184 



ELECTROTONUS 



[CH. XVI. 



the electrodes are applied, and extends in gradually diminishing 

 intensity on each side of them. Between the electrodes the increase 

 shades off into the decrease, and it is evident that there must be a 

 neutral point where there is neither increase nor decrease of irritability. 

 The position of this neutral point is found to vary with the intensity 



FIG. 199. Diagram illustrating the effects of various intensities of the polarising current, n, ri, nerve ; 

 a, anode ; fc, 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 

 a, and as the current is increased in strength, the changes in irritability are greater, and the neutral 

 point approaches fc. 



of the polarising current when the current is weak the point is 

 nearer the anode, when strong nearer the kathode. 



Pfliiger's law of contraction. The constant current sometimes 

 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 ex- 

 citability 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. 200) : 



Cell 



Muscle , 



Key 



FIG. 200. Arrangement of apparatus for demonstrating Pfliiger's law. 



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 

 reverser lead wires to the binding screws of the rheochord ; from these 

 same screws of the rheochord the non-polarisable electrodes lead to 

 the nerve of a nerve-muscle preparation. The strength of the current 

 is varied by the slider S. The nearer S is to the binding screws the 



