iv GENEEAL PHYSIOLOGY OF NEEYOtfS SYSTEM 249 



break of weak currents produces no excitation, and why at break 

 the contraction is relatively less marked with moderate currents 

 than it is at make. 



The excitation caused by stimulating with strong or very 

 strong currents requires a more elaborate explanation. In this 

 case, also, the nerve is excited at the kathode at make and at the 

 anode at break of the circuit. The excitation, moreover, increases 

 in proportion to the strength of the current. But for a motor 



Ascending currents Descending currents 



t I 



Weak r M B^ 9 ^ Weak 



Medium V I~' : '^V ' ^ S ^ Medium 



Strong f I SB BBS SMS , ] ^Strong 



strong V,;,,..v ' - strong 



FIG. 160. Pfluger's Law of Contractions. 



nerve, the kathodal excitation at make with an ascending 

 direction of current must, in order to reach the muscle, pass 

 through the anodal region, in which as we have seen con- 

 ductivity is greatly depressed. This explains why in such a 

 case the make contraction is either very feeble with strong 

 currents, or fails altogether with very strong currents. So, too, 

 the anodal excitation at break, in order to reach the muscle, 

 must with a descending current traverse the kathodal region, in 

 which (owing to the disappearance of katelectrotonus) conductivity 

 is much diminished. This explains the weak contraction that 

 appears at break of strong descending currents, and which may 

 fail altogether when very strong currents are employed. 



