692 COMPARATIVE ELECTRO-PHYSIOLOGY 



attendant expansion, galvanometric positivity, and enhanced 

 power of recovery. At the maximum point that is to say, 

 at the top of the tetanic curve the two forces are balanced ; 

 arid at this point, if the stimulus be suddenly withdrawn, the 

 particular state of unstable balance is often manifested by a 

 brief overshooting in one or other direction. This effect is 

 often noticed in the retina and in certain vegetable structures 

 under the action of light (figs. 244, 247, 254, and 260), and 

 in nerve under electric tetanisation (p. 536). That it is not 

 primarily dependent on assimilation and dissimilation, but 

 on the molecular factor, is seen in the fact that similar effects 

 are also to be observed, under corresponding circumstances, 

 in the response of inorganic substances (figs. 258 and 383). 



The phasic molecular alternation so conspicuously ex- 

 hibited under continuous stimulation may also be seen in the 

 record of responses to successive stimuli. The phenomenon 

 is then regarded as an after-effect and shown by the shifting 

 of the base-line of the record (figs. 208 and 396). 



Since the effect of stimulus is to induce a molecular 

 upset, the change in question must be attended by various 

 concomitant physical changes. It will therefore be possible 

 to record the excitatory effect by recording the attendant 

 variations of any one of these. The effect of stimulus may 

 thus be recorded by (i) the accompanying change of form, 

 in contraction or expansion ; (2) an attendant secretion or 

 absorption ; (3) a variation of electric resistivity by dimi- 

 nution or increase of resistance ; and (4) electro-motive 

 changes of galvanometric negativity or positivity. The 

 changes in the responding substance as a whole, may be 

 recorded by any one of the first three methods. But in the 

 last, or that by the electro-motive variation, the method 

 depends on the relative variations of the electric potential at 

 two different points. For if the substance be isotropic and 

 subjected to diffuse stimulation, the electro-motive change at 

 the two contacts being similar, there will be no resultant 

 effect to record. For the recording of electro-motive response, 

 then, it is necessary to obtain an effect which is differential. 



