424 THE FUNCTIONS OF CROSS-STRIATED MUSCLES 



It will be understood that the terms weak, medium, and strong as applied 

 to the current in this discussion do not designate any absolute values of the 

 current, since what is medium current for one nerve-muscle preparation may 

 be strong for another. The terms are purely relative to any given preparation. 



This peculiar behavior of a nerve-muscle preparation to currents of dif- 

 ferent strength, which finds expression in the law of contraction, depends 

 upon another law enunciated by Pfliiger, namely, that a constant current has 



FIG. 161. Catelectrotonus. The tracing is to be read from right to left. The nerve was first 

 stimulated in the neighborhood of the cathode of the polarizing current with stimuli too weak 

 to produce any effect while the polarizing current was not running. The polarizing current 

 was then turned on, and, without changing the strength of the stimuli, they became effective. 

 When the polarizing current was again turned off, the stimuli were again subminimal. 



no stimulating action on the nerve between the poles, but acts only at the 

 poles. On closing the current the stimulus starts from the cathode, on opening 

 from the anode. 



This polar law of excitation may be illustrated by the following experimental 

 facts. If in stimulating with the constant current the electrodes be applied to 

 the nerve as far apart as possible, and the latent period of the closing contrac- 

 tions be determined both for the ascending and descending currents, we find 

 this period to be longer for the former than for the latter (v. Bezold). With 

 the descending current the cathode is nearer the muscle than with the ascend- 

 ing current; hence the stimulus has a shorter distance to travel to reach the 

 muscle with the former than with the latter. In a similar way it can be shown 

 that the stimulus starts from the anode when the current is broken. 



The law of contraction may be illustrated by carrying the experiment far- 

 ther. Thus if from the determinations of the latent period just mentioned 

 the rate of transmission of the stimulus be calculated, it will be found consider- 

 ably lower than when it is determined on the same nerve, by the method (de- 

 scribed on page 417) of stimulating at two points with the induction cur- 

 rent. The reason is that with the ascending current the stimulus on its way 

 to the muscle has experienced some resistance at the anode. This resistance 

 varies considerably in amount according to the strength of the stimulating cur- 

 rent. When the current is weak or of medium strength the stimulus at the 

 cathode on closing the current is strong enough to overcome the resistance at 

 the anode. But when the current is strong the resistance at the anode is too 

 great to be overcome by the stimulus at the cathode, and it constitutes therefore 

 a complete block. It can be shown also that when the cathode intervenes between 

 the anode and the muscle, it creates a resistance to the anodic stimulus. 



The polar law of excitation was deduced by Pfliiger mainly on the ground 

 of the alterations in excitability produced in the nerve by a constant current. 



