1.1 TIIK NERVOUS SYSTEM AM) ITS C< >.\SKUYATI< >\ 



IK-CM expressed by Thom.->on. It is, briefly. a> follow.-: 

 Sleep become- necessary at the saMic stage in develop- 

 ment where there is the earliest entertainment of conscious 

 purpose-. Animals which are never actuated by such pur- 

 poses will not overdo. Their metabolism, their waste, 

 and repair will go on in a fairly even manner, each output 

 of energy belli"' followed by a prompt subsidence of 

 activity to permit recuperation. But as soon as there i- 

 aiiy germ of will, any interest in the pursuit of an object, 

 the organism will be spurred on until the compensatory 

 change has to be of a profound and long-continued kind, 

 and. moreover, has to abolish the entertainment of the 

 conscious purpose while it is in progress. 



\Yhile we cannot help emphasizing the relation of sleep 

 to consciousness, and, therefore, on the physiologic side, 

 to the cerebrum, we an- obliged to concede that other 

 parts of the nervous system are involved. This is plain 

 from the fact that decerebrate birds and mammals still 

 exhibit an alternation of two conditions which have the 

 outward aspect of sleeping and waking. The periodic 

 depression is not confined to the cortex, though a slight 

 change in the chemical condition of the cerebral gray 

 matter would seem to be sufficient to produce the purely 

 psychic experience of going to sleep. 



Whatever may be the essential condition of sleep, it is 

 favored by the cessation ,>f external stimulation and post- 

 poned by the action of such disturbances. It is probably 

 for this reason that most animals are disposed to sleep 

 at night. This will most surely be the case with those 

 which receive a large share of their guiding impressions 

 through the eye. Normal man falls in this class. His eyes 

 are his nio-t important receptors, but because of his com- 

 mand of artificial lighting he is not compelled to submit 

 lamely to the suggestion that he go to sleep at sundown. 

 lie has developed a curious habit of being active far into 

 the night and sleeping far into the day, at least in the 

 spring and summer of the temperate /one. This tend- 

 ency may suggest a whimsical citation of I he first law 



