184 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



instances in which a single nerve-cell, or part of such a cell, has been the 

 object of direct physiological experiment. We shall therefore approach the 

 question indirectly by showing- what the histological relations have to suggest. 



Direction of the N6rve-irnpulse. — In the case of a given nerve-cell the 

 impulses which we usually consider, pa>> in one direction only. For instance, 

 along the ventral nerve-roots of the spinal cord, the impulses pass from the 

 cord to the periphery, while in the dorsal roots, so far as the fibres take 

 origin from the cells of the spinal ganglia, these impulses travel in the oppo- 

 site direction. At the same time experiment has shown that if a nerve- 

 trunk be stimulated at a given point, then the nerve-impulse can be demon- 

 strated as passing away from the point of stimulation in both directions. 



We are therefore led to inquire what limits are set to the passage of 

 impulses in a direction opposite to the usual one. The narrowest limits, it 

 appears, are those of the single cell in which the impulse has originated. 

 The experimental observations are as follows : When the fibres forming the 

 ventral root of a spinal nerve are stimulated electrically, and the cross- 

 section of the spinal cord, somewhat cephalad to the level at which the root 

 joins it, is explored with an electrometer, there is not found any evidence of 

 nerve-impulses passing cephalad in the substance of the cord. The arrange- 

 ment of the cells in the cord is such, however, that the cell-bodies which give 

 origin to the fibres forming the ventral root are physiologically controlled by 

 fibres running toward them from every portion of the cord, and under normal 

 conditions these fibres convey impulses to the cell-bodies in question. The 

 experiment shows that when an impulse enters the cell-body by way of the 

 ventral root-fibre, to which it gives origin, the impulse* does not stimulate the 

 other elements of the cord. 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 

 have the cell-body located in the spinal ganglion. The cells are essentially 

 diaxonic (Fig. 72); one axone extends from the point of division toward 

 the periphery and the other enters the spinal cord, where it forms two 

 branches, both of which course longitudinally for some distance within it 

 (see Fig. 75). In this case, therefore, 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 there- 

 fore to be expected 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 variation is to be detected in the nerve- 

 fibers on the distal side of the spinal ganglion. These impulses have there- 

 fore passed in a direction the reverse of that usually taken. The fibres 

 which in this instance are stimulated in the cross-section of the cord are, 

 however, outgrowths of tin; spinal ganglion-cells, and thus, although the 

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



