60 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



tion, illustrated by/, though not affecting nerves, might suffice for striated 

 muscles and be favorable to the excitation of non-striated muscles. 



In the case of nerves, duration of current is less important than a rapid 

 change of intensity. In the case of striated muscles the advantage to be 

 gained by rapid variations can be easily overstepped, and the importance of 

 the duration of the current is greater ; while in the case of non-striated muscles 

 duration of current is of the first importance and rapid variation may fail to 

 excite. In the case of all tissues, strength and density of current, what we 

 may call intensity of current, is favorable to excitation. 



(/) Effect of the Direction in which the Current flows along the Nerve. 

 The result of the irritating change produced in a nerve by a battery current 

 has been found to depend upon whether the current flows toward or away 

 from the organ stimulated by the nerve. This fact can be most readily ob- 

 served in the case of isolated motor nerves. In the case of these nerves, the 

 effects produced by opening and closing the current are different according as 

 the current is descending, i. e. flows through the nerve in the direction of the 

 muscle, or ascending, *. e. flows through the nerve in the opposite direction. 

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

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

 graph, Untersuchungen uber die Physiologie des Elektrotonus, published in 

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

 under varying conditions : 



Pfluger's Law of Contraction. 



Ascending Current. Descending Current. 



Closing. Opening. Closing. Opening. 



Weak current Contr. Rest. Contr. Rest. 



Medium " Contr. Contr. Contr. Contr. 



Strong " Rest. 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. 53). 



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

 developed at the anode (see p. 53). 



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. 



