206 PHYSIOLOGY CHAP. 



From these facts we derive the important conclusion that 

 the internal excitatory process, or active state of the nerve, 

 is transmitted at a rate that is, comparatively speaking, so low 

 that it must undoubtedly consist in a physico-chemical change of 

 the living substance of the axis cylinder, propagated by contiguity 

 from one part to the next. The conduction of excitation in the 

 nerve is analogous to the transmission of excitation in the muscle, 

 although it occurs much more rapidly. We may assume with 

 Pfliiger that potential energy is liberated during activity in nerve 

 as in muscle, this chemical process being propagated from segment 

 to segment till it reaches the muscle, where it excites the 

 mechanical process of contraction just as the spark of a match 

 produces an explosion when it reaches the powder in a mine. 



As in muscle so in nerve, it can be proved that excitation 

 is a diphasic cyclic process, whatever concept be formed of the 

 hitherto unknown chemical changes aroused by . the stimulus. 

 Just as in muscle the phase of relaxation follows the phase of 

 contraction, and the whole cycle of muscular excitation results 

 from these two factors, so in nerve the active state results, as can 

 be demonstrated, from a physico-chemical, presumably katabolic, 

 change, followed after a brief interval by the opposite (anabolic) 

 change, which represents the return of the protoplasm of the 

 nerve to the molecular equilibrium proper to the resting state. 

 Our physiological analysis of the phenomena of excitation will 

 yield constant confirmation of this law. 



IV. We have seen that the excitation or active state of a 

 muscle is expressed in three orders of effects; in mechanical, 

 chemical, and electrical phenomena. The active state of a 

 nerve induced by various stimuli is, on the contrary, so far as we 

 know, expressed solely by alteration of its electrical potential. 



The chemical composition of the axis-cylinder (the only really 

 and specifically active part of a nerve) is totally unknown to us. 

 Under the microscope it gives the xanthroproteic reaction and 

 other indications of a protoplasmic character. From this single 

 fact we may conclude, with Foster, that there is a generic 

 analogy between the chemical composition of the active sub- 

 stance of muscle and that of nerve, and conjecture that the 

 transmission of excitation along the nerve- fibre is accompanied 

 by chemical changes similar to those which take place in the 

 muscle fibre. It is, however, certain that the nutritive exchanges 

 and metabolic phenomena which are theoretically probable in 

 nerve must be extremely small, since it has so far been impossible 

 to obtain any direct demonstration of them. 



A. D. Waller, starting from the observation (which we shall 

 discuss below) that there is a relation between the functional 

 capacity of the nerve and the variations produced experimentally 

 in the C0 2 content of the surrounding atmosphere, concludes that 



