STIMULATION OF MUSCLES AND OF NERVES 427 



series of shocks of increasing strength we have therefore a gap in the resulting 

 contractions (Fick). This is only observed with the ascending currents, and 

 is the result of a block at the anode. The absence of the contractions is there- 

 fore entirely analogous to the corresponding phenomena for the strong ascending 

 constant current. The contractions coming after the gap and gradually increas- 

 ing in size are produced really by the excitation taking place at the disappear- 



FIG. 164. Schematic representation of the distribution of an electric current in a human arm 

 on application of two electrodes over a nerve, after de Watte ville. "". 



ance of the induction current, and are to be regarded for this reason as a sort 

 of opening contractions. But further discussion of their nature here would 

 carry us too far afield. 



The stimulating effects and the alterations of excitability produced by 

 the electric current follow the same laws in human nerves as in the exsected 

 frog's nerves (Waller and de Watteville). 



In experiments on living men, the electrodes of course cannot be applied to 

 the nerves themselves, but can only be placed on the skin; the nerve is stimu- 

 lated then only by the threads of current which penetrate that far. It will be 

 clear at once that the density of that portion of the current reaching a par- 

 ticular nerve will be greater the nearer the nerve lies to the surface of the skin. 

 Consequently in using the current for therapeutic purposes the electrodes are 

 applied to the skin at those points where the nerve, which it is desired to stimu- 

 late, can be reached most directly. 



The effective anode is of course the place where the current enters the nerve 

 itself, the effective cathode, the place where it leaves the nerve. If both poles 

 were to be placed on the skin over the nerve, as in Fig. 164, anodes and cathodes 

 would be present at almost every possible point along the nerve, as indicated 

 by the radiating lines. Evidently such an experiment would not be adapted to 

 the study of electrical effects on human nerves. The monopolar method is there- 

 fore used, the current being conveyed to and away from the body by electrodes 



