1036 



HUMAN ANATOMY. 



the conspicuous processes of the formatio reticularis and the projecting lateral horn 

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

 ensheathed by interlacements of neuroglia fibrilke. 



In the several parts of the posterior horn the amount of neuroglia varies. 

 Thus, the apex consists almost exclusively of glia tissue, while within the Rolandic 

 substance the number of glia fibres and cells is unusually small. Within the 

 caput and remaining parts of the posterior horn the neurogliar elements are similar 

 in quantity and disposition to those in the anterior horn. 



The ependyma cells lining the central canal of the cord are the direct descendants of the 

 radially arranged embryonal supporting elements (page 1004) ; they may, therefore, be regarded 

 as specialized neuroglia cells. Although most advantageously studied in the fcetus and the 

 child, in favorable preparations from adult cords they are seen as a single row of pyramidal 

 cells, from .030-.050 mm. long and from one-fourth to one-third as broad, whose bases are 

 directed towards the lumen of the canal and beset with cilia. Their pointed distal ends, or 

 apices, are prolonged into a long delicate ependyma! fibre, that in the adult is soon lost in the 

 surrounding neuroglia, but in the fcetus extends through the entire thickness of the cord. The 

 ependyma cells are not all of equal size, those occupying the ventral mid-line, especially in the 

 cervical region, being about twice as long as those on the opposite wall of the canal. The epen- 

 dymal fibres proceeding from these cells are of special length and thickness, the ventral ones con- 

 verging to form a wedge-shaped mass that in the young subject continues as far forward as the 

 bottom of the anterior median fissure. The dorsal ependymal fibres are prolonged through the 

 gray commissure into the posterior median septum, some diverging into the columns of Goll. 



FIG. 891. 



Substantia gelatinosa centralis is the name given to a zone of peculiar trans- 

 lucency that immediately surrounds the central canal. This annular area consists of 



modified neuroglia in which radial ependymal fibers are 

 interwoven with circularly disposed neurogliar fibrillae, 

 the whole giving rise to a compact stratum, interspersed 

 with an unusual number of glia cells, upon which arrange- 

 ment, in conjunction with the absence of nerve-fibres, 

 the characteristic appearance of the gelatinous substance 

 depends. In addition to the branched glia elements, a 

 number of radially directed spindle cells are present in 

 this zone ; they send delicate processes between the 

 ependyma cells, of which they are probably outwardly 

 displaced members. In marked contrast with the Ro- 

 landic substance, which caps the posterior horn, the 

 substantia gelatinosa centralis contains only a few small 

 nervous elements, in recognition of which the term, sub- 

 stantia gliosa centralis, has been proposed by Ziehen. 

 The Nerve-Fibres of the Gray Matter. 

 Within all portions of the gray core a considerable part 

 of the intricate ground-work in which the nerve-cells lie 

 embedded is contributed by the processes of neurones 

 situated at the same, different or even remote levels. 

 These processes, which constitute the nerve-fibres, 

 medullated and nonmedullated, that are seen traversing 

 the gray matter, in all directions, include:(i) the collate- 

 rals and the terminal branches of the dorsal root-fibres that enter the gray ma'trr ; 

 M Tve-fibres of the descending tracts that terminate in relation with the ventral 

 (motor) horn cells ; (3) the axones and collaterals given off by the numerous pos- 

 terior horn cells, that traverse the gray matter to and from the respective columns into 

 which they pass. The dendritic processes, as well as the axones of the type II cells, 

 also contribute to the sum of nervous fibrilla- encountered within the gray matter of 

 the cord. 



WHITE MATTER OF THE SPINAL CORD. 



Tin predominating components of the white substance being the longitudinal 

 nerve-fibres which pass fora longer or shorter distance up and down in the columns of 

 the cord, in cross-sections the outer field, between the gray core and the periphery 



Central canal and surrounding 

 substantia gelatinosa centralis, from 

 child's cord ; canal is lint-d with 

 ependvnia < rlK. outside of which 

 lies neuroglia with glia cells. X 135. 



