114 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



nerve impulse affect the intensity of the action current in the same 

 manner. It is beheved generally, therefore, that the electrical 

 change is an invariable accompaniment of the excitatory wave, 

 and the demonstration of an action current in a nerve is tanta- 

 mount to a proof of the passage of a nerve impulse.* 



Direction of Conduction in the Nerve. — The fact that under 

 normal conditions the motor fibers conduct impulses only in one 

 direction — i. e., toward the periphery — and the sensory fibers in 

 the opposite direction — that is, toward the nerve center — suggests, 

 of course, the question as to whether the direction of conduction is 

 conditioned by a fundamental difference in structure in the two 

 kinds of fibers. No such difference in structure has been revealed 

 by the microscope. It is the accepted behef in physiology that 

 any nerve fiber may conduct an impulse in both directions, and 

 does so conduct its impulses when the fiber is stimulated in the 

 middle of its course. An entirely satisfactory proof for this 

 belief is difficult to furnish unless the conclusion in the preceding 



Fig. 52. — Schema to show the arrangement for proving the propagation of the negative 

 charge in both directions: a, The stimulating electrodes; g and g', galvanometers with leading 

 off electrodes arranged to show the negative variation on each side. 



paragraph is admitted — the conclusion, namely, that the electrical 

 change is a necessary and invariable accompaniment of the nerve 

 impulse. It is not difficult to show by means of a galvanometer 

 that when a nerve trunk is stimulated the wave of negativity 

 spreads in both directions from the point stimulated and gives 

 an action current on either side, as indicated in the accompany- 

 ing diagram. This fact holds true for motor or for sensory 

 fibers. The older physiologists attempted to settle this question 

 in a more direct way, but by methods which later experiments 

 have proved to be insufficient. They attempted, for instance, to 

 unite a motor and sensory trunk directly, to cut the hypoglossal 

 (motor) and the lingual (sensory) and suture, say, the central stump 

 of the lingual to the peripheral stump of the hypoglossal. If stimu- 

 lation of this latter trunk, after union had been established, gave 

 signs of sensation it was considered as proof that the efferent hypo- 

 glossal fibers were now conducting afferently. We now know that in 

 such a case the old hypoglossal fibers degenerate completely, and 



*For a more extended discussion see Keith-Lucas, Croonian Lecture, 

 "Proceedings of the Royal Society," B. 85, 1912. 



