GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 61 



The twelve cases included in the above table can be represented in the fol- 

 lowing diagram (Fig. 23), in which a cross is marked at the part of the nerve 



Ascending Current. 

 K A 



Descending Current. 

 A K 



Weak current. 



Medium current. 



Strong current 



FIG. 23. Diagram illustrating Pflviger's law. 



from which the irritation which 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 contractions, i. e. those originating at the kathode, are obtained 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 fails 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 

 the battery shall flow through one of 

 nerves in an ascending direction and through 

 the other in the descending direction (see 

 Fig. 24). 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, 



A * K 



FIG. 24. Effect of direction of current 

 as shown by simultaneous excitation of 

 two nerve-muscle preparations. 



