NATURE OF THE NERVE IMPULSE. 115 



than in the surrounding sheath.* The point of importance is that, 

 with a core model (see Fig. 46) consisting of a glass tube with a 

 core of platinum wire and a sheath of solution of sodium chlorid, 

 0.6 per cent., electrical phenomena can be obtained similar to those 

 shown by the stimulated nerve. 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 stimu- 

 lus. It is evident, therefore, that in such a model, as in an ocean 

 cable, an electrical charge maybe 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 experiment among specialists. We are far from having an 

 explanation of the nerve impulse resting upon such an experimental 

 basis as to command general acceptance, f 



Bethe has proposed a 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 internal energy. To account for 

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

 bine, electronegative ions anions are liberated, so that as the wave of 

 affinity progresses it is accompanied by an electronegative condition. J 



Other somewhat similar theories have been proposed which attempt to 

 account for the negative electrical charge (nerve impulse) on the theory 

 of an electrolytic dissociation and an unequal velocity of migration in the 

 resulting ions. Thus Macdonald shows by microchemical reactions that the 

 axis cylinder contains potassium salts in combination with colloidal material. 

 On injury or stimulation the compound is dissociated and the colloid passes 

 possibly into a gel condition. The more rapid diffusion of the cations (K) 

 leaves an excess of anions which accounts for the negative change. The 

 process is reversible, so that an electrically neutral molecule is re-formed 

 after the passage of the impulse. 



* Macallum, "Proceedings of the Royal Society," 1906, B, Ixxvii., 165. 



t 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 Physiologic," vol. ii, part n, 1903; Hering, 

 " Zur Theorie der Nerventhiitigkeit," 1899; Gotch, Schiifer's "Text-book of 

 Physiology," vol. ii, 1900. 



J Bethe, " Allegemeine Ahatomie u. Physiologic des Nervensy stems." p. 

 301, 1903. 



Macdonald, 'Proceedings of the Royal Society," B. Ixxvi., 322. 



