THE CKANIAL NERVES 581 



but at certain points, the gray substance takes on a special degree of 

 development, and presents an abundant collection of nerve cells. These 

 collections are called the "nuclei" of the nerves, on account of their 

 evident importance as points of origin from which the nerve roots can 

 be traced to their points of emergence at the base of the brain. The 

 foregoing diagram shows the changes in external form of the cerebro- 

 spinal axis, and in the position of its gray matter, as examined at differ- 

 ent levels in the cranium and spinal canal. 



