108 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



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, although in two respects at least it will be re- 

 membered that the sensory nerve fibers react differently from the 

 motor fibers namely, in the fact that they are readily stimulated 

 by high temperatures and that during the passage of a galvanic 

 current of constant strength they are stimulated continuously in- 

 stead of only at the opening or closing of the current. These latter 

 differences, however, may rest simply upon a difference in irrita- 

 bility and have no bearing upon the question in hand. It is the 

 accepted belief 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 para- 

 graph is admitted, 

 the conclusion, 

 namely, that the 

 electrical change is 

 a necessary and in- 

 variable accompani- 



Fig. 48. Schema to show the arrangement for proving ment of the nerve 

 the propagation of the negative charge in both directions: i -r, x 



a. The stimulating electrodes; g and g f , galvanometers impulse, 



with leading off electrodes arranged to show the negative Hiffirmlf tn Vinw Vv 



variation on each side. OimCUlt IO snow Dy 



means of a galva-: 



nometer that when a nerve trunk is stimulated the negative 

 charge spreads in both directions from the point stimulated and 

 gives an active current on either side, as indicated in the accom- 

 panying 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 afferent ly . We now know that in 

 such a case the old hypoglossal fibers degenerate completely, and 

 the new ones that are eventually formed in their place are out- 

 growths from the lingual stump, or at least are not the old efferent 

 fibers, and hence experiments of this kind are not so conclusive' 



