SPINAL CORD 417 



pigment are sometimes conspicuous. All of these features may be ob- 

 served in the smaller nerve cells, but they are most evident in the large 

 motor cells. The dendrites of the motor cells extend far into the dorsal 

 columns (horns), and they even pass out of the gray substance into the 

 ventral and lateral funiculi. 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 colla- 

 terals; when present they are very small. None of the neuraxons cross 

 to the opposite side of the cord before entering the motor roots. 



The nerve cells of the second type, usually smaller than the motor 

 cells but more abundant, are distributed throughout the gray substance 

 either singly or in groups. Definite groups of nerve cells in the spinal 

 cord and brain are known as nuclei, and at the root of the dorsal column 

 (horn) near its junction with the gray commissure, there is the important 



FIG. 429. NERVE CELL OF THE SPINAL CORD FIG. 430. NERVE CELL OF THE SPINAL CORD OF 

 OF A DOG. X 600. A CHILD. X 430. 



dorsal nucleus (column of Clarke). It is composed of cells which send 

 their neuraxons into the lateral funiculus, in which they ascend to the 

 cerebellum. The dorsal nucleus is limited to the thoracic portion of the 

 cord, and adjacent parts of the lumbar and cervical regions. 



The fibers of the ground bundles are derived from scattered cells of 

 the second type. Their dendrites are long but sparingly branched. 

 The neuraxons give off collaterals in the gray substance, and enter the 

 ventral and lateral funiculi (rarely the dorsal) of the same or opposite 

 side. In the white substance most of them divide into ascending and 

 descending fibers, which send collaterals back into the gray, either singly 

 or in bundles, and the main branches finally terminate like the collaterals. 

 After re-entering the gray substance they ramify freely around the motor 

 cells. 



In transverse sections the dorsal column appears capped by the zona 

 spongiosa which covers the substantia gelatinosa (Fig. 424). The former 

 contains spindle-shaped "marginal cells" which send fibers into the 

 white substance. The substantia gelatinosa contains a limited number 

 of very small nerve cells which send processes into the zona terminalis 



27 



