DISSECTING MANUAL. 



postero-median fissure. The white matter of the halves is 

 connected by a transverse band (anterior white commissure) 

 forming the bottom of the antero-median fissure. [456] 



The anterior end of each gray crescent is the anterior cornu; 

 this is constricted (cervix) near the commissure but is thick- 

 ened (caput) at its end, which does not reach the surface; the 

 anterior nerve roots arise from it. The posterior end of each is 

 the posterior cornu; this is narrower, constricted (cervix) near 

 the commissure, enlarged (caput) outwardly, and ends in a 

 point (apex) which almost touches the bottom of the postero- 

 lateral furrow; its apex is somewhat translucent and consists 

 of substantia gelatinosa Rolandi: the posterior nerve roots 

 arise from it. The lateral horn is a projection outward from the 

 concavity of each crescent, opposite the gray commissure, and 

 most marked in the upper dorsal region ; it blends above and 

 below with the anterior horn. In the cervical region the 

 anterior horns are very large, while the posterior are narrow. 

 In the dorsal region both horns are narrow, but especially the 

 posterior. In the lumbar region the difference is less marked 

 as the posterior horns widen out. [456] 



The white matter of each lateral half is in three columns; 

 the posterior lies behind the posterior horn, the lateral column 

 occupies the concavity of the crescent, and the anterior lies in 

 front of the anterior horn. The central canal runs throughout 

 the cord, in the gray commissure, and ends, in the conus medul- 

 laris, in an expansion (ventriculus terminalis) . The filum ter- 

 minale is in two parts; the filum terminale externum is the 

 part which has pierced the dura, and is a mere fibrous thread; 

 the filum terminale internum is the part within the dura, 

 consists largely of pia, and, in its upper half, contains the end 

 of the central canal, surrounded by gray matter. [459] 



THE ORBIT. 



The eyeball with its muscles, vessels, and nerves lies in a 

 mass of fat. A smooth membrane (capsule of Tenon) sur- 



[52] 



