THE MUSCULAR SYSTEM. 431 



more potent than the ascending (and with still weaker currents is the 

 only one which produces any effect), since the kathode is near the mus- 

 cle; whereas in the case of the ascending current the stimulus has to 

 pass through a district of diminished irritability, which with a very 

 strong current acts as a block, but with a weak only slightly affects the 

 contraction. As the current is stronger recovery from anelectrotonus 

 is able to produce a contraction as well as kathelectrotonus, a contrac- 

 tion 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 irrita- . 

 bility at the kathode blocks the stimulus of the rise in irritability a t 

 the anode. 



Thus we have seen that two circumstances influence the effect of the 

 constant current upon the 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 spe- 

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

 make or break of the constant current. This is especially liable to oc- 

 cur 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. 



