I have before said that if a living and entire frog be set upon a plate of zinc, contractions can very seldom be produced in any part of its body by passing a rod of silver over it, so that the silver, the frog, and the zinc, may be all in contact with each other. But, I have found in upwards of twenty experiments, that when inflammation had been excited in one of the hind legs of a frog, by irritating it with a brush, contractions uniformly took place in that leg when the metals were applied to it, although none had been produced in it before it was inflamed, nor could still be produced in the other leg which remained in its natural state.
Having previously excited inflammation, by means of a brush, in the foot and leg of a healthy and large frog, I cut off its head. The contractions excited by the metals in the inflamed leg were in vigorous and instantaneous jirks; those in the sound leg more languid and difficultly excited. Spontaneous motions continued at this time nearly the same in both. Till the end of the second day, after this frog’s head had been taken off, the contractions excited in the inflamed leg continued uniformly, and beyond all comparison more vigorous than what I could by any means excite in the sound leg. But, after this time, the inflamed leg became hard as a piece of wood; probably in consequence of the effusion to which the inflammation had given rise.
The event of five similar experiments was so nearly the same, that I should be thought unnecessarily minute, were I to relate them in detail.
We are now perhaps prepared to account for the deficiency of contractile power in those legs, whose sciatic nerves had been divided, the one, between three weeks and a month, the other, six weeks, compared with its continuance in the leg, whose nerve had been divided upwards of three months. It appears, from the circumstances of those experiments, that some of the arteries, appropriated to the supply of the sciatic nerves of frogs, have the same course with the nerves themselves; since the deposition of new matter could in all be traced from the upper division of the nerves. It is obvious, therefore, that the part of the nerves below the division, must have been deprived of so considerable a portion of their usual arterial supply, as in time would occasion some alteration in their structure, and consequently in their powers. We accordingly find that such alteration of structure, and such deficiency of power, had actually taken place. It is further probable, that, in proportion as the supply from the arteries was restored, the powers of that nerve, which had been three months divided, had been likewise restored. This supposition is countenanced by every instance in which nerves are reproduced; as we find the functions of the parts in which they had been divided, are not immediately, but gradually restored.
M. Fontana seems too hastily to have adopted the opinion, that the sciatic nerves, when divided, are probably never reunited by truly nervous structure, because no reunion took place during the very short period which he suffered to elapse between their division, and their subsequent examination. In the experiments, which I have related, the progress towards reunion seems to have borne a very exact proportion to the time the nerves had remained divided; and, in an experiment related by Dr Monro, where the sciatic nerve of a frog had been divided a year previous to the death of the animal, the reproduction was advanced so far as to have the appearance of being perfect. Nor can I doubt, that both the sensibility and the motion of the limb would have been restored, had the animal been permitted to live a sufficient length of time. The following fact renders the supposition at least extremely probable.
In the first volume of the Edinburgh Medical Essays, the case of a Captain of a man of war is related, who entirely lost the use of his right arm, in consequence of a gun-shot wound received in his neck. The circumstances of the case are such as leave no reason to doubt, that the loss of the power of motion, in this gentleman’s arm, was owing to the division of the cervical nerves proceeding to the arm: yet both the full use, and strength of this arm, were restored, after a period of about two years and a half. A proof perfectly satisfactory that an actual regeneration of nerves had, in this case, taken place; and if in this, one sees no reason why it should not equally take place in any other part of the body.
It might be difficult to assign a satisfactory reason for the very speedy reproduction of the intercostal, parvagum, and recurrent nerves, when compared with the great length of time required for the reproduction of others. May it not be owing to the very profuse manner in which they are supplied with arteries, probably both in an ascending, and in a descending direction; from above, by the superior, and from below, by the inferior laryngeal arteries?
It appears upon the whole, therefore, tolerably certain, that the sanguiferous system contributes more immediately than the brain to the support of that condition of muscles and of nerves, upon which the phenomena of contraction depend; since that condition is much more injured by intercepting the influence of the former than of the latter.
Every experiment and observation, which has been made upon the subject of nutrition, and of the reproduction of parts, clearly demonstrates that nerves and muscles, in common with every other part of the body, derive their structure from the arteries; and it is evident, that upon this structure their several properties must in some measure depend. But M. Galvani’s discovery of a subtile influence, which may be transmitted apparently from one part of an animal to another through foreign media, may reasonably give rise to a conjecture that the phenomena exhibited by nerves and by muscles may perhaps depend more immediately upon some such influence; and reasons exist, which might induce some to suspect that even this is derived from the blood.