CH. XVI.] 



LAW OF CONTRACTION. 



bility or power of responding to stimuli altered in the way just 

 described, but its conductivity or power of conducting nerve im- 

 pulses is altered as well. Moreover, the alteration in conductivity 

 does not run parallel to that of excitability. 



If weak currents are used, there is but little change in the 

 conducting power of the nerve, and what little change there is, 

 is in the direction of diminution near the kathode. But when 

 the strength of the current is increased, the diminution of con- 

 ductivity is very marked round the kathode, and gradually fades 

 away towards the anode (fig. 192, i). On increasing the 

 strength of the current still more, the loss of conductivity is 

 greater, and spreads over both the anodic and kathodic regions 

 (fig. 192, 2). 



Fig. 191. Diagram illustrating the effects of various intensities of the polarising current. 

 H, *', nerve ; a, anode ; k, kathode ; the curves above indicate increase, and those 

 below decrease of irritability, and when the current is small the increase and decrease 

 are both small, with the neutral point near </, and as the current is increased in 

 strength, the changes in irritability are greater, and the neutral point approaches k. 



When the current is taken out, the reverse changes take place, 

 but this after-effect only lasts a short time. In fig. 192, 3, we 

 see the effect of breaking a current of moderate strength, viz., a 

 fall of conductivity in the post-anodic region which gradually 

 fades away towards the kathode. 



Pfluger's law of contraction. The constant current some- 

 times causes a contraction both at make and break, sometimes at 

 make only, sometimes at break only. The difference depends on 

 the strength and direction of the current ; and follows from the 

 electrotonic changes of excitability and conductivity we have been 

 studying. Increase of excitability acts as a stimulus ; so that- at 

 the make the kathode is the stimulating electrode, and at the 

 break the anode is the stimulating electrode. 



The facts may be demonstrated in the following way (fig. 193) : 

 from a battery lead the wires to the middle screws of a reverser (with 

 cross wires), interposing a key ; from one pair of end screws of the 



