NATURE OF THE NERVE IMPULSE. 121 



phenomenon known to occur in the stimulated nerve, it has been 

 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." 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- 



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



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. 



