CHAP, i.] THE SPINAL CORD. 863 



matter, on the other hand, appear to be continuous. It may be 

 added that the grey matter is well supplied with blood vessels, 

 these being in it, as stated above, relatively much more numerous 

 than in the white matter. 



565. The central canal is lined by a single layer of columnar 

 epithelial cells, which are generally described as bearing cilia ; 

 but it is not certain that the processes which may be seen project- 

 ing from the surfaces of the cells are really cilia. These epithelial 

 cells rest not on a distinct basement membrane but on a bed of 

 neuroglia, free apparently or nearly so from nervous elements, 

 which surrounds the central canal and is sometimes spoken of as 

 the substantia gelatinosa centralis (Fig. 96, c. g. s.}. The attached 

 bases of the epithelial cells are branched or taper to a filament, 

 and become continuous with the branched cells or fibres of the 

 neuroglia below. As we said above the neuroglia elements are 

 transformed epithelial cells ; and the continuity of the cells, which 

 retaining the characters of epithelial cells form a lining to the 

 canal, with the cells which have become branched and lost their 

 epithelial characters indicates the epithelial origin of the latter. 



The central canal with the surrounding area of neuroglia 

 forms the central part of the isthmus uniting the two lateral 

 halves of the cord. Posterior (dorsal) to this central mass lies the 

 posterior grey commissure (Figs. 96, 98, 99, p. g. c.), composed 

 chiefly of fine filaments running transversely, and anterior 

 (ventral) to it lies first the thinner anterior grey commissure 

 (Figs. 96, 98, 99, a. g. c.) of a similar nature, and then the 

 relatively thick white commissure (Figs. 9(5, 98, 99, a. c.) which 

 is formed by medullated fibres crossing over from one side of 

 the cord to the other, and thus constitutes a decussation of 

 fibres along the whole length of the cord. On each side, the 

 central mass of neuroglia of which we are speaking gradually 

 merges into the central grey matter of the corresponding lateral 

 half. 



The end or head (caput) as it is frequently called of the 

 posterior horn is occupied not by ordinary grey matter, but by a 

 peculiar tissue, the substantia gelatinosa of Rolando, which forms 

 a sort of cap to the more ordinary grey matter but differs in 

 size and shape in different regions of the cord. Cf. figs 96, 97, 

 98, s.g. In carmine and some other modes of preparation it is 

 frequently stained more deeply than is the ordinary grey matter, 

 and in such preparations is very conspicuous. It may be described 

 as consisting of a somewhat peculiar neuroglia traversed by fibres 

 of the posterior root, and containing a large number of cells, which, 

 for the most part small, the cell-bodies being small relatively to 

 the nuclei, are not all alike, some being probably nervous and 

 others not. It takes origin from the cells forming the immediate 

 walls of the embryonic medullary canal. In the embryo, this 

 canal is relatively wide, though compressed from side to side, and 



F. 55 



