GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 51 



from which the irritation that is effective in producing a contraction takes 

 its rise. 



In the case of fresh motor nerves of the frog, when the current is weak, 

 only closing excitations, i. c, those originating at the kathode, are effective by 

 both directions of the current. As the strength of the current is increased, at 

 the same time that the closing kathodic contractions grow stronger, opening 

 anodic contractions begin to appear; and with currents of medium strength 

 both closing and opening contractions are obtained with both directions of the 

 current. If the strength of the current be still further increased, a change is 

 observed ; with a strong current, the closing of the ascending and the opening 

 of the descending current tails to excite a 

 muscular contraction. This fact is demon- 

 strated most clearly if we employ two 

 nerve-muscle preparations, and lay the nerves 

 in opposite directions across the non-polar- 

 izable electrodes, so that the current from 

 the battery shall flow through one of the 

 nerves in an ascending direction and through 

 the other in the descending direction (see 

 Fig. 26). If under these conditions a strong 

 battery current be employed, muscle a (through 

 the nerve of which the current is descending) 

 will contract only when the circuit is closed, 

 and muscle b (through the nerve of which the current is ascending) will con- 

 tract only when the circuit is opened. 



Since in the case of currents of medium strength, both opening and clos- 

 ing the circuit, when the current is ascending and when it is descending, 

 develops a condition of excitation in the nerve sufficient to cause contractions, 

 the failure of the contraction by the closing of the strong ascending current, 

 and by the opening of the strong descending current, can scarcely be supposed 

 to be due to a failure of the exciting process to be developed in the nerve ; and 

 it would seem more likely that the nerve-impulse is tor some reason prevented 

 from reaching the muscle — which, as has been said, is the fact, the region of the 

 anode being incapable of conducting during the flow of a strong current, and 

 the region of the kathode losing its power to conduct at the instant such 

 a current is opened. 



Effect of Battery Currents upon Normal Human Nerves. — In experi- 

 ments upon normal human nerves, the current cannot be applied directly to the 

 nerve, but has to be applied to the skin over the nerve. As it passes from the 

 anode, the positive electrode, through the skin, the threads of current spread 

 through the fluids and tissues beneath, somewhat as the bristles of a brush 

 spread out, and the current flows in a more or less diffuse stream toward the 

 point of exit, where the threads of current concentrate again to enter the 

 kathode, the negative electrode. This spread of the current is illustrated in 

 Figure 27. 



\? — \r 



A >—* K 

 Fig. 26.— Effect of direction of current 

 as shown by simultaneous excitation of 

 two nerve-muscle preparations. 



