122 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



conductor, the efficiency of which depends upon its having a 

 structure similar to that of a " core conductor." Other theories 

 of a physico-chemical character have been proposed especially 

 to explain the initial excitation caused by a stimulus and the 

 electrical phenomena responsible for the action current. Nernst 

 has supposed that the electrolytes contained in the axis cylinder 

 lie within membranous partitions which are impermeable to the 

 passage of certain ions. When an electrical current is passed 

 through a nerve, it is conveyed of course by the dissociated elec- 

 trolytes, and in consequence of the impermeable character of the 

 septa, there will be a concentration of positively charged ions at 

 one face of the membranes and of negatively charged ions at the 

 other. When the concentration of the ions reaches a certain point, 

 excitation occurs. The nature of the excitation under such 

 circumstances has been further imagined by Hill, who suggests 

 that some sensitive substance, presumably a colloid, exists in the 

 nerve in combination with certain ions. This combination is in 

 an unstable or critical state, and when, in consequence of a stimulus 

 of any kind, the concentration of ions in combination with it is 

 increased, it breaks down and this act constitutes the excitation, 

 which is then propagated along the nerve. This author has 

 treated his assumption mathematically to ascertain how far it 

 accords with the known facts of the stimulation of nerves with 

 electrical currents. It should be added that these 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.* 



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 hearing 

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

 of ending of the nerve, one terminating in a special part of the cortex 



40, 190, 1910; Lucas, ibid., p. 224; and Croonian Lecture, "Proceedings Royal 

 Society," B. 85, 582, 1912; Tashiro, "Proceedings of the National Academy of 

 Sciences," 1, 110, 1915. 



