4io 



HISTOLOGY 



nerve cells. The processes of the nerve cells, in so far as they are within 

 the limits of the gray substance, are non-medullated. The outer layer is 

 at first entirely free from nuclei, and later it contains only a few cell 

 bodies, belonging with the neuroglia and with the endothelium of vessels 

 which penetrate the cord; it contains no nerve cells. This layer consists 

 of a network of neuroglia fibers through which nerve fibers extend in vari- 

 ous directions, but chiefly up and down the cord. As these fibers become 

 medullated, the layer becomes white macroscopically, and it forms the 

 white substance of the adult cord. In preparations in which myelin is 



Dorsal Median 1 Portion of 



Entrance median Dorsal [ dorsal 



zone. septum, funiculus | Lateral j root. Dorsal root. 



Dorsal column. 



Groups of nerve cells. 



Central canal. 



Ventral root. Whjte Ventral Ventral funiculus. 



commissure. median 

 fissure. 

 FIG. 424. CROSS SECTION OF THE LUMBAR ENLARGEMENT OF THE HUMAN SPINAL CORD. X8. 



deeply stained, the white substance appears darker than the gray sub- 

 stance (Fig. 424). From what has been said, it appears that the med- 

 ullary tube early becomes divisible into inner, middle, and outer 

 layers, which give rise to ependyma, gray substance and white substance 

 respectively. 



As the medullary tube enlarges, ventral swellings are formed on either 

 side of the median line (Fig. 423). These later project so far ventrally 

 that the flloor of the medullary tube is found at the dorsal end of a ventral 



