THE SPINE. 375 



8 cervical, 12 dorsal, 5 lumbar, 5 sacral, and i coc- 

 cygeal. Each nerve is attached to the cord by a 

 dorsal and a ventral root. Each root, before uniting 

 with the cord, breaks up into secondary bundles and 

 spreads out like a fan, making a continuous linear 

 attachment. The dorsal ganglion is located upon 

 the dorsal within the vertebral canal, near the union 

 of the two roots. 



Gray Matter of the Cord. The gray matter of the 

 cord is centrally located and takes the form of a 

 capital letter H. The gray matter in each lateral 

 half resembles a crescent which is joined to the op- 

 posite side by an isthmus, in the center of which is 

 the central canal. The latter is usually obliterated 

 in the adult man, and is filled as well as surrounded 

 by the gelatinous substance of Rolando, which is a 

 reticular structure. That part of the isthmus above 

 the central canal is called the posterior gray com- 

 missure, while the gray matter ventral to the canal 

 is the anterior commissure. 



Each crescent may be divided into a posterior, 

 lateral, and anterior horn. The anterior horn is the 

 largest and the lateral horn is the smallest. The 

 posterior horn is pointed and approaches near to the 

 posterior lateral groove. The apex of this horn is 

 called the zona terminalis. At the base of this apex 

 there is a reticular substance called the zona reticu- 

 laris, while next to this and apparently capping the 

 posterior horn is a gelatinous mass, similar to that 

 which surrounds the central canal, called the sub- 

 stantia gelatinosa of Rolando. 



The nerve cells of the posterior horn are irregularly 



