CH. XVI.] 



LAW OF CONTRACTION. 





a descending current; and only at break with an ascending 

 current. 



The contractions produced in the muscle of a nerve-muscle 

 preparation by a constant current have been arranged in a table 

 which is known as Pfliiger's Law of Contraction. It is really 

 only a statement as to when a contraction may be expected : 



The increase of irritability at the kathode when the current is 

 made, is greater and so more potent to produce a contraction 

 than the rise of irritability at the anode when the current is 

 broken ; and so with weak currents the only eft'ect is a contraction 

 at the make. There is little or no alteration in the conductivity 

 of the nerve to hinder the propagation of the impulse so started. 



With a current of moderate strength and in an ascending 

 direction, the excitation which occurs at the anode at break starts 

 ;in impulse in a part of the nerve near the muscle, and not 

 separated from it by any region of diminished conductivity ; at 

 make also there is no block to prevent the excitation which occurs 

 ut the kathode from reaching the muscle, since with moderate 

 currents the fall of conductivity does not reach the anodic 

 region (fig. 192, i). When the current is descending, the excita- 

 tion at the make is at the kathode, and as this is near the muscle 

 there is no block in the nerve between it and the muscle ; at 

 the break, the kathodic block is removed, and so the anodic 

 excitation is readily propagated to the muscle. 



With strong currents the case is a little more complicated, 

 because here the diminution of conductivity is so great that 

 certain regions of the nerve become impassable by nerve-impulses. 

 When the current has an ascending direction, the impulse at the 

 break is started at the anode, and as this is next to the muscle 

 there is no hindrance to the propagation of the impulse, but at 

 the make the impulse started at the kathode is blocked by the 

 lowering of conductivity which as we have seen with strong 

 currents spreads and reaches the anode (fig. 192, 2). When the 

 current is descending the kathode is near the muscle, and so the 



