MUSCLE-NERVE PHYSIOLOGY. 531 



notent than the ascending (and with still weaker currents is the only 

 one which produces any effect), since the kathode is near the muscle. 

 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, being of considerable amount and extent, but with a 

 weak current being less considerable both in intensity and extent, only 

 slightly affects the contraction. As the current is stronger however, 

 recovery from anelectrotonus is able> to produce a contraction as well 

 as katelectrotonus ; 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 iwcending current maybe explained by sup- 

 posing 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 should ~be 

 normal; not increased or diminished. Sometimes (when the prepara- 

 tion 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 di- 

 rection or stopped by passing a current in the same direction. 



THE EFFECT OF BATTERY CURRENTS ON NORMAL HUMAN NERVES. 



The following account is condensed from Lombard in " An American 

 Text-book of Physiology." 



As an electric current cannot be applied to living human nerves di- 

 rectly, it is applied to the skin along the course of the nerve. The cur- 

 rent passes from the anode or positive pole through the skin, and spreads 

 out in the tissues much as the bristles of a brush; it then gradually 

 concentrates and leaves the skin at the kathode or negative pole. 



In addition to the physical anode and kathode of the battery, there 

 x are what are called physiological anodes and kathodes. There is a 

 physiological anode at every point where the current enters a nerve, and 

 "a physiological kathode at every point where it leaves it. 



Generally when the current is applied to nerves through the skin, 

 only part of it flows longitudinally along the nerves; most of it passes 

 diagonally through them to the tissues below. Thus it happens that in 

 that part of the nerve beneath either the physical anode or kathode, 

 groups of physiological anodes and kathodes are found. 



