SLEEP 1 i "> 



of mechanics: "A body in motion continues in motion, 

 a body at rest continues at rest, unless acted on by an 

 external force." 



The purposes served by sleep are plain. It is a state 

 in which the local and general losses of the tissues, which 

 have not been fully met as they have taken place, can be 

 offset. Bodily rest without sleep gives an opportunity 

 for such repair in the skeletal muscles, but not neces- 

 sarily in the receptor system, and certainly not in the 

 cerebrum. Even the skeletal muscles are probably better 

 rested in sleep than they can be otherwise, for their relax- 

 ation is more complete. As to the autonomic mechan- 

 isms, it is harder to determine how far they are granted 

 a remission of activity during sleep. We have to bear in 

 mind the fact that many of the autonomic influences are 

 inhibitory in character, and the interruption of such cur- 

 rents would lead to an actual increase of activity in the 

 organs reached by the nerve-fibers. No case of this kind 

 is certainly known, but the principle helps us to under- 

 stand how there may be no interference with digestion 

 while an animal is sleeping, as it is likely to after eating 

 heavily. 



Many theories have been advanced to account for the 

 passage from the waking to the sleeping state. It will be 

 helpful to consider what conditions other than sleep com- 

 monly suspend consciousness. Two suggest themselves, 

 the one mechanical and the other chemical. One is inter- 

 ference with the circulation in the brain, and the second 

 is anesthesia by drugs carried through the medium of the 1 

 blood. Though we may call the first a mechanical cause 

 of unconsciousness, it is, doubtless, a chemical one at bot- 

 tom, for lack of blood-supply must lead to chemical altera- 

 tions in the neurons. Fainting is an instance of this action. 

 The other type of influence may be illustrated by ether or 

 chloroform narcosis, as well as by the coma of diabetes, 

 where the acidity of the blood is the source of trouble. 

 Normal sleep has sometimes been thought of as a swooning 

 and sometimes as an anesthesia. When the former cause 



