268 



THE HUMAN MECHANISM 



into folds, or convolutions-, and this increase in size and 

 surface folding carried yet farther in the monkey reaches 



its highest development 

 in the human brain. 



3. The cranial nerves. 

 Nerves enter the 'tween- 

 brain, midbrain, and hind- 

 brain somewhat as they 

 enter the spinal cord ; 

 and although their sepa- 

 ration into dorsal and 

 ventral roots is not ob- 

 vious, the neurones to 

 which their nerve fibers 

 belong are in all respects 

 analogous to the neu- 

 rones of the spinal nerves. 

 They may serve as the 

 paths of reflexes (for ex- 

 ample, a wink is a re- 

 flex from the optic or the 

 trigeminal nerve to the 

 facial nerve), and their 

 relation to the cells of 

 the cerebrum and other 

 higher portions of the 



FIG. 109. A portion of the gray matter bram * essentially the 

 (cortex) of the cerebrum (highly magni- same as that of the spinal 



fied). After Kolliker 



nerves. Fig. 107 will give 



Note the large number of dendrites. The the pointg Q f entrance or 



axons are the fibers of uniform diameter 



running lengthwise of the drawing. One of 



these cells is shown in Fig. 41, D 



exit of these nerves from 

 the human brain. 

 4. Histological structure of the brain. Microscopic study 

 of the brain shows an aggregation of neurones similar to 

 that seen in the spinal cord. These neurones differ greatly 



