METABOLISM DURING NERVE ACTIVITY. 453 



fibre to another is associated with very special characteristics, which 

 will be referred to later in considering the excitatory changes present 

 in nerve-endings. It is mentioned here, in order to guard against the 

 inference that the functional isolation now under consideration is an 

 absolute one ; it is evident that it is only relative isolation ; wherever 

 the axis cylinder path exists, the propagation is restricted to it, when 

 it ceases a passage can be forced through other routes. 



Phenomena of the excitatory state. — The excitatory state evoked 

 by a stimulus manifests itself in nerve fibre by electromotive 

 changes, and, as far as our present knowledge goes, by these only. 

 This state may spread to successive portions of the nerve beyond 

 the region subjected to the action of the stimulus, the spread being- 

 accompanied by similar electromotive changes. All theories as to the 

 nature of the hidden events which take place in the living tissue, and 

 constitute this excitatory state, must embrace these demonstrable 

 facts. 



It is further demonstrable that any alteration in the nerve, evoked 

 by the operation of an external agent, is succeeded on the removal of 

 the producing cause by a reversed state. Just as, in a muscle, relaxation 

 succeeds contraction, and the whole cycle constitutes the response, so, 

 in a nerve, the excitatory state is in fact, as in conception, composed of 

 changes in one direction, succeeded after a very brief interval by others 

 of opposite character. 



The conception of such an excitable living tissue as nerve, implies 

 that of a molecular state which is in stable equilibrium ; this equilibrium 

 can be readily upset by an external agency, the stimulus, but the term 

 "stable" expresses the fact that a change in any direction must be 

 succeeded by one of opposite character, this being the return of the 

 living structure to its previous state. Thus the electrical manifestation 

 of the excitatory state is one whose duration depends upon the time 

 during which the external agent is able to upset and retain in a new 

 poise the living equilibrium, and if this is extremely brief, then the 

 recoil of the tissue causes such manifestation to be itself of very short 

 duration. Whether our conception of the actual physico-chemical 

 conditions of this living equilibrium, whose disturbance is excitation, 

 be mechanical (du Bois-Reymond, Pfluger), chemical (Hering, Bieder- 

 mann), electrochemical (Bernstein, Hermann, Boruttau), the above 

 statement is equally true. No departure from the normal in conse- 

 quence of excitation takes place without being succeeded by the opposite 

 condition. Throughout the whole of the phenomena of nerve this 

 consideration must be always kept in view. Such well-known experi- 

 mental investigations as those of the polar changes in excitability are 

 framed for its direct demonstration, and there is little likelihood in these 

 of its being forgotten, but in others, in which its determination is not 

 the direct object of inquiry, it is apt to be passed over. It is for this 

 reason that this fundamental conception is introduced at this early 

 stage in the consideration of nerve phenomena. 



Metabolism during nerve activity. — During the activity of a 

 muscle, changes of four different kinds are revealed — mechanical, 

 electrical, chemical, and thermal. In nerve the electrical changes are 

 readily demonstrable, but the chemical ones, upon which these are 

 presumably based, are extremely slight, whilst any thermal effects, in 

 consequence of such chemical processes, have not hitherto been ascer- 



