280 MICROSCOPIC AX ATOMY OF THE ORGANS. 



yrisea central-is. In adults it is often partly obliterated on 

 account of the growth of ependymal cells and neuroglia fibres. 

 The gray commissure is divided by the central canal into a 

 dorsal and a ventral gray commissure. 



The white matter, as already mentioned, surrounds the gray 

 matter, and is separated into right and left halves by the^/x- 

 sura mediana ventralis in front, and the septum medianum 

 dorsale behind. The former is a longitudinal fissure which 



FIG. 210. 



Longitudinal sections of Medullated nerve fibres 



' mednllated fibres j cut across 



White matter 



Nerve 



Gray matter 



Glia cell 



The ventral half of the ventral horn from a calf's spinal cord. Section through the cervical 



enlargement. X 80. 



extends the whole length of the cord, but never is quite deep 

 enough to reach the gray matter. A thin strand of white 

 matter intervenes, and is known as the white commissure (Figs. 

 205 and 212). Each of these halves of the white matter is 

 divided by means of the ventral and dorsal nerve roots into 

 ventral, lateral, and dorsal columns (Figs. 205 and 212). On 

 the surface of the cord this division is marked by the sulci 

 (sulcus later alls ventralis and dorsali*). 



