278 



THE NERVOUS SYSTEM 



by a little connective tissue in which run blood vessels 

 and lymphatics, the whole inclosed in a sheath called the 

 perineurium. The nerves from the central system are 

 composed chiefly of medullated fibers, with which are 

 bound up a few nonmedullated ones, while the nerves 

 of the sympathetic system are made up almost wholly of 

 the gray nonmedullated fibers. 



412. A Ganglion is a little group of nerve cells forming 

 a nervous center. As a rule (to which the spinal ganglia 

 form an exception), the nerve fibers running from the cen- 

 tral system to a ganglion are medullated, while those 

 passing from the ganglion toward the periphery are non- 

 medullated and also more numerous. 



413. Neuroglia. The fibers and cells of both the gray 

 and the white nervous matter are supported by a tissue 



called neuroglia (Fig. 125), 

 which is composed of ex- 

 tremely fine fibers and cells. 

 It differs chemically and in 

 origin from the connective 

 tissues, though like them in 

 function. 



414. The Nerve Unit or Neu- 

 ron. A nerve cell, with its 

 two sorts of processes, con- 

 stitutes a nerve unit^ or neuron 

 (Fig. 126). That is, the whole nervous system is built 

 up of an indefinite but enormous number of these units, 

 supported by neuroglia and connective tissue. A nerve 

 cell sends off one process (rarely more), which is pro- 

 longed into the axis cylinder, or neuraxon* while the 

 numerous other branches, called dendrons or dendrites, 

 almost immediately break up into fine twigs or brushlike 



Fig. 125. Neuroglia cells. 



