STRUCTURE OF THE NERVOUS SYSTEM 



723 



processes are of two kinds: one of them is termed the axis-cylinder process or axon 

 because it becomes the axis-cylinder of a nerve fiber (Figs. 626, 627, 628). The 

 others are termed the protoplasmic processes or dendrons ; they begin to divide and 



FIG. 624. Various forms of nerve cells. A. Pyramidal cell. B. Small multipolar cell, in which the axon quickly 

 divides into numerous branches. C. -Small fusiform cell. D and E. Ganglion cells (E shows T-shaped division of 

 axon). ax. Axon. c Capsule. 



Nucleus 



Cell protoplasm 



" " Axon 



FIG. 626. Motor nerve cell from ventral horn of medulla 

 spinalis of rabbit. The angular and spindle-shaped Nissl 

 bodies are well shown. (After Nissl.) 



subdivide soon after they emerge from the 

 cell, and finally end in minute twigs and be- 

 come lost among the -other elements of the 

 nervous tissue. 



The body of the nerve cell, known as the 

 cyton, consists of a finely fibrillated proto- 

 plasmic material, of a reddish or yellowish- 

 brown color, which occasionally presents 

 patches of a deeper tint, caused by the ag- 

 gregation of pigment granules at one side of 

 the nucleus, as in the substantia nigra and 

 locus cseruleus of the brain. The protoplasm also contains peculiar angular gran- 

 ules, which stain deeply with basic dyes, such as methylene blue; these are known 

 as Nissl's granules (Fig. 626). They extend into the dendritic processes but not 

 into the axis-cylinder; the small clear area at the point of exit of the axon in 



Dendron 

 Myelin sheath 



FIG. 625. Bipolar nerve cell from the spinal gan- 

 glion of the pike. (After Kolliker.) 



