620 



AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



the conditions named, an impulse enters the cell-body by way of the ventral 

 root-fibre to which it gives origin, it does not pass out of this cell-body into 

 the other elements of the cord causing an electric change detectable as a nega- 

 tive variation. It appears, therefore, that the connection between the fibres 

 of the cord and the cell-bodies in question is such that though impulses 

 readily pass from the former to the latter, they do not pass in the reverse 

 direction, thus showing that in this instance the cell-boundary sets a limit to 

 the reversed impulse. 1 



With the elements forming the dorsal spinal root, the case is at first glance 

 apparently different, though in reality it is the same. These elements are 



those having the cell-body located in the spinal 

 ganglion. The cells are essentially diueuric 

 (Fig. 148) ; one neuron extends from the 

 point of division toward the periphery, and 

 the other enters the spinal cord to distribute 

 itself as a fibre coursing longitudinally for some 

 distance within it (see Fig. 151). The normal 

 direction of the effective impulses is from the 

 periphery toward the cord, and within the cord 

 they are delivered to other elements which carry 

 them in all directions. It is therefore to be ex- 

 pected that the stimulation of the dorsal root- 

 fibres would give rise to impulses passing in both 

 directions in the dorsal columns of the cord. 

 When, however, the dorsal columns of the cord 

 are electrically stimulated in a cross section made 

 just above the level of the entrance of a dorsal 

 root, then it is found that the electrical varia- 

 tion is to be detected in the nerve-fibres on the 

 distal side of the spinal ganglion. These im- 

 pulses have therefore passed in a direction the 

 reverse of that usually taken. The fibres which 

 are stimulated in this instance in the cross sec- 

 tion of the cord are, however, outgrowths of the 

 spinal ganglion-cells, and thus, although the 

 stimulation of the cord does give rise to an im- 

 pulse in the peripheral nerve, nevertheless the 

 impulse is continually within the limits of one 

 cell-element. The question of whether the re- 

 versed impulse can traverse the cell-body is 

 here answered in the affirmative, for these cells 

 are virtually dineuric, and everything points to the passage of the impulse 

 through the cell-body in passing from one neuron to the other. There is, 

 however, no evidence that the stimulation of the dorsal columns of the cord 

 1 Gotch and Horsley : Proceedings of the Royal Society, 1888. 



FIG. 151. A longitudinal section 

 of the cord to show the branching of 

 incoming root-fibres in dorsal col- 

 umns. At the left are three D R 

 root-fibres, each of which forms two 

 principal branches. These give off 

 at right angles other branches, col- 

 laterals, Col, which terminate in 

 brushes. C C, central cells, whose 

 neurons give off similar collaterals 

 (Ram6n y Cajal). 



