50 



AN AMERICAN TEXT- BOOK OF PHYSIOLOGY. 



Moreover, by a given rate of change of intensity, the stimulating effect varies 

 with the Btrength of the current employed. Pfliiger in his celebrated mono- 

 graphj Untermchungen uber die Physiologie des Eleldrotonus, published in 

 Berlin, 1859, p. 454, formulated the following rule for the result of excitations 

 under varying conditions: 



Pfliiger's Law of Contraction. 



Ascending Current. 

 Closing. Opening. 



Weak current Contr. Eest. 



Medium " Contr. Contr. 



Strong " Eest. Contr. 



Descending Current. 

 Closing. Opening. 



Contr. Rest. 



Contr. Contr. 



Contr. Rest. 



To understand this so-called " law of contraction " we must bear in mind 

 certain fundamental facts, namely: 



a. When a nerve is subjected to a battery current, an excitatory process is 

 developed in the part of the nerve near the kathode when the current is 

 closed, and in the part of the nerve near the anode when the current is opened 

 (see p. 38). 



b. The excitatory process developed at the kathode is stronger than that 

 developed at the anode (see p. 38). 



c. A third fact which is of no less importance, and which will be considered 

 in detail when we study the effects of the constant current on the irritability 

 and conductivity of nerve and muscle (see p. 95), is the following: During 

 the time that a strong constant current is flowing through a nerve, the conduct- 

 ing power is somewhat lessened in the part to which the kathode is applied, and 

 is greatly decreased, or altogether lost, in the region of the anode ; moreover, 

 at the instant that the current is withdrawn from the nerve the conducting 

 power is suddenly restored in the region of the anode, and greatly lessened, or 

 lost, in the region of the kathode. 



Ascending Current. 

 K A 



Descending Current. 

 A K 



Weak current 



Strong current 



Fig. 25.— Diagram illustrating Pfliiger's law. 



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

 lowing diagram ( Fig. 25), in which a cross is marked at the part of the nerve 



