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AN AMERICAN TEXT-BOOK OE PHYSIOLOGY. 



terminate somewhere between this extreme limit and their point of en- 

 trance. 



Since the fibres 1 in the dorsal funiculi of the cord degenerate on destruc- 

 tion of the dorsal roots, it is inferred that they must be morphologically con- 

 tinuous with certain fibres in the roots, and, since the dorsal roots are afferent 

 pathways, they too musl form pari of the afferent pathway in the cord. 



The continuation of the other paths for the afferent impulses must, how- 

 ever, be formed by the axones of the central cells with which the dorsal 

 root-fibres conned a- they terminate at the several levels of the cord. 



Fig. '.'7.— The sections are from five levels of the spinal cord of a dog. The dorsal roots on one side 

 had heen sectioned in two groups: first, the twenty-eighth, twenty-seventh, and twenty-sixth spinal 

 nerves; and second, the twenty-second and twentieth: a, shows a schematic picture, representing a 

 cross-section of the spinal cord taken just below the level of the twenty-second spinal root. The black 

 spot represents the principal bundle of degenerated fibres as it appears in the dorsal column. At this 

 level the bundle is rather near the median septum, but if sections further cam lail were examined in series 

 it would be found thai the bundle constantly approached the dorsal horn, and finally fused with it at the 

 level where the injured nerves joined the cord. If, on the other hand, a section be taken from the level 

 between the t\\ enty-second and the twentieth nerves— that is, after passing the level at which the second 

 group of sectioned nerves joins the cord — there arc to be seen two bundles of degenerated tihres marked 

 by black spots in the sections ''*, C, d). The last bundle to enter the cord, and the one lying nearer the 

 dorsal horn, is, of course, formed by the degenerating fibres from the second group of roots. In the sec- 

 tions c, d, e, taken respectively at the level of the eighteenth nerve, the middle of the thoracic cord and 

 the cervical enlargement, it is seen that both degenerated bundles grow smaller: that they shift toward 

 the median Beptum and approach one another; and. finally, that they completely fuse in the cervical 

 region 



Degeneration after Hemisection of Cord. — Upon hemisection of the 

 cord involving one lateral half, the ascending fibres which degenerate appear 

 in the dorsal columns, in the dorso-lateral ascending tract, and in the ventro- 

 lateral ascending tract. The number of degenerated fibres is large on the 

 side of the lesion, hut on the opposite side there are also degenerated fibres in 

 all these localities, although tiny are by no means s<> numerous. It is inferred 

 that all the fibres which thus degenerate form path.- for the afferent impulses. 



The impulses which come in over a dorsal root on one side can, therefore, 

 find their way cephalad either by the direci continuation- of the dorsal root- 

 fibres running in the dorsal column mainly on the side of the lesion or 

 through the interpolation of central cells, the axones of which appear degen- 



1 The bundles of "endogenous fibres" not arising from spinal ganglion-cells are neglected 

 here. 



