CHAP, ii.] THE CONTRACTILE TISSUES. 117 



condition to a state of katelectrotonus, or from a state of anelec- 

 trotonus back to a normal condition ; but that the passage from 

 a normal condition to anelectrotonus or from katelectrotonus 

 back to a normal condition is unable to generate an impulse. 

 Hence, when a constant current is ' made,' the impulse is gen- 

 erated only at the kathode where the nerve passes suddenly into 

 katelectrotonus ; when the current, on the other hand, is ' broken,' 

 the impulse is generated only at the anode where the nerve passes 

 suddenly back from anelectrotonus into a normal condition. We 

 have an indirect proof of this in the facts to which we drew 

 attention a little while back, viz. that a contraction sometimes 

 occurs at the ' breaking ' only, sometimes at the ' making ' only 

 of the constant current, sometimes at both. For it is found that 

 this depends partly on the strength of the current in relation to 

 the irritability of the nerve, partly on the direction of the current, 

 whether ascending or descending ; and the results obtained with 

 strong, medium and weak descending and ascending currents have 

 been stated in the form of a ' law of contraction.' We need not 

 enter into the details of this ' law,' but will merely say that the 

 results which it formulates are best explained by the hypothesis 

 just stated. We may add that when the constant current is 

 applied to certain structures composed of plain muscular fibres, 

 whose rate of contraction we have seen to be slow, the making 

 contraction may be actually seen to begin at the kathode and 

 travel towards the anode, and the breaking contraction to begin 

 at the anode and travel thence towards the kathode. 



Since in katelectrotonus the irritability is increased, and in 

 anelectrotonus decreased, both the entrance from the normal 

 condition into katelectrotonus, and the return from anelectrotonus 

 to the normal condition, are instances of a passage from a lower 

 stage of irritability to a higher stage of irritability. Hence, the 

 phenomena of electrotonus would lead us to the conception that a 

 stimulus in provoking a nervous impulse produces its effect by, in 

 some way or other, suddenly raising the irritability to a higher 

 pitch. But what we are exactly to understand by raising the 

 irritability, what molecular change is the cause of the rise, and 

 how either electric or other stimuli can produce this change, are 

 matters which we cannot discuss here. 



Besides their theoretical importance, the phenomena of electro- 

 tonus have also a practical interest. When an ascending current 

 is passed along a nerve going to a muscle or group of muscles, the 

 region between the electrodes and the muscle is thrown into 

 anelectrotonus, and its irritability is diminished. If the current 

 be of adequate strength, the irritability may be so much lessened 

 that nervous impulses cannot be generated in that part of the 

 nerve, or cannot pass along it. Hence, by this means the irregular 

 contractions of muscles known as ' cramp ' may be abolished. 

 Similarly, by bringing into a condition of auelectrotouus a portion 



