THE SPINAL CORD. 



273 



cornu, where the glia fibrils form septa of considerable thickness that 

 diverge into the surrounding white columns; moreover, the conspicuous 

 processes of the formatio reticularis and the projecting lateral horn consist 

 largely of neuroglia. The larger nerve-cells and their more robust processes 

 are ensheathed by interlacements of neuroglia fibrillae. The amount of 

 neuroglia varies in the several parts of the posterior horn; thus, the extreme 

 apex consists almost entirely of glia tissue, although within the substantia 

 gelatinosa Rolandi the number of glia fibres is unusually meagre. Within 

 the caput and remaining parts of the posterior horn, the neurogliar elements 

 are similar in quantity and disposition to those in the anterior cornu. 



Substantia gelatinosa centralis is the name given to a zone of 

 peculiar translucency that immediately surrounds the central canal. This 

 annular area consists of modified neuroglia in which radial ependymal fibres, 



continued from the ependymal cells lining the 

 central canal, are interwoven with circularly 

 disposed neuroglia fibrillae, the whole being a 

 compact stratum interspersed with an unusual 

 number of glia cells. In marked contrast to 

 the substantia gelatinosa which caps the pos- 

 terior horns, that surrounding the central 

 canal contains few nerve-cells and chiefly glia 

 elements. 



The nerve-fibres of the gray matter 

 constitute a considerable part of the intricate 

 groundwork in which the nerve-cells are em- 

 bedded. The fibres, seen traversing the gray 

 matter in all directions, are the processes con- 

 tributed by neurones situated at the same, dif- 

 ferent or even remote levels, and include: (i) 

 the collaterals and terminal stems of the dor- 

 sal root-fibres that enter the gray matter; (2) 

 terminal fibres of the descending tracts that 

 end in relation with the ventral (motor) radic- 

 ular cells; (3) the axones and collaterals from 

 the numerous cells of the posterior horn that 

 traverse the gray matter to and from the respective columns in which they 

 course. The dendritic processes also contribute to the complex. 



The White Matter. Since the predominating components of the 

 white substance are longitudinal nerve-fibres that pass for a longer or shorter 

 distance up and down in the columns of the cord, in transverse sections the 

 field between the gray core and the surface is made up chiefly by the cross- 

 sections of the medullated nerve-fibres. These appear as small, closely set 

 and somewhat compressed rings (Fig. 320), enclosing deeply stained dots, 

 surrounded by faint annular striations. The dots are sections of the axis- 

 cylinders, the annular striations the remains of the framework of the medul- 

 lary coat, and the rings the slight condensations of the neuroglia that take 

 the place of a neurilemma, which is wanting on the fibres of the cerebro- 

 spinal axis. The individual nerve-fibres vary greatly in size, even within 

 the same tract large and small ones often lying side by side. The smallest 

 may be less than 5 ;t in diameter and the largest over 25 /*. In general, the 

 thickest fibres are found in the anterior and in the lateral part of the posterior 

 columns, and the thinnest ones in the posterior and lateral columns in the 

 immediate vicinity of the gray matter. 

 18 



FIG. 319. Central canal and sur- 

 rounding substantia gelatinosa cen- 

 tralis from child's cord ; the canal is 

 lined with ependymal cells, outside of 

 which lies the neuroglia. X 135. 



