236 ELECTRIC CURRENT. LECT. XII. XIII. 



ployed, and even then the contractions soon cease; and 

 that, whether we act on the nerves or on the muscles. 



Effect of tying the Nerve. I wish also to show you some 

 differences observed in the physiological action of the cur- 

 rent when the nerve on which we act has been tied. I 

 expose and isolate the crural nerve of a rabbit, put a liga- 

 ture on the nerve at about its middle, and afterwards 

 transmit the current through the part above the ligature, 

 namely, towards the brain. I obtain contractions of the 

 back and signs of pain, both when I open the circuit and 

 when I close it, whatever may be the direction of the current. 

 Very soon, these effects are produced only at the com- 

 mencement of the passage of the inverse current, and at 

 the cessation of the direct one. If, on the contrary, I 

 transmit the current below the ligature, I first obtain con- 

 tractions of the leg, whether I open or close the direct or 

 inverse current, and very soon, as usual, contractions cease, 

 except at the first moment of the direct current, and at the 

 termination of the inverse one. From these facts, then, it 

 appears that a ligature on a nerve produces no other in- 

 fluence than that of insulating the effects of the current ; 

 that is, of producing the effects of its action on the nervous 

 centres, separately from those which it has when acting 

 upon the extremities of nerves. It is unnecessary to add, 

 that if we operate on a dead animal, we obtain no signs 

 indicative of pain. 



In order to avoid errors in repeating these experiments, 

 care must be taken to insulate the nerve completely from 

 the moist parts which surround it, and to apply the ligature 

 tightly. The best way of proceeding is to use a frog pre- 

 pared in the usual manner, and then to suspend it by its 

 nerve with a silk thread. In this way we are sure that no 

 moist part around the nerve can divert any portion of the 

 current ; but if we neglect these precautions, a certain por- 



