THE CENTRAL NERVOUS SYSTEM. 



295 



within the white substance, while the coarser alone penetrate 

 into the gray matter, the outer zone of which they in part supply. 

 The vessel occupying the posterior median septum, the arteria fis- 

 surae posterioris, is the most important of the peripheral branches : 

 twigs also accompany the anterior and posterior root-bundles. 



At the bottom of the anterior median fissure the arteria sulci 

 divides into two sulco-commissural branches, which, diverging 

 slightly, enter the gray matter to the inner side of the base of the 

 anterior horn. After a short course within the gray substance, these 

 vessels break up into a number of twigs, which soon form close cap- 

 illary net-works within the anterior and middle parts of the gray 



FIG. 328. 



Section of injected spinal cord of child : s, sulcal branch of anterior spinal artery occupying anterior 

 median fissure ; c, c, sulco-commissural vessels from sulcal artery passing to gray matter to form 

 dense net-work ; p, posterior spinal artery, sending off twigs to white matter ; around margin of cord 

 numerous peripheral vessels enter white substance to form open net-work. 



crescents ; a branch of some size passes backward to supply the 

 region corresponding to Clarke's column. The sulco-commissural 

 artery likewise gives off vertical anastomosing branches, one 

 passing brainward, the other caudalward, to unite with similar off- 

 shoots from the corresponding arteries of different planes. The veins 

 follow in general the course of the corresponding arteries : some of 

 the blood, however, brought by the sulcal artery is carried off by the 

 peripheral veins. 



THE MEDULLA. 



The differences between the medulla and the spinal cord are 

 rather of arrangement than of any great variation in structural 



