PFLUGER'S LAW 



93 



Start with the slider c close to the screws s,, s 2 , in which position the current 

 through the nerve is practically nil. Open and close the circuit by the key 

 K 2 . With this very weak current no contraction occurs on make nor on break. 

 Arrange the commutator k, to give an ascending current, and slide c a little 

 further along the wires, so as to increase the current through the nerve. At 

 a certain position of the slider a contraction will be obtained at make but 

 none at break. Reverse the current. The same result is obtained. A 

 current of this length is spoken of as a weak current. It will vary in strength 

 according to the excitability and electrical resistance of the particular nerve 

 experimented upon. Increase the current still further, when contractions 

 will occur on both make and break of ascending and of descending currents. 

 A current of this strength is spoken of as a current of medium strength. 

 To complete the series remove the rheochord and attach the electrode wires 

 directly to the commutator. It will usually be necessary to further increase 

 the strength of the current to four, six, or even eight cells. Increase the 

 current till one is found which gives a contraction on make only of a descend- 

 ing, and on break only of an ascending current. Such a current is spoken of 

 as a strong current. 



The result of this experiment may be arranged in tabular form, 

 and the statement of results is commonly termed the law of 

 contraction. (Pfliiger's Law.) 



This ' law ' can be explained on the basis of the facts we have 

 already studied with regard to changes of excitability and conductivity 

 brought about by the constant current and by our knowledge of the 

 position at which excitation occurs. Thus, the results with a weak 

 current mean that the production of a certain degree of katelectrotonus 

 can stimulate, whereas the disappearance of the anelectrotonus pro- 

 duced by precisely the same current cannot stimulate. If the kathode 

 be nearer the muscle (descending current) there is nothing to prevent 

 the impulse reaching the muscle when the stimulus starts from the 

 kathode. The muscle therefore contracts on closing the current. 

 When the current is reversed the stimulus now has to travel through 

 the intrapolar piece of nerve, and through that part in anelectrotonus. 

 In both these parts the conductivity is diminished but not sufficiently 

 to block the impulse. The muscle therefore contracts. An exactly 

 analogous explanation holds for a descending weak current. 



With currents of medium strength the disappearance of anelec- 

 trotonus can stimulate. At make of a descending current the stimulus 



