NATURE OF THE NERVE IMPULSE. 115 



If an induction shock is sent into such a model at one end and 

 two leading off electrodes are connected at another point, an 

 action current may be detected for each stimulus. It is 

 evident, therefore, that in such a model, as in an ocean cable, an 

 electrical charge may be transmitted in a wave-like form when a 

 current is applied at one end. And, as such a moving electrical 

 disturbance is the only objective phenomenon known to occur in 

 the stimulated nerve, it is assumed that it constitutes the nerve 

 impulse. When this electrical disturbance reaches the end- 

 organ, the muscle, for instance, it initiates the chemical changes 

 that characterize the activity of the organ. This kind of theory 

 makes the nerve impulse an electrical phenomenon, and assumes 

 that the nerve fibers have become differentiated to form a specific 

 kind of conductor, the efficiency of which depends upon its having 

 a structure similar to that of a "core conductor." It should be 

 added that this and, indeed, all specific theories of the nature of 

 the nerve impulse are, at present, matters for discussion and experi- 

 ment among specialists. The subject is referred to here solely to 

 indicate the trend of modern discussion. We are far from having 

 an explanation of the nerve impulse resting upon such an experi- 

 mental basis as to command general acceptance.* 



Bethe has proposed a new theory of the production and conduction of 

 the nerve impulse which varies somewhat from the types given above. It 

 is founded upon an observed histological fact already referred to (p. 102) The 

 nerve impulse is defined in his hypothesis as a wave of chemical affinity be- 

 tween the fibrils and fibril acid which, starting at the point stimulated, is 

 transmitted along the nerve. There is thus conceived a kind of chemical 

 reaction which involves no liberation of combined energy. To account for 

 the electrical changes, it is assumed that, when the fibril and fibril acid 

 combine, electronegative ions anions are liberated, so that as the wave 

 of affinity progresses it is accompanied by an electronegative condition. f 



Qualitative Differences in Nerve Impulses and Doctrine of Spe- 

 cific Nerve Energies. Whether or not the nerve impulses in vari- 

 ous nerve fibers differ in kind is a question of great interest in physi- 

 ology. The usually accepted view is that they are identical in 

 character in all fibers and vary only in intensity. According to 

 this view, a sensory nerve the auditory nerve, for instance car- 

 ries impulses similar in character to those passing along a motor 

 nerve, and the reason that in one case we get a sensation of hearirg 

 and in the other a contraction of a muscle is found in the manner 



* For a summary of the literature upon the nature of the nerve impulse 

 consult Boruttau, " Zeit. f. allg. Physiologic," 1, 1, Sammelreferate, 1902; 

 Biedermann, " Ergebnisse der Physiologie," vol. ii, part ti, 1903; Hering, 

 "Zur Theorie der Nerventhatigkeit," 1899; Gotch, Schafer's "Text-book 

 of Physiology," vol. ii, 1900. 



t Bethe, " Allgemeine Anatomie u. Physiologie des Nervensystems," p. 

 301, 1903. 



