CH. XVI.] 



PFLtJGER's LAW OP CONTRACTION 



185 



less is the resistance in the rheochord circuit, and the less the current 

 through the nerve. With a weak current, a contraction occurs at 

 make only, but more readily, i.e. with a weaker current, when its 

 direction is descending, i.e. towards the muscle. With a stronger 

 current (ascending or descending) contraction occurs both at make 

 and break. With a very strong current (six Groves), the contraction 

 occurs only at make with 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. 



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 effect is a contraction at the make. 

 But when the strength of the current is increased the rise of 

 excitability is in all cases sufficient to provoke a contraction 

 (moderate effect in above table). The alteration in conductivity 

 is not sufficient to prevent the impulses being 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 extreme lowering 

 of conductivity at the anode. When the current is descending the 

 kathode is near the muscle, and so the impure at make reaches the 

 muscle without hindrance ; but at the break, the impulse started at 

 the anode has to traverse a region of nerve, the conductivity of which 

 is so lessened that the excitation is not propagated to the muscle. 



G. N. Stewart has stated in opposition to the foregoing statements that at the 

 make conductivity is most lowered at the kathode, and at the break at the anode. 

 In other words, conductivity and excitability vary in opposite directions. His 

 results have, however, not been accepted by other physiologists, and are due to a 

 complex set of excitatory and polarisation changes produced by the galvanometric 



