CHAP, i.] THE SPINAL CORD. 897 



The several instances in which there seemed to be evidence that 

 ^planchnic 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 grey 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 grey matter ; division here seems 

 to serve the purpose of union. The efferent fibre 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 on as an axis-cylinder process stretching across the 

 grey 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 nervous fibrils lost to view in the grey 

 matter. Conversely, though our knowledge of the junction of the 

 posterior fibres with the grey 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 

 mediation of cells, or by direct division of the axis-cylinder 

 without the mediation of cells, similarly break up into fibrils and 

 are similarly lost in the grey matter. All the evidence goes 

 to shew 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 manner 

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

 into the nervous tangle which forms such a special feature of the 

 grey matter of the cord. We may perhaps venture to regard the 

 grey matter of the segmental 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 grey matter 

 presents to view much more than this nervous groundwork. 

 To say nothing of the indubitable neuroglia and the obscure 

 structures, including small cells, which are claimed now to be 

 neuroglia, now to be nervous in nature, the grey matter in every 



572 



