ACTION OF ELECTRICITY UPON THE NERVES. Ill 



rent, it is restored more promptly by stimulation with the 

 inverse current than by absolute repose, and vice versa. 

 This phenomenon, observed by Yolta, is sometimes known 

 as " voltaic alternation." * It is very strikingly illustrated in 

 frogs prepared as above described, with the two posterior 

 extremities, the nerves attached through a portion of the 

 spinal cord, placed in vessels of water so tl^at a current may 

 be simultaneously passed through both nerves, being 'direct 

 for the one and inverse for the other. As we have already 

 seen, after a time, contraction occurs only in one leg, for 

 which the current is direct, on making the circuit, and in the 

 other, only on breaking the circuit. By repeatedly passing 

 the current in this way, after a time there will be no con- 

 traction in either leg, the irritability of the nerves having 

 become exhausted. If the poles of the battery be now re- 

 versed, so as to make the inverse current take the place of 

 the direct, contractions with making and breaking the cir- 

 cuit will again occur. The irritability may again be ex- 

 hausted and restored by changing the poles, and this may 

 be repeated several times with the same preparation. 



There can be no doubt with regard to the action of the 

 direct and inverse currents, as above described, applied to 

 nerves exclusively motor, as well as to the mixed nerves. In 

 the mixed nerves separated from the centres, it is evident 

 that the motor elements only are acted upon ; and it would 

 be difficult to understand how the action of these currents 

 could be different when applied to the anterior roots of the 

 spinal nerves. Longet and Matteucci, however, in their 

 earlier experiments upon the anterior roots of the spinal 

 nerves, observed that contraction of muscles took place on 

 breaking the circuit, with the direct current, and on making 

 the circuit, with the inverse current ; precisely the opposite 

 of the phenomena noted in experiments on the mixed 

 nerves ; and Longet proposed from this to draw a distinc- 

 tion between the ordinary nerves and those possessed of ex- 



1 LONGET, Traite de physiologic, Paris, 1869, tome ill, p. 199. 

 108 



