243 



PHYSIOLOGY 



CHAP. 



unaltered. The reversed polar changes on opening the circuit 

 also affect the rate of conductivity ; in the region in which 

 excitability is increased conductivity is also accelerated. 



All these data in regard to the polar effects of the constant 

 current are founded on experiments specially made on frogs' nerves. 

 Many workers since Helmholtz have attempted to reproduce the 

 same electrotonic phenomena upon man, but the results have been 

 variable and uncertain. Waller and De Watteville alone succeeded 

 in showing that electrotonus follows the same laws in man as in 

 other animals, the only difference being that the polar changes are 

 less marked with different modes of sending in the current. 



VIII. In speaking of the polar changes of excitability and 

 conductivity in nerve during the passage of a constant current 

 we have confined ourselves to the excitatory influence of this 

 current upon the nerve at make and break, i.e. when its action 

 upon the nerve begins and ceases. These excitatory effects are 

 expressed in the muscular contractions that occur at these two 

 moments. According to the strength of the polarising current, 

 and its ascending or descending direction in the nerve, it is 

 possible to obtain break as well as make contractions, or break or 

 make contractions only. The regular order in which these signs 

 of nervous excitation occur, and the explanation of their occurrence 

 by the laws of electrotonus, constitute what is known as " Pfliiger's 

 law of contractions," as in the following table : 



These experimental data, which together constitute the Law of 

 contractions, are expressed in the diagram of Fig. 160. 



The results obtained with weak and moderate currents are 

 readily interpreted if we assume with Pfliiger that they depend on 

 rise of excitability in the nerve at the kathode (katelectrotouus), 

 which takes place so abruptly on closure of the circuit that it 

 causes excitation, no matter what the direction of the current may 

 be. The anodal rise of excitability which occurs on opening the 

 circuit, owing to the disappearance of anelectrotonus, is less 

 effective than the kathodal rise at closure. This explains why the 



