THE ELASMOBRANCH FISHES 



229 



NERVOUS SYSTEM OF ELASMOBRANCHS IN GENERAL 



The central nervous system, comprising the brain and the spinal cord, is com- 

 posed of neurones. A neurone consists of a nerve cell from which proceed the 

 axone and dendrites. The nerve cells themselves are of three general types. 

 One of these is a multipolar cell (fig. 206) with numerous dendritic processes 



Fig. 20 



Fig. 208 



rig. 207. Bipolar cells of cord, Pi-istiurus. (Modified from Yon Leiihossek.) 



a-c, stages in development of a bipolar cell; ax., axone or fiber; c, adult type of cell; 

 d.r., dorsal root; g., dorsal root ganglion. 



Fig. 208. Supporting elements, Mustelus cants. (From Houser.) A. Neurogleal cell. B. 

 Ependymal cell. 



(dt.) an axone (ax.), and a large nucleus (n.); another is the bipolar type 

 with two processes to the cell body. Figure 207 shows the development of 

 some bipolar cells in the dorsal root ganglion of Pri.stiiirus. An early stage (a) 

 shows the two processes, one of which, the axone proper, passes in toward the 

 cord; the other process extends toward the periphery. Stage (c) shows the 

 union of the two processes to form the stem of a T. In this stage the cell appears 

 to be unipolar or with a single process extending from it. 



In addition to the cells and fibers found in the central nervous system, cer- 

 tain supporting elements are also present. The first of these are the ependymal 



