68 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



have shown that even this if long continued first increases and later decreases 

 the irritability of muscles. The cause of this is, however, probably the removal 

 of other salts which are essential to the irritability, or the presence of carbonic 

 acid. 



From all these results it becomes evident that the normal irritability of 

 nerves and muscles requires that a certain chemical constitution be maintained, 

 and that even slight variations from this suffice to alter, and if continued to 

 destroy, the irritability. Further, it is noticeable that in most cases the first 

 step toward deterioration is a rise of irritability, which, if marked, is accom- 

 panied by a condition of irritation. If the cause of the increase in irritability 

 and excitation be continued, sooner or later exhaustion supervenes, the irrita- 

 bility lessens, and finally is lost. 



Effect of the Electric Current upon Muscles. If a constant-battery current 

 of medium strength be sent through a muscle for a short time, the muscle will 

 give a single short contraction at the moment that the current enters it, and 

 again when the current leaves it. If a strong current be used, the short 

 closing contraction may be followed by a prolonged contraction (Wundt's closing 

 tetanus), which, though gradually decreasing, may last as long as the current 

 is closed ; and when the current is broken, the usual opening contraction may 

 be likewise followed by a prolonged contraction (Hitter's opening tetanus), 

 which only gradually passes off. The closing contraction originates at, and 

 the closing continued contraction may be limited to, the region of the kathode ; 

 and the opening contraction originates at, and the opening continued contrac- 

 tion may be limited to, the region of the anode. 



In case a very weak current is used, no contraction will be observed ; 

 nevertheless, while the current is flowing through the muscle it modifies its 

 condition ; a state of latent excitation is produced at the kathode, which shows 

 itself in an apparent increase of irritability of that part of the muscle. On 

 the other hand, the irritability of the muscle at the kathode will be found to 

 be lessened after the withdrawal of the polarizing current, because the condi- 

 tion of excitation which it causes fatigues that part of the muscle. 



The effects of the battery current at the region of the anode are just oppo- 

 site to those produced at the kathode. While the current is flowing, the irri- 

 tability at the anode is lessened, and when the polarizing current is removed, 

 irritability at the anode is found to be greater than it was before the battery 

 current was applied. 



The lessened irritability which is produced at the anode during the flow 

 of the battery current may be shown by an inhibition of a condition of exci- 

 tation which may be present at the time that the current is applied to the 

 muscle. For example, in the case of unstriated muscles, not only does closing 

 the battery circuit never cause a contraction at the anode, but if the part of 

 the muscle exposed to the influence of the anode happens to be at the time in 

 a condition of tonic contraction, the entrance of the current causes that part 

 of the muscle to relax. The inhibitory influence exerted by the anode, as a 

 result of the lowering of the irritability, is seen to a remarkable degree in its 



