LECT. XII. XIII. EFFECTS ON DEAD ANIMALS. 231 



continue the experiment, we soon perceive the alteration 

 already described ; that is to say, at the moment when the 



Fig. 17. 



Experiment to illustrate the Action of a direct and an inverse Current on the Nerves 

 of a Frog. 



circuit is completed, one limb only contracts, namely, that 

 in which the current is direct ; while, on the contrary, when 

 we interrupt the circuit, the contraction takes place in the 

 other limb, namely, in the one traversed by the inverse cur- 

 rent. This series of phenomena is manifested, more or less 

 speedily, according to the strength of the current, and the 

 vivacity of the animal, but it is never absent. Thus, then, 

 the frog is not only a galvanoscope of extreme sensibility, 

 but it is also an instrument which may perform the office of 

 a galvanometer, and like this, indicate the direction of the 

 current which circulates in a portion of its nerves. 



It was Marianini who first observed that contractions were 

 obtained at the interruption of the circuit, and not at the 

 moment when it was closed. To succeed with this expe- 

 riment, it is necessary to introduce a frog into the circuit 

 of a pile, and to close this by touching one pole with one 

 hand, and plunging the fingers of the other into the liquid 

 in which one of the extremities of the frog are also im- 

 mersed. At first the current which circulates is very feeble, 

 in consequence of the bad conducting power of the hand, 

 but it goes on increasing in proportion as the fingers imbibe 



