THE EXCITATION OF NERVE FIBRES 



299 



strength, it will give rise to no contraction. The degree of suddenness 

 of the rise, which is most beneficial in causing contraction, varies with 

 the nature of the tissue stimulated. Thus it is more rapid in nerve 



Sf 



FIG. 115. Diagram to show the variations of irritability in a nerve during the passage 

 of polarising currents of different strengths. The degree of change] is represented 

 by the distance of the curves from the base line ; the part of the curve, below 

 the line signifying decrease, that above the line increase of irritability. 



A, anode ; B, cathode ; y v effect of weak current ; y. 2 , medium current ; y s , strong 

 current. It will be noticed that the indifferent point, x, where the curve crosses 

 the horizontal line, approaches nearer and nearer the cathode as the current is 

 increased in strength. (From FOSTER, after PFLUGER.) 



than in muscle, and in pale muscle than in red muscle, and in voluntary 

 muscle than in unstriated muscle. 



It is evident that there must be, somewhere between the anode 

 and cathode, an indifferent point that is to say, a region where the 

 irritability is neither increased nor diminished. We find experimentally 



ascendmq current 



make excitation blocked 

 at anode. 



kath. 



an. 



an. 



kath 



break excitation at anode 

 blocked at kathode- 



FIG. 116. Diagram to show the blocking effect of a strong constant current 

 passed through the nerve of a nerve-muscle preparation. 



i 



that this indifferent point is nearer the anode when the polarising 

 current is weak, and gets nearer to the cathode as the current is 

 strengthened, so that with very strong currents nearly the whole intra- 

 polar length is in a condition of anelectrotonus (Fig. 115). When a 

 strong polarising current is used, the depression of irritability at the 

 anode is so marked that no impulse can pass this region. Thus if we 

 send a very strong ascending current through the nerve, there is no 

 contraction at make. This is owing to the fact that the impulse 

 started at the cathode on make of the current cannot reach the muscle, 

 its passage down the nerve being blocked in the region of the anode 

 (Fig. 116, A). 



