156 



SPINAL MARROW. 



The pia mater is the immediate covering. This is a cellular 

 membrane consisting almost entirely of vessels. It terminates in a 

 round cord-hke process, reaching from the end of the spinal mar- 

 row to the bottom of the canal, being concealed in the roots of the 

 nerves which constitute the caucla equina. 



SPINAL MARROW. 



The spinal marrow is the medullary column included in the spinal 



canal of the vertebrse. 

 ^'^•^^^- ^ig-138. It is cylindrical, and 



reaching from the atlas, 

 terminates in a point at the 

 first or second lumbar ver- 

 tebra. It is flattened an- 

 teriorly and posteriorly, 

 and has an enlargement in 

 the neck and loins. It is 

 divided longitudinally into 

 symmetrical halves, by 

 an anterior and a pos- 

 terior fissure. The ante- 

 rior^ fissure is broad and 

 shallow; the posterior^ is 

 deep and narrow. Each 

 half is divided unequally 

 by a lateral fissure; the 

 anterior column being the 

 larger.* 



A transverse section of 

 the spinal marrow exhibits 

 the arrangement of its me- 

 dullary and gray matter. 

 The medullary matter is 

 external, and the gray, in 

 this section, resembles two 

 crescents united by a com- 

 missure. 



The posterior horns of 

 the crescent extend to the 

 surface in the lateral fis- 

 sure, in which originates 

 the posterior root of the 

 nerves. 



From the sides of the 

 spinal marrow proceed 30 

 pairs of nerves, each ori- 





