266 



PHYSIOLOGY 



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 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 



FIG. 114. 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 lt 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.) 



is neither increased nor diminished. We find experimentally that this in- 

 different point is nearer the anode when the polarising current is weak, and 

 approaches the cathode as the current is strengthened, so that with very 

 strong currents nearly the whole intrapolar length is in a condition of an- 

 electrotonus (Fig. 114). When a strong polarising current is used, the de- 

 pression of irritability at the anode is so marked that no impulse can pass 



ascending current 



make excitation blocked 



kath 



break excitation at anode 

 blocked at kathode. 



katr 



FIG. 115. Diagram to show the blocking effect of a strong constant current 

 passed through the nerve of a nerve-muscle preparation. 



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 tte region of the anode 

 (Fig. 115, A). 



The results of stimulating motor nerves by means of constant currents were studied 

 by Pftiiger and, embodied in a Table, make up what is known as Pfluger's law. The 

 result of stimulating varies with the strength of a current. 



