186 



THE ESSENTIALS OF HISTOLOGY 



posterior is the narrower and comes near the surface of the cord ; 

 opposite to it the bundles of the posterior nerve-roots enter the cord. 

 The bundles of the anterior nerve-roots enter the anterior cornu. 



The white matter of each half of the cord is subdivided by the 

 passage of the nerve-roots into the cornua into three principal columns 

 anterior, lateral, and posterior." It is composed of longitudinally 

 coursing medullated nerve-fibres, which in stained sections appear as 

 clear circular areas with a stained dot, the axis -cylinder, near the 

 middle (fig. 222). 



A g 



FIG. 221. SECTION OF THE SPINAL, CORD IN THE LOWER DORSAL REGION. (Cadiat.) 



A, B, C, anterior, lateral, and posterior columns ; S, s', anterior and posterior median fissures. 

 a, b, c, ceils of anterior cornu ; d, posterior cornu and substantia gelatinosa of Rolando ; 

 e, central canal ; /, veins ; g, anterior root-bundles ; h, posterior root-bundles ; i, white 

 commissure ; j, grey commissure ; /, reticular formation. 



The medullated fibres are supported by a peculiar reticular tissue 

 {neurogiia) which contains a number of nuclei embedded in it and 

 appears to be composed of branched fibrillated cells (neuroglia-cells). 

 The neurogiia is accumulated in greater amount at the surface of the 

 cord underneath the pia mater (fig. 222), and it extends into the grey 

 matter, of which it may be said to form the basis, and in which it is 

 especially accumulated at the apex of the posterior cornu and around 

 the ceniral canal (substantia gelatinosa of Eolando). 



