534 THE SPINAL CORD. 



other, and the connective-tissue septa of the latter run right into the former 

 the outline of the gray matter is not smooth and even, but broken by tooth- 

 like processes due to the septa. Since, as we have just said, some of the true 

 nerve-cells are very small, and since the nerve-filaments like the neuroglia 

 fibres are very fine and take like them an irregular course, it often becomes 

 very difficult in a section to determine exactly which is neuroglia and which 

 are nervous elements. The neuroglic cells may, however, be distinguished 

 perhaps from the smaller nerve-cells by their nuclei not being so conspicuous 

 or so relatively large as in a nerve-cell, and by their staining differently. 



The gray matter, then, may be broadly described as a bed of neuroglia, 

 containing a certain number of branching nerve-cells, for the most part 

 though not exclusively large and conspicuous, but chiefly occupied by what 

 is not so much a plexus as an intricate interweaving of nerve-filaments 

 running apparently in all directions. Some of these filaments are fairly con- 

 spicuous naked axis cylinders, and a few are easily recognized medullated 

 fibres of ordinary size ; but by far the greater number are either exceedingly 

 fine medullated fibres, whose medulla is only made evident by special modes 

 of preparation, or delicate fibrils devoid of medulla. With the nervous web 

 formed by these filaments the branching processes of the nerve- cells, on the 

 one hand, and the divisions of nerve-fibres passing into or out of the gray 

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

 the gray matter is well supplied with bloodvessels, these being in it, as stated 

 above, relatively much more numerous than in the white matter. 



478. 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 projecting 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. 119, c. g. .). The attached 

 basis 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. Pos- 

 terior (dorsal) to this central mass lies the posterior gray commissure (Figs. 

 119, 121, 122, p. g. c.) composed chiefly of fine filaments running trans- 

 versely, and anterior (ventral) to it lies first the thinner anterior gray com- 

 missure (Figs. 119, 121, 122 a. g. c.) of a similar nature, and then the rela- 

 tively thick white commissure (Figs. 119, 121, 122, 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 gray 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 gray matter, but by a peculiar tissue, the sub- 

 stantia gelatinosa of Rolando, which forms a sort of cap to the more ordinary 

 gray matter, but differs in size and shape in different regions of the cord. 

 (Cf. Figs. 119, 120, 121, s. g.) In carmine and some other modes of prepa- 

 ration it is frequently stained more deeply than is the ordinary gray matter, 

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



