ELECTRO-PHYSIOLOGY. 307 



mity of the negative pole. This result, it Avill be seen, bears an analogy to 

 that of Marianiui, and, without resorting to other hypotheses, we may establish 

 ourselves on that analogy. On this proposition is founded the use of the pre- 

 pared frug, to which I have given the name of the galvanoscopic frog, for detect- 

 ing not only the existence of the smallest variations of the electric state, but 

 discovering also the direction in which the discharges or electric currents flow. 

 The galvanoscopic frog consists of a leg of a frog to which is united a long 

 nervous filament, being the sciatic portion, and also, if possible, the lumbar 

 portion. It is necessary to make this preparation rapidly in order that the nerve 

 and muscle may have the greatest possible vitality. When this instrument. is 

 to be used, we place the leg on a surfsice of gutta-percha, or in the interior of 

 a glass tube which is held in the hand, and we touch two different points of the 

 nervous filament with any two points of the body in which the electro-motor 

 or power is supposed to exist. This being done, however weak, the electric 

 current there will be a contraction in the galvanoscopic frog either at the closing 

 or the opening of the circuit. By considering the position of the nerve and 

 the moment of the contraction, we can judge of the direction of the current in, 

 the nerve, whether it be direct or inverse. 



We shall see more fully, in the next lecture, why it is proper, in using the 

 galvanoscopic frog to support it on an isolating surface ; the frog, like every 

 muscle, is an electro-motor, and without that precaution we should obtain the- 

 contractions and signs of the current through the eff'ect of the muscular electro- 

 motor, and therefore through a cause which resides in the instrument itself. 

 When the above precaution is used, the galvanoscopic frog becomes the most 

 delicate instrument which we know, and it will be seen in the ensuing lecture, 

 on animal electricity what frequent use is made of it. In the mean time I pro- 

 pose to off'er a single experiment with the galvanoscopic frog for the purpose, 

 not only of conveying an idea of the sensibility of that instrument and of the 

 mode of using it, but of exhibiting an instance of electricity developed entirely 

 without metals. In this experiment I employ two long glass tubes filled with 

 quartz or powdered glass moistened with water, one of which is plunged in a 

 solution of potassa, and the other in a vessel of porous earth immersed in that 

 solution and full of nitric acid. With the nerve of the galvanoscopic frog I 

 touch the extremities of the columns of powdered glass or quartz which com- 

 municate with the two liquids of the so-called battery of Becquerel. We then 

 see the frog contract, now at the closing, now at the opening of the circuit, 

 according to the position of the nerve. The current, we know, passes from the 

 potassa to the acid in the liquid, and circulates in the nerve from the tube of 

 glass immers'ed in the acid to the tube of glass immersed in the potassa. Hence 

 the frog immediately contracts when the free portion of the nerve touches the 

 tube immersed in the acid, and the portion next to the leg touches the tube im- 

 mersed in the potassa. We are justified, therefore, in the assei'tion that not only 

 is the galvanoscopic frog in the greater number of cases the most delicate 

 instrument we possess for discovering the presence of electricity, but that, prop- 

 erly used, it serves also to indicate the direction of the current. 



A second proposition of electro-physiology is this : " That by using, for the 

 excitation of a nerve, an electric current of very slight intensity, and such, there- 

 fore, that, being still further diminished, thei-e would be perceived a correspond- 

 ing diminution in the muscular contraction, if this current be forced to divid*} 

 itself in half between two nerves, the eifect excited in the muscle is reduced to 

 half what it was at the first instant when the current passed entire in the 

 nerve." 



At first sight this proposition seems contrary to the general definition which 

 we have given of electro-physiological effects : it has been our object to show 

 that these eftects arising only at the beginning and at the end of the current, 

 and not during thetime when the circuit remained clossd, they were in some 





