120 HISTOLOGY. 



processes run in every direction. It differs from the white substance, 

 therefore, in the absence of myelin, the presence of nerve cell bodies and 

 the confused courses of the nerve fibers. 



The cell bodies belong with three types of cells. The largest are the 

 motor cells, 67 to 135 ( in diameter, which form a group in the ventral 

 column. (In the cervical and lumbar enlargements of the cord (Fig. 140) 

 the group is divided into dorso-lateral and ventro-medial portions.) Cell 

 bodies like those of the motor cells are represented in Figs. 145 and 146. 

 The former shows the fibrillar structure of their protoplasm, and the latter 

 the groups of granules, chromatic bodies (Nissl's bodies) which may 

 occur between the fibrils. These are rounded or angular masses which 

 are not limited to motor cells. They become reduced or disappear with 



Nissl's bodies. 



PIG. 145. NERVE CELL OP THE SPINAL CORD FIG. 146. NERVE CELL OP THE SPINAL CORD 

 OP A CHILD. X 430. OP A Doc. X 600. 



fatigue, in old age, and in certain diseases and poisonings. It is supposed 

 that they are nutritive rather than nervous elements. After preservation 

 in alcohol they may be stained with methylene blue. In the motor cells 

 the fatty pigment may be abundant, but often in ordinary specimens 

 these special features are invisible and the protolasm seems densely 

 granular. The processes of the motor cells are dendrites, which may 

 extend into the ventral and lateral funiculi, and even into the dorsal 

 funiculi, and neuraxons which leave the cord in the ventral roots and 

 proceed to the striated muscles. The neuraxon begins as a slender non- 

 medullated fiber at the tip of a clear 'implantation cone' and acquires 

 its myelin sheath as it crosses the white layer. Ordinarily it has no 

 collaterals; when present they are very small. 



Cell bodies of the second type are more numerous and smaller than 

 the motor cells. They occur singly and in groups throughout the gray 

 substance. Their dendrites are long but with comparatively few branches. 

 Their neuraxons give off many collaterals in the gray substance and enter 



