126 GENERAL PROPERTIES OF LIVING TISSUES 



tensity is increased, the anodal rise becomes also 

 effective, and contraction is secured by both mak- 

 ing and breaking the current. 



But we have to deal also with a decrease in 

 irritability, and, still more important for the 

 explanation of the effects of strong currents, with 

 a decrease in conductivity. The irritability and 

 conductivity are decreased on closure at the anode 

 and on opening at the cathode. If the anode is 

 next the muscle (Fig. 34), the decrease in con- 

 ductivity on closure of a strong 

 current will block the nerve im- 

 pulse coming from the cathode ; 

 it will therefore never reach the 

 muscle, and there will be no 

 Fig. 34. contraction on closure. If the 

 cathode is next the muscle, the 

 conductivity may be so decreased on opening that 

 the nerve impulse coming from the anode may be 

 blocked. The decrease at cathode, when the cur- 

 rent is broken, is, however, less marked than the 

 decrease at anode when the current is made, so 

 that the cathodal decrease, even with strong 

 currents, sometimes fails to block the impulse 

 entirely. In that case, a weak contraction may 

 be obtained at the break of the descending 

 current. 



