NATURE OF THE NERVE IMPULSE. 121 



change which in a very general way may be compared to the pas- 

 sage of a spark along a line of gunpowder. A fundamental ob- 

 jection to such a view is the uncertainty of the proof regarding the 

 consumption of material in a nerve during activity, as has been ex- 

 plained in the preceding sections. Quite the opposite point of 

 view has also been held, namely, the idea that the nerve impulse 

 is a purely physical process, which involves no chemical change 

 and no using up of materials Various suggestions have been 

 offered as to the character of this physical change, but the one that 

 is perhaps most worthy of consideration identifies the nerve im- 

 pulse with the negative electrical change that is known to pass 

 along the fiber. It is assumed that this electrical change consti- 

 tutes the nerve impulse, and to explain its occurrence and propaga- 

 tion from a physical standpoint it has been supposed that the 

 nerve fiber has a structure essentially similar to the " core conduc- 

 tor " (see p. 108), in that it contains a central thread surrounded by 

 a liquid sheath of less conductive material. The central thread 

 may be supposed to be the axis cylinder and the less conductive 

 sheath the surrounding myelin, or, perhaps, to follow another sug- 

 . gestion that fits the non-medullated as well as the medullated fibers, 

 the central threads are represented by the neurofibrils within the 

 axis cylinder and the surrounding sheath by the perifibrillar 

 substance. That the axis cylinder is a better conductor than 

 the myelin sheath has been indicated by the microchemical 

 researches of Macallum. This observer has shown that in the 

 axis cylinder the chlorids exist in greater concentration than in 

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

 a core model (see Fig. 50), 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 current, 

 serving as a stimulus, is sent into one end of such .an artificial 

 nerve and from the other end two leading off electrodes are 

 connected with a galvanometer, then we can demonstrate by 

 means of the galvanometer that an electrical charge is propagated 

 along the model at each application of the stimulus. And, as 

 such a moving electrical disturbance is the only objective 

 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 

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



