148 



THE PHYSIOLOGY OF NERVE 



bility at the cathode is then much diminished. Strong currents, 

 therefore, cause a depression at both poles but the cathodic depression is 

 always less than that developed at the anode. Werigo^ expresses this 

 fact by saying that the cathodic depression is initiated by a brief 

 period of excitation. It is to be remembered, however, that Fig. 87 

 represents the conditions prevailing during the passage of the constant 

 current, when the term anelectrotonus is synonymous with depres- 

 sion and the term catelectrotonus with excitation, and does not portray 

 the conditions existing subsequent to the breaking of the current. 

 The post-anelectrotonic and post-catelectrotonic effects are the reverse 

 of those just described, i.e., while strong currents cause a depression 

 at both poles, the cathodic region is now more highly depressed. 

 At this time, the stimulus is derived from the anodic excitation still 

 remaining. 



These electrotonic differences are responsible for the occurrence 

 of the phenomenon known as ''secondary tetanus of nerve." If a 

 long piece of the sciatic nerve of a frog {A) is placed beside the nerve 

 of a nerve-muscle preparation {B), as is indicated in Fig. 81, the 

 excitation of the central end of nerve {A) with a constant current 

 invariably results in a contraction of the muscle. By making and 

 breaking the current more rapidly, the muscle may be thrown into 

 a complete state of tetanus. In this case, it is the electrotonic current 

 in nerve (A) which produces the nerve impulse in {B) and the subsequent 

 muscular reaction. It will be remembered from the previous discus- 

 sion that this result may also be obtained with the aid of an ordinary 

 action current. 



It has been found by Pflliger that the making and breaking of a 

 weak galvanic current gives rise to a contraction only on the make. In 

 this case, it is immaterial whether the anode be situated near to or far 

 away from the muscle, i.e., the results are the same whether the current 

 be ascending or descending. With a medium current a contraction 

 is produced on the make as well as on the break, and this holds true 

 for the ascending as well as for the descending current. With a 

 strong current, the results are more complex, because the ascending 

 current gives a contraction only on the break, and the descending 

 current only on the make. These effects have been formulated into 

 what is known as Pfliiger's Law of Contraction which may be summar- 

 ized as follows: 



Werigo, Pfliiger's Archiv, Ixxxiv, 1901, 547. 



