INFLUENCE OF ELECTRICAL CURRENTS. 



495 



current, is allowed to traverse the nerve by the same electrodes as 

 those employed for the galvanic current, and is so directed as always 

 to excite at the pole nearest the muscle. 



The changes in an afferent nerve are demonstrable by similar 

 experiments on the brainless frog, the more central pole and its neigh- 

 bourhood being the seat of the uniform exciting stimulus, and the reflex 

 response the index of the extent of the excitation thus produced. 



The investigation of the changes in the intrapolar region is more 

 difficult, since the employment of electrical modes of stimulation is 

 impracticable, owing to the formation, at the exciting contacts, of a 

 subsidiary intrapolar anode and cathode. By the use of mechanical or 

 chemical modes of excitation, the intrapolar anodic and cathodic changes 

 can be ascertained, and the position of the indifferent point determined. 1 



The degree of anelectrotonic fall and catelectrotonic rise of excita- 

 bility during closure, together with the extent of the spread of the two 

 states, varies with the intensity and duration of the polarising current, 

 and with the length of the nerve traversed by the current (i.e. the 

 distance between the poles). 



The influence of varying current intensity is shown diagrammatical ly in 

 the left-hand portion of Fig. 259 ; the curves below the base line indicate anodic 



Fig. 259. 



fall, those above cathodic rise, of excitability, with I. weak, and 77. strong 

 currents respectively. It will be observed that the anodic fall tends to increase 

 more than the cathodic rise, so that the indifferent point gradually gets closer 

 to the cathode. The right- 



hand portion of the diagram A B ex 



(Fig. 259) shows the influence 

 of varying duration ; here 

 again the anodic change tends 

 to spread at the expense of 

 the cathodic. With currents 

 of considerable intensity and 

 prolonged duration, as in III., 

 the cathodic change may be 

 completely swamped by the Fig. 260.— The closure of K, allows a descending 

 spread of the anodic into both current to traverse the length of nerve from A to 



extrapolar regions. The influ- B ^ °P enin g K x and closing I the current 



ence of increasing the length 

 of traversed nerve is easily 

 demonstrated by passing the current through a long length of nerve, AC 

 (Fig. 260), and testing the extrapolar excitability by induced currents at X. 

 This is compared with that produced when the same current traverses two 

 1 Tigerstedt, Mitth. v. physiol. Lab. d. Carotin, med.-chir. Insf. in Stockholm, 1882, Bd. i. 



C. 



traverses the two lengths of nerve (A'B' and BC). 



The length AC = A / B' + BC. 



