CHAP. XI. §. I. THE GAXVANOMETEE. 1 37 



My more recent experiments may be classed 

 under three heads : 1st, Those in which the galva- 

 nometer was employed; Snd, Those in which the 

 galvanoscopic frog was used ; Srd, Those in which a 

 magnetised needle was used. 



Sect. I. The Galvanometer. 



The animal, most frequently a rabbit, but occa- 

 sionally a guinea-pig or a frog, was pithed, or 

 rendered insensible by means of prussic acid. The 

 sciatic nerve was carefully exposed throughout its 

 whole course, and a plate of thin glass passed 

 beneath it. All traces of blood being carefully 

 removed, the pointed extremities of platinum elec- 

 trodes were inserted at the extreme ends of the 

 exposed nerve, as far apart as possible, leaving, 

 however, a small portion, just at its exit from the 

 pelvis, for the purpose of serving as a , point of 

 irritation. The leg of the limb was fastened down 

 so as to prevent a too great motion during con- 

 traction, which might otherwise disturb the position 

 of the electrodes. The other ends of the electrodes 

 rested on and dipped into wooden cups containing 

 mercury, the cups being placed upon glass for 

 insulation : by these means a slight motion of the 

 electrodes would not disturb the galvanometer, and 

 it remained perfectly steady. The galvanometer 

 consisted of several coils. 



At first the nerve was irritated with the point of a 

 steel needle, which was insulated, being inserted by 



