THE CENTRAL NERVOUS SYSTEM. 203 



The accompanying diagram, after Lenhosse"k, illustrates the vari- 

 ous groups of nerve-cells now recognized as taking part in the com- 

 position of the gray matter, as well as the assumed communications 

 established by the collateral fibres within the cord. 



In addition to the nerve-cells and the fibres, the gray matter is 

 everywhere pervaded by the supporting and uniting substantia 

 spongiosa ; this ground-substance is composed of neuroglia fibres 

 and richly-branched neuroglia cells, the latter being rather more nu- 

 merous than in the white matter. Covering the posterior cornu 

 and immediately surrounding the central canal of the cord, the 

 ground-substance is modified to become the apparently almost homo- 

 geneous substantia gelatinosa ; the mass capping the posterior 

 horn, the substantia gelatinosa Rolandi, contains some few fusi- 

 form nerve-cells and presents a striation produced, in part at least, 

 by the course of the posterior root-fibres. The zone of clear ground- 

 matrix surrounding the central canal of the cord, the substantia 

 gelatinosa centralis, markedly differs from that on the posterior 

 cornu, in being composed principally of closely-felted neuroglia 

 fibres ; in certain regions (cervical and dorsal) the gelatinous sub- 

 stance encroaches somewhat upon the gray commissure. 



The central canal occupies the gray commissure, being contin- 

 uous with the cavity of the fourth ventricle above, and ending blindly 

 below in the upper half of the filum terminale. The canal does not 

 occupy the centre of the gray commissure, since it lies rather ven- 

 trally to that point, especially in the lower part of the cord. 



The columnar epithelium lining the canal is an extension of 

 that of the cerebral ventricles. In children, and in many animals 

 at all ages, the surface of the cells directed towards the lumen is 

 clothed with cilia ; the opposite ends of the cells terminate in long 

 slender processes, which extend deeply into the surrounding struct- 

 ures. The lining cells represent the spongioblasts, which in the 

 embryonic cord closely crowd around the central canal and send long 

 delicate fibres from their outer ends through the cord as far as the 

 pia, while from their inner surface the hair-processes, the cilia, pro- 

 ject into the central canal. The epithelium, with the subjacent 

 neuroglia layer on which the cells rest, constitutes the ejjendvma. 



The form and size of the central canal, which represents the 

 remains of the primitive neural tube, vary in the different divisions : 

 in the upper cervical region its cross-section is somewhat quad- 

 rilateral, from the level of the fifth cervical nerve becoming oval 

 or slit-like, with the cleft placed parallel with the commissure. In 

 the dorsal region the canal gradually approaches the circular form, 

 while in the lumbar it once more becomes a compressed oval, 

 with, however, the long diameter coinciding with the sagittal plane. 



