being moved by the will : the mus- 

 cles which move the lungs are of this order, for we can if we wish 

 suspend their action for a short time or quicken it; these muscles we 

 call semi-voluntary. In sleep, then, the voluntary muscles sleep, and 

 the nervous organs which stimulate the voluntary muscles sleep ; but 

 the involuntary and the semi-voluntary muscles and their nerves merely 

 rest : they do not veritably sleep. 



This arrangement will be seen, at once, to be a necessity, for upon 

 the involuntary acts the body relies for the continuance of life. In 

 disease the voluntary muscles may be paralyzed, the brain may be 

 paralyzed, but, if the involuntary organs retain their power, the animal 

 is not dead. Sir Astley Cooper had under his care a man who had re- 

 ceived an injury of the skull causing compression of the brain, and the 

 man lay for weeks in a state of persistent unconsciousness and repose ; 

 practically he slept. He did not die, because the involuntary system 

 remained true to its duty ; and, when the great surgeon removed the 

 compression from the brain of the man, the sleeper woke from his long 

 trance and recovered. Dr. Wilson Philip had a young dog that had 

 no brain, and the animal lay in profound insensibility for months, prac- 

 tically asleep ; but the involuntary parts continued uninfluenced, and 

 the animal lived and, under mechanical feeding, grew fat. Flourens 

 had a brainless fowl that lived in the same condition. It neither saw 

 nor heard, he says, nor smelled nor tasted nor felt; it lost even its in- 

 stincts; for however long it was left to fast, it never voluntarily ate; 

 it never shrunk when it was touched, and, -when attacked by its fellows, 



