THE SPINE. 



377 



the cord consists of medullated nerve fibers. Most of 

 these fibers have no neurilemma, and the cord there- 

 fore is soft and pulpy, in contrast to the nerve 

 trunks whose fibers have a neurilemma which with 

 the connective-tissue elements make nerves tough 

 and strong. The white matter practically encloses 

 the gray. The fibers which compose it vary con- 



Postero-lateral horn. 



Posterior horn. 



Posterior fissure. 



Lissaur's marginal 



ground bundle. 

 Comma tract. 

 Pia mater. 



Direct cerebellar 

 tract. 



Gowers's tract. 

 Processus reticularis. 



Lowenthal's tract. 



Anterior commissure. Anterior fissure. Direct pyramidal tract. 



Fig. 266. Cross section of the spinal cord, dorsal region, i, Zona 

 terminalis; 2, zona reticularis; 3, substantia gelatinosa of Rolando; 4, 

 stellate cells of posterior horn; 5, column of Clarke; 6, Waldeyer's 

 central cell column; 7, cells of lateral horn; 8, central canal; 9, antero- 

 mesial cells; 10, postero-mesial cells; n, antero-lateral cells; 12, pos- 

 tero-lateral cells. 



siderably in size, both large and small being mixed 

 up together. In sections of the adult healthy cord 

 no evidence of definite tracts of fibers can be seen. 

 We know, however, that longitudinally arranged 

 groups of fibers run a definite course, have definite 

 connections, and carry impulses that result in defi- 

 nite sensations and actions. The physiological evi- 



