THE STRUCTURE OF THE SPINAL CORD. 561 



as the outcome of its own intrinsic changes. Nor have \ve any evidence 

 that the cell can exert any marked transforming power over the impulses 

 passing along the fibre ; the impulses which travel away from the cell do not 

 appear to differ markedly from those which travel toward it. The several 

 instances in which there seemed to be evidence that splanchnic ganglia acted 

 as centres either of reflex or of automatic action, have, as we have seen, 

 broken down ; and it is not even suggested that the ganglia of the posterior 

 roots possess any such powers. The gray matter of the spinal cord, on the 

 other hand, as we have already seen, and as we shall see more in detail, is 

 especially characterized by the possession of reflex and automatic as well as 

 of other powers. 



In structure, moreover, such a spinal segment differs strikingly from a 

 ganglion and exhibits features unknown in ganglia. In a ganglion the 

 nerve-fibres may divide, and in a small peripheral ganglion the division may 

 give rise to very delicate fibrils ; but the fibres or fibrils resulting from the 

 division leave the ganglion to follow their appropriate courses ; the division 

 serves for dispersion only. In the spinal cord, on the other hand, both 

 efferent and afferent fibres divide in such a way that their divisions are lost 

 to view in the gray matter ; division here seems to serve the purpose of union. 

 The efferent fibres of the anterior root may be traced back as a process of a 

 cell in the anterior horn. That cell gives off other processes, but no one of 

 these processes is continued as an axis-cylinder process stretching across the 

 gray matter until it becomes a fibre of the posterior root, or as anything like 

 such an axis-cylinder process. On the contrary, all the processes, except 

 the axis-cylinder process, divide into branches, and appear to end in ner- 

 vous fibrils lost to view in the gray matter. Conversely, though our know- 

 ledge of the junction of the posterior fibres with the gray matter is much more 

 imperfect than that of the junction of the anterior fibres, what we do know 

 leads us to believe that the fibres of the posterior root, either by the media- 

 tion of cells, or by direct division of the axis-cylinder without the media- 

 tion of cells, similarly break up into fibrils and are similarly lost in the 

 gray matter. All the evidence goes to show that the anterior and posterior 

 roots are functionally continuous ; this functional continuity is, however, 

 effected not by a gross continuity of axis-cylinders, but in a peculiar man- 

 ner through the division of branches of nerve-cells or of axis-cylinders into 

 the nervous tangle which forms such a special feature of the gray matter of 

 the cord. We may, perhaps, venture to regard the gray matter of the seg- 

 mental groundwork, of which we are now alone speaking, as constituting a 

 nervous network or web, formed certainly in part by the rapidly dividing 

 branches of nerve-cells, and probably in part by the divisions of directly 

 dividing nerve-fibres. 



In any ordinary section of the spinal cord the gray matter presents to 

 view much more than this nervous groundwork. To say nothing of the in- 

 dubitable neuroglia and the obscure structures, including small cells, which 

 are claimed now to be neuroglia, now to be nervous in nature, the gray 

 matter in every section shows numerous distinct nerve-fibres crossing it in 

 various directions ; of these fibres a few are ordinary medullated fibres, some 

 are non-medullated fibres, that is to say, are naked axis-cylinders, while 

 others, and these the more numerous, are the peculiar medullated fibres of 

 small diameter spoken of in 476. A large number of these fibres, indeed 

 all the larger ones, though they go to make up what we call gray matter, 

 are not continuous with, and do not belong to, the groundwork or nervous 

 web, at all events do not form part of the groundwork seen in the same 

 section as themselves. They are simply fibres traversing the groundwork, in 

 spaces of the neuroglia bed, on their way up or down the cord, or across the 



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