560 THE SPINAL COED. 



posterior roots. If, then, it is to be regarded as a channel of afferent im- 

 pulses passing into it from the posterior roots, those impulses must pass into 

 it along those fibres of the posterior root which find secondary trophic cen- 

 tres in some part of the gray matter ; in this respect this tract resembles the 

 cerebellar tract, and differs from the median posterior tract. The latter 

 is the direct continuation up the cord to the bulb of such fibres as are still 

 trusting for their nutritive activity to the cells of the ganglion on the pos- 

 terior root ; the fibres of both the former trust for their nutritive activity to 

 some part of the gray matter of the cord, and presumably to the nerve- 

 cells of that gray matter. A further resemblance between the antero-lateral 

 ascending and cerebellar tracts must be admitted, if future researches con- 

 firm the opinion of those who hold that the former like the latter, at the top 

 of the cord, pass along the restiform body to the cerebellum. Indeed, under 

 such a view it would appear probable that the antero-lateral tract is simply 

 a more diffuse and outlying part of the cerebellar tract. 



492. We may now briefly pass in review, somewhat as follows, the chief 

 facts which we have learned concerning the structure of the spinal cord, 

 always keeping in view their physiological meaning. 



The important feature of the spinal cord is the presence of what we have 

 called " gray matter," and all our knowledge goes to show 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 gray matter. 



With this gray 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 gray matter close to the attachment of the root, and the rest 

 most probably join the gray 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 pos- 

 terior tract right up the cord to the bulb, without joining the spinal gray 

 matter at all, we may say that the fibres of the posterior root also join the 

 gray 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 ground- 

 work, obscured though this is by fusion and overlaid by the several commis- 

 sural tracts. Each segment of this groundwork we may conceive of as a 

 central mass of gray 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 gray 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 



