238 



COMPARATIVE ZOOLOGY 



nucleated cells, which give off one to several processes, 

 the latter serving as paths of communication between 

 the cells themselves or between the cells and the various 

 motor, sensory, and other organs with which they are 

 connected. Such threads of nerve tissue are called 



R 



[R 



FIG. 210. Portion 

 of two Nerve 

 Fibers: a, medul- 

 lary sheath; c, nu- 

 cleus of neuri- 

 lemma; R, annu- 

 lar constriction. 



Dendrites 



Nerve process 

 Collateral branch 



Axis cylinder 



Neurilemma 



Terminal branches 



FIG. 211. Diagram of a Neuron: a, nerve pro- 

 cess; b, neurilemma; c, medullary sheath; 

 d, neurilemma and medullary sheath, combined. 



nerve fibers, and each consists essentially of a prolonga- 

 tion of the protoplasrnic substance of which the cell 

 body is composed. The cells vary from 35*5-0 to %^Q of 

 an inch in diameter, and are found in the nerve centers 

 (Fig. 329), the gray portion of the brain, spinal cord, 

 and other ganglia. The fibers vary in structure. In 

 the lowest Metazoa they are merely naked threads of 



