324 PEACTICAL PHYSIOLOGY 



The effect of the constant current upon the conductivity of the nerve 

 is determined upon the same preparation. The stimulating electrodes 

 are placed upon the central part of the nerve ; a minimal stimulus 

 is found, and its effect is recorded upon the stationary drum. The 

 polarising circuit is now closed through the nerve in either the 

 ascending or descending direction, and then the minimal stimulus is 

 again applied. It is no longer effective owing to the decrease in the 

 conductivity of the nerve. This change in the conductivity of nerve 

 is also shown in the third stage of Pfl tiger's Law of Contraction, and 

 in the experiment upon the absence of fatigue in a stimulated nerve 

 (Chapter XVII., p. 330). 



CHAPTER XV. 

 LAW OF EXCITATION. PFLUGER'S LAW OF CONTRACTION. 



ACCORDING to Du Bois Reymond's Law of Excitation the efficiency of 

 an electrical current as a stimulus to a nerve depends upon the rapidity 

 of the change in the strength of the current passing through a given 

 sectional area of the nerve. The changes in the electrical current are 

 most abrupt at the point of entry and the point of exit, the anode and 

 kathode respectively. Excitation starts at these points ; thus at the 

 make of a galvanic current the excitation takes place at the kathode, 

 and at the anode when the circuit is broken. Excitation is produced 

 by a sudden rise in excitability produced by the appearance of katelectro- 

 tonus, or the disappearance of anelectrotonus. In both cases the molecular 

 stability of the nerve is suddenly lessened. 



The effect will vary according to the strength and direction of the 

 galvanic current, for if there be an intervening region of anelectrotonus 

 or of subsiding katelectrotonus, the conductivity of the nerve in this 

 region will be diminished, and the transmission of the excitatory state 

 will be blocked. 



Pfliiger's Law of Contraction, as stated below, can be deduced from 

 the above facts and can be proved by simple experiments. 



[Pfliiger's Law of Contraction. 



