896 THE NATURE OF THE GREY MATTER. [BOOK HI. 



The important feature of the spinal cord is the presence of 

 what we have called ' grey matter, ' and all our knowledge goes to 

 shew that the important powers of the spinal cord, by which it 

 differs from a thick multiple nerve, and by virtue of which we 

 speak of it as a nervous centre or series of centres, are in some way 

 or other associated with this grey matter. 



With this grey matter the fibres of the spinal nerves are 

 connected. The greater part of the fibres of the anterior root 

 certainly end in or rather take origin from the grey matter close 

 to the attachment of the root, and the rest most probably join 

 the grey matter at no great distance. The fibres of the posterior 

 root run, as we have seen, for some little distance in the white 

 matter, but, if we except the special bundle which runs in the 

 median posterior tract right up the cord to the bulb without 

 joining the spinal grey matter at all, we may say that the fibres 

 of the posterior root also join the grey matter not far from the 

 attachment of the root. 



Morphological reasons lead us, as we have seen, to regard the 

 spinal cord as a series of segments, each segment corresponding 

 to a pair of nerves ; and even in the spinal cord of man we may 

 recognize a segmental groundwork, obscured though this is by 

 fusion and overlaid by the several commissural tracts. Each 

 segment of this groundwork we may conceive of as a central mass 

 of grey matter, connected on each side with an anterior and a 

 posterior root, thus constituting a segmental nervous mechanism 

 capable of carrying out certain functions. 



Such a segment has been compared to a ganglion, but it differs 

 strikingly from a ganglion, whether of the posterior root or of the 

 splanchnic system, both in structure and in function. A ganglion 

 and the grey matter of a spinal segment both contain nerve-cells, 

 and so far resemble each other; but there the resemblance for 

 the most part ends. In a ganglion the constituent nerve-cell is a 

 development of the axis-cylinder of a fibre into a nucleated cell- 

 body which lies on the course of the fibre, and may, as in a 

 splanchnic ganglion, be placed just where one fibre divides into 

 two or more. We have clear evidence that the cell, that is to say, 

 the nucleus with the adjacent cell-substance, exercises an important 

 influence on the nutrition, and so on the functional activity of the 

 nerve-fibre ; it acts as we have seen as a ' trophic centre.' There 

 are also reasons for thinking that the cell-substance is more 

 sensitive, more readily responsive to changes in its circumstances 

 than is the axis-cylinder at some distance from the cell. But we 

 have no satisfactory evidence that the cell can automatically 

 originate nervous impulses in itself, as the outcome of its own 

 intrinsic changes. Nor have we 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 towards it. 



