294 NORMAL HISTOLOGY. 



The canal of the sacral cord and of the conus medullaris assumes a 

 _L-form, consisting of a ventral wider arm and a narrow dorsal ex- 

 tension ; an irregular dilatation in the lower part of the conus has 

 received the name ventriculus terminalis. The cords of chil- 

 dren and of many animals contain a completely pervious central 

 canal ; in the human cord in later life this is usually more or less 

 occluded, although much variation exists in this respect. The upper 

 cervical, lower lumbar, and sacral regions usually contain, even in 

 the adult, a partially pervious tube. Overgrowth of the lining 

 cells, as well as of the subepithelial substantia gelatinosa, is the prin- 



FIG. 327. 



Section of spinal cord of human embryo stained by Golgi silver method ; the left half of the figure 

 exhibits the neuroglia-cells, while the right shows the elements constituting the framework of the epen- 

 dyma. (After Lenhosstk.) 



cipal factor in the closure of the central canal, which, however, must 

 be regarded as a normal change and not a pathological process. 



The blood-vessels of the substance of the cord are arranged in 

 two groups : those entering at the periphery, including the 

 larger branches which follow the connective-tissue septa ; those de- 

 rived from the arteria sulci, given off from the anterior spinal 

 artery and lodged within the anterior median fissure, from which 

 branches are distributed to the gray matter. Of the numerous 

 arteries which enter at the circumference, the finer usually terminate 



