io RESPONSE IN THE LIVING AND NON-LIVING 



sion. As the excitatory state disappears with the 

 return of the excitable tissue to its original condition, 

 the current of action will gradually disappear. 1 The 

 movement of the galvanometer needle during excita- 

 tion of the tissue thus indicates a molecular upset by 

 the stimulus ; and the gradual creeping back of the 

 galvanometer deflection exhibits a molecular recovery. 



This transitory electrical variation constitutes the 

 * response,' and its intensity varies according to that of 

 the stimulus. 



Electric recorder. We have thus a method of 

 obtaining curves of response electrically. After all, 

 it is not essentially very different from the mechanical 

 method. In this case we use a magnetic lever (fig. 4, a), 

 the needle of the galvanometer, which is deflected 

 by the electromagnetic pull of the current, generated 

 under the action of stimulus, just as the mechanical 

 lever was deflected by the mechanical pull of the 

 muscle contracting under stimulus. 



The accompanying diagram (fig. 4, b) shows how, 



' The exciting cause is able to produce a particular molecular rearrange- 

 ment in the nerve ; this constitutes the state of excitation and is accompanied 

 by local electrical changes es an ascertained physical concomitant.' 



' The excitatory state evoked by stimulus manifests itself in nerve fibres 

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

 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.' Text-book of Physiology, ed. by Schafer, 

 ii. 453. 



