Galvanism. 121 



contractions may be excited by one metal, as- 

 sisted by other substances, or even without any 

 metal whatever. The metals, however, are the 

 most certain agents, but they will produce no 

 effect without the intervention of some fluid which 

 has a chemical action on one or both of the 

 metals. 



The experiment may be tried upon any animal 

 recently dead; but what are called the cold- 

 blooded animals, that is, those which have their 

 blood of a temperature not higher than that of 

 the atmosphere, such as reptiles and fishes," retain 

 this sensibility much longer than others ; dead 

 frogs for instance will retain it for several hours, 

 and sometimes for a day or two. 



To give the experiment proper effect some 

 preparation is however requisite; and as the gal- 

 vanic influence acts principally on the nerves, it 

 is necessary that they should be exposed to one 

 of the metals: it is made most successfully on 

 the hind legs of a dead frog. To this end we 

 have only to cut them off with a small bit of the 

 spine attached to the nerves of the thigh, as in 

 plate X. fig. 41, where GH are the lower limbs, 

 thus adhering to a small piece of the spine AB, by 

 means of the crural nerves CD. The legs must 

 be skinned in order to lay bare the muscles, and 

 a small piece of tin-foil wrapped round the spine 

 A, B. If we then hold one of the legs in our 

 fingers, and let the other be suspended with the 

 bundle of nerves and spine hanging upon it, and 



VOL. i. G 



