CAUSES AND PHENOMENA OF MOTION. 49 



current becomes stronger, recovery from anelectrotonus is able to produce 

 a contraction as well as katelectrotonus, and a contraction occurs both 

 at the make and the break of the current. The absence of contraction 

 with a very strong current at the break of the ascending current may be 

 explained by supposing that the region of fall in irritability at the kathode 

 blocks the stimulus of the rise in irritability at the anode. 



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 sud- 

 denly and not gradually, and that the irritability of the nerve be normal, 

 and not increased or diminished. Sometimes (when the nerve is specially 

 irritable?) instead of a simple contraction a tetanus occurs at the make 

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

 the break of a strong ascending current which has been passing for some 

 time into the preparation; this is called Ritter's tetanus, and may be 

 increased by passing a current in an opposite direction or stopped by 

 passing a current in the same direction. 



VOL. II. 



