It is evident, that in whatever direction we suppose the influence to have passed in its Circle, it must, in Experiment 20th, have passed up one Leg and down the other in the same Frog: And, in Experiment 21st, if it passed from one end of the Chain to the other end of it, it must have passed upwards in two of the Frogs, and downwards in the other two; or if the influence passed from the two ends of the Chain towards its middle, where the Spines of the two middlemost Frogs were contiguous, it must have passed upwards in all of them.
23. When after cutting four living Frogs transversely at the middle of their Spines, but without cutting at their Pelves all the Parts but the Sciatic Nerves, I placed the hind Parts of them in a Chain, as in Experiments 20th, 21st and 22d, the Muscles were not convulsed on applying the Gold to the Zinc.
I next found, that after placing in contact with each other the several Muscles which had been cut transversely in Experiments 20th, 21st and 22d, allowing the Nerves to remain undivided, the muscles were not convulsed when I touched the Plate of Zinc with the gold Probe held in my right hand, although I touched the other end of the Chain of Frogs with my left hand.
The reason why the Muscles were convulsed in Experiments 20th, 21st and 22d, and not in Experiment 23d, evidently is, that in the former, the influence was concentrated in the Nerve, in the latter the influence was diffused; that is, was in part conveyed by other Organs, as well as by the Trunks of the Nerves.
24. After finding that I could readily excite Convulsions in the hind Legs of a Frog, without cutting it, by laying its Back on a Plate of Zinc, and introducing a gold Probe within its Intestinum Rectum and touching the Zinc with the side of the Probe, I produced two or three hundred Convulsions, succeeding each other quickly, and observed that its Legs were, by these means, so much weakened, that it could not jump, and crawled with difficulty, but in a few minutes it recovered nearly the full force of its Muscles.
In other Frogs I passed a gold Wire between their Sciatic Nerves and Os Sacrum, and twisted together the two ends of the Wire over the Backs of the Animals. I then put them into a Zinc Vessel filled with Water, or into a Glass Vessel filled with Water, in the bottom of which I laid a large Plate of Zinc: So that every time the Animals by moving separated the Gold from the Zinc, and again brought them into contact, their hind Legs were convulsed. I allowed them to remain three or four days in this situation, and found that their Limbs were weakened considerably, but not exhausted of their Power of Motion; and, after removing the gold Wire, the Limbs by degrees recovered their strength.
I made the same Experiment on those Frogs in which I had, six weeks before, cut out, from behind, all that part of their Spinal Marrow which is covered by the six undermost Vertebræ, and found, several days after the Frogs had been subjected to the Experiment, that, by pinching their Sciatic and Femoral Nerves, and still more readily by the application of the Gold and Zinc, weak convulsions of the Muscles were excited.
25. After Frogs were prepared as above described, by cutting their Spines transversely, and then all the parts of their Pelves, except their Sciatic Nerves, I found that slight Electrical Shocks, or a Leyden Phial discharged directly through the Limbs of a Frog, or indirectly by the medium of water, produced convulsions in their Muscles, exactly resembling those excited by the Metals. And when, after moderate Electrical Shocks had been passed repeatedly through their Legs, the Metals were applied to their Nerves, in the manner before mentioned, the Muscles were convulsed. I found, likewise, that after cutting the Nerves transversely, and tying them together, Electrical Shocks were conducted by the Nerves, and occasioned convulsions of the Muscles.