STIMULUS BY ELECTRIC CURRENT. 105 



p. p', the " currents of rest " obtainable from the various points of the nerve will 

 be 'different during the passage of the polarizing current from those which were 

 manifest before or after the current was applied ; and, moreover, the changes in the 

 nerve-currents produced by the polarizing current will not be the same in the 

 neighborhood of the anode (p) as those in the neighborhood of the kathode (p / ). 

 Thus, let G and H be two galvanometers so connected with the two ends of the 

 nerve as to afford good and clear evidence of the ''currents of rest." Before the 

 polarizing current is thrown into the nerve, the needle of H will occupy a posi- 

 tion indicating the passage of a current of a certain intensity from h to h' 

 though the galvanometer (from the positive longitudinal surface to the negative 

 cut end of the nerve), the circuit being completed by a current in the nerve from 

 A x to /i, i. e., the current will flow in the direction of the arrow. Similarly the 

 needle of G will, by its deflection, indicate the existence of a current flowing from 

 g to //through the galvanometer, and from g / to g through the nerve, in the 

 direction of the arrow. 



At the instant that the polarizing current is thrown into the nerve at p p', the 

 currents at gg', hh' will undergo a "negative variation," that is, the nerve at 

 each point will exhibit a "current of action" corresponding to the nervous im- 

 pulse, which, at the making of the polarizing current, passes in both directions 

 along the nerve, and may cause a contraction in the attached muscle. The cur- 

 rent of action is, as we have seen, of extremely short duration, it is over and 

 gone in a small fraction of a second. It, therefore, must not be confounded with 

 a permanent effect which, in the case we are dealing with, is observed in both 

 galvanometers. This effect, which is dependent on the direction of the polariz- 

 ing current, is as follows : Supposing that the polarizing current is flowing in the 

 direction of the arrow in the figure, that is, passes in the nerve from the positive 

 electrode or anode p to the negative electrode or kathode p', it is found that the 

 current through the galvanometer G is increased, while that through His dimin- 

 ished. The polarizing current has caused the appearance, in the nerve outside 

 the electrodes, of a current having the same direction as itself, called the "elec- 

 trotonic" current; and this electrotonic current adds to, or takes away from, the 

 natural nerve-current or "current of rest" according as it is flowing in the same 

 direction as that or in an opposite direction. 



The strength of the electrotonic current is dependent on the strength of the 

 polarizing current, and on the length of the intrapolar region which is exposed 

 to the polarizing current. When a strong polarizing current is used, the electro- 

 motive force of the electrotonic current may be much greater than that of the 

 natural nerve-current. 



The strength of the electrotonic current varies with the irritability, or vital 

 condition of the nerve, being- greater with the more irritable nerve ; and a dead 

 nerve will not manifest electrotonic currents. Moreover, the propagation of the 

 current is stopped by a ligature, or by crushing the nerve. 



We may speak of the conditions which give rise to this electrotonic current as 

 a physical electrotonus analogous to that physiological electrotonus which is made 

 known by variations in irritability. The physical electrotonic current is probably 

 due to the escape of the polarizing current along the nerve under the peculiar 

 conditions of the living nerve ; but we must not attempt to enter here into this 

 difficult subject or into the allied question as to the exact connection between the 

 physical and the physiological electrotonus, though there can be little doubt that 

 the latter is dependent on the former. 



75. These variations of irritability at the kathode and anode respect- 

 ively, thus brought about by the action of the constant current, are inter- 

 esting theoretically, because we may trace a connection between them and 

 the nervous impulse which is the result of the making or breaking of a 

 constant current. 



For we have evidence that a nervous impulse is generated when a portion 

 of the nerve passes suddenly from a normal condition to a state of katelec- 

 trotonus or from a state of anelectrotonus back to a normal condition, but 

 that the passage from a normal condition to anelectrotonus or from katelec- 

 trotonus back to a normal condition is unable to generate an impulse. 

 Hence when a constant current is " made " the impulse is generated only at 



