1 84 ELECTROTONUS. [CH. xvi. 



impulse 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 anodic end of which has a smaller con- 

 ductivity immediately after opening than during the flow of the 

 current (fig. 192, 3); the kathodic end of this region also does 

 not become immediately passable after a strong current. 



Thus we have seen that two circumstances influence the effect 

 of the constant current upon a nerve, viz., the strength and 

 direction of the current. It is also necessary that the stimulus 

 should be applied suddenly and not gradually, and that the irri- 

 tability of the nerve should be normal ; not increased or diminished. 

 Sometimes (when the preparation is specially irritable) instead of 

 a simple contraction a tetanus occurs at the make or break of the 

 constant current. This is liable to occur at the break of a 

 strong ascending current which has been passing for some time 

 into the preparation, or at the make of a strong descending 

 current ; both being conditions which increase the excitability of 

 the piece of nerve nearest to the muscle ; this is called Bitter's 

 tetanus, and may be stopped in the first case by throwing in the 

 current in the same direction, or in the second case by throwing 

 in a current in the opposite direction. 



The same general laws hold for muscle as well as for nerve, 

 but are more difficult to demonstrate ; the main fact, however, 

 that the kathode is the stimulating electrode at the make, and 

 the anode at the break, may be shown by the following experi- 

 ment ; if a curarised, that is, a physiologically nerveless muscle, 

 is arranged as in the experiment for demonstrating the muscle 

 wave (see fig. 149, p. 125), and a non-polarisable electrode placed 

 at each end, the muscle wave at the make of a constant current 

 starts at the negative electrode (kathode) and at the break at the 

 positive electrode (anode). 



An induced current in the secondary circuit of an inductorium 

 may be regarded as a current of such short duration that the 

 opening and closing are fused in their effects. This is true for all 

 induction currents, whether produced by the make or break of 

 the primary circuit. The kathode will always be the more 

 effective in causing contraction. 



RESPONSE OF HUMAN MUSCLES AND NERVES TO ELECTRICAL 

 STIMULATION. 



Perhaps the most important outcome of this study of the 

 response of muscle and nerve to electrical stimulation is its 

 application to the muscles and nerves of the human body, 



