THE BRAIN 109 



The Cranial Nerves. Of the cranial nerves, only 

 one pair join the cerebrum. These are the olfactory 

 nerves, diffuse collections of fibers which lead into the 

 brain from microscopic receptors in the upper part 

 of the nasal passages. As the name implies, these nerves 

 are responsible for the sense of smell. The olfactory 

 nerves are the farthest forward of all in the cranial 

 series, so they are spoken of as the first cranial nerves. 

 The second pair are the optic nerves, trunks of large size 

 coming from the eyeballs to a place on the under sur- 

 face of the brain where they appear to cross in the form 

 of a letter X. Their fibers enter the brain-stem close 

 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 



