M. Galvani is said to have observed the effects of the influence, which he discovered, diffused over the whole body of a frog, when the metals were applied to a nerve merely laid bare, without being either divided or separated from surrounding parts. If we are allowed to infer this diffusion of the influence from the restlessness expressed by the animal, M. Galvani’s observation may be just. If from the contractions produced, I suspect it is by no means so; since, in every experiment which I have made upon the subject, the contractions have been confined to those parts to which the nerve touched by the metals was distributed.
That this influence, however, may pass in a direction contrary to the course of nerves, is evident from some of the experiments which I have related relative to its effects upon the senses, but is still more clearly demonstrated by the following.
If, after having divided at the pelvis a frog recently killed, the sciatic nerves be freed from cellular membrane up to their origin from the spine, and all the parts below this, except themselves, be cut away, the muscles on each side of the spine, for some little way up, may be brought into contraction by touching the nerves alone with the two metals in contact. This experiment has not always succeeded with me, and never unless the frog had been recently killed. So long as the hind legs remain undivided from the nerves, it never succeeded; the only contractions produced being in the legs.
OF THE BLOOD VESSELS.
We are told by Dr Valli, that no contractions are excited by arming the blood vessels; but as he has not told us whether his experiments were made upon them while the blood still continued to flow through them, or after they had been deprived of their blood, I determined to make the following experiment.
Having laid bare, and separated from surrounding parts and from each other, the crural artery, and nerve, in the thigh of a full grown frog, I cut out the whole of the nerve between the pelvis and the knee. I then insinuated beneath the artery a thin plate of sealing wax, spread upon paper, and broad enough to keep a large portion of the artery completely apart from the rest of the thigh. The blood still continued to flow, through the whole course of the artery, in an undiminished stream. The artery, thus partially insulated, was touched with silver and zinc, which were then brought into contact with each other; but no contraction whatever was produced, in any muscle of the limb. This experiment was frequently repeated upon several different frogs, both in whom the nerve was, and in whom it was not, divided. The result was uniformly the same. But vivid contractions were produced in the whole limb, when an electrical spark, or even a full stream of the aura, was passed into the artery.
It, however, by no means follows from this experiment, that the sanguiferous system of animals bears no relation whatever to the influence discovered by Galvani. I have already shewn, that the heart may be affected by it, and have given reason to believe, that the smallest arteries of the body are not exempted from its action. Should it ever be proved to be an exclusive property of animals, it is not impossible but that even its origin may be traced to their sanguiferous system.
[11]. I have not been at the pains to inform myself, who first was the author of this doctrine; but its adoption by Caldani, by Haller, and by Fontana, and by all upon the faith of experiment, was certainly sufficient to give it currency, in opposition to that of Willis, Lower, Kaau, Boerhaave, Laghi, and even of the ingenious Whytt.