THE BRAIN 111 



to the cerebrum. The remaining cranial nerves are 

 connected with the brain-stem at short intervals be- 

 tween the place of entrance of the optic fibers and the 

 opening at the base of the skull through which the spinal 

 cord goes out. 



We need not make a complete list of the cranial nerves 

 and the regions with which they stand in relation but 

 certain ones may be noticed. The fifth pair are the 

 important trigeminal nerves, bringing in sensory fibers 

 from the face, the outer surface of the eyeballs, the teeth, 

 and the linings of the mouth and nose. The seventh 

 nerves are the facial, controlling the small muscles on 

 which the play of facial expression depends. The dis- 

 abling of one of these nerves, or rather of the gray 

 matter related to it, causes a drooping of one side of 

 the face, a condition which is not uncommon. The 

 eighth pair of cranial nerves are the auditory, valuable 

 not only as mediating the sense of hearing but bearing 

 an important part in shaping the reflexes that serve to 

 maintain equilibrium. The tenth or vagus nerves, 

 already mentioned, have a manifold service, as we shall 

 see. 



White and Gray Matter in the Brain. The brain- 

 stem resembles the spinal cord in having white matter 

 at the surface. But the H-figure is not to be traced 

 within; the gray matter occurs, rather, in more or less 

 isolated collections. Some of these are distinctly 

 tributary to certain cranial nerves and are described 

 as the nuclei of these nerves. The cerebrum and the 

 cerebellum have gray matter spread in a thin layer 

 over their surface, a development called the cortex. 

 We have every reason to believe that it is upon the 

 organization of the cortex, especially of the cerebrum, 

 that the rank of an animal depends. The human 

 cortex is thus of extraordinary interest to us. The 

 convolutions seem to be devices to increase the extent 

 of cortex. The interiors of the cerebrum and the cere- 

 bellum are taken up chiefly with white matter, fibers 



