of an animal entrusted to, or depending more particularly upon, one sense than the rest, we are sure to find that sense proportionably perfect; as in quadrupeds, the organ of smelling is remarkably perfect, and leads them to their prey; so the eyes of birds are proportionably perfect, being the means, not only of their support, but from them they receive the first intimation of approaching danger.

The eyes of birds, like those of other animals, consist of three coasts, the sclerotica, choroides, and retina, The human eye, as well as those of quadrupeds, is nearly spherical; in birds the sphere is more oblate, the sclerotica, as it approaches the cornea, becoming suddenly flat. The cornea, though small, when compared with the size of the whole eye is more convex, as it forms the segment of a smaller circle added to the larger formed by the sclerotica. The reason or advantage of this flatness is not very evident. It prevents them, perhaps, from projecting so far as to expose them to danger from the trees and grass amongst which these animals live.

After having examined the eyes of birds, and seeing this curious apparatus, I was next led to the examination of the eyes of quadrupeds, that I might see in what manner they resemble the eyes of birds, and if I could account for their being able to accommodate their eyes to objects at different distances.

This was a subject I found involved in much difficulty, as the eyes of quadrupeds appeared on examination not to have these im

and by tearing and dissecting, I found that it terminated in, and with the other parts composed, the cornea; so that on the first volition of the mind, the recti muscles on contracting will have the power of fixing the eye, and keeping it steady, and at the same time by contracting more or less, will adapt the focus of the eye to the distance of the object, but in a less degree than in birds. On these muscles giving over acting, the eye will be restored to its former state by the elasticity of the sclerotic coat.

From a knowledge of these cir cumstances, we may, from rational principles, explain, why people, by being long accustomed to view small objects, obtain, in time, a sort of microscopic power, if it may be so called; that is, the muscles which contract the cornea, will, by custom, increase their power of action, and grow stronger, like the other muscles of the body; other phenomena of vision on these principles may be explained.

erroneous, but has also made known several important facts, in addition to those which had been discovered by that author. As he appears, however, from these letters, to have fallen into some mistakes himself, and has certainly not exhausted the subject which he has treated in them, I shall venture to communicate to this learned body a few observations I have made respecting it, which may contribute both to correct his errors, and to increase our knowledge of the cause of those motions, which have been attributed by Mr. Galvani and others to an animal electricity. These observations will be so arranged, as to furnish answers, more or less satisfactory, to the following questions: does the incitement of the influence which, in Mr. Galvani's experiments, occasions the muscles of animals to contract, either wholly, or in part, depend upon any peculiar property of living bodies? What are the conditions necessary for the excitement of this influence? Is it electrical?

When a muscle contracts upon a connection being formed, by means of one or more metals, between its external surface and the nerve which penetrates it, Mr. Galvani, contends that, previously to this effect, the inner and outer parts of the muscle contain different quantities of the electric fluid; that the nerve is consequently in the same state, with respect to that fluid, as the internal substance of the muscle; and that, upon the application of one or more metals between its outer surface and the nerve, an electrical discharge takes place, which is the cause of the contraction of the muscle. In short, he supposes a complete similarity to

exist between a muscle, in a proper condition to exhibit this appear. ance, and a charged Leyden phial; the nerve of the former answering, as far as his experiments are concerned, the same purpose as the wire, which is connected with the internal surface of the latter.

Now, if this were just, such a muscle ought to contract, whenever a communication is formed between its internal surface and the nerve, by means of any conductor of electricity; and accordingly Mr. Volta, who to a certain extent adopts Mr. Galvani's theory, asserts this to be the case, as often as the experiment is made upon an animal which has been newly killed. But I am inclined to believe that he rests this assertion upon some general princi. ple, which he thinks established, and not upon particular facts; for he gives none in proof of it, and I have often held a nerve of an animal newly killed in one hand, while with the other I touched the muscle to which the nerve belonged, but never saw contractions by this means excited. I have also frequently taken hold of a nerve of an animal, which was recently killed, with a non-conductor of electricity, and have in this way applied its loose end to the external surface of the muscle which it entered, without ever observing motion to follow. I think, therefore, I am entitled to conclude, not only that the theory advanced by Mr. Galvani, respecting the cause of the muscular motions in his experiments, is erroneous; but also, that the influence, whatever its nature may be, by which they are excited, does not exist in a disengaged state in the muscles and nerves, previously to the application of metals. Should it be urged urged against this conclusion, that, since metals are much better conductors of electricity than moist substances, the charge of a muscle may be too weak to force its way through the latter, though it may be able to pass along the former; my answer is, that, in all Mr. Galvani's experiments, the nerve makes a part of the connecting medium between the two surfaces of the muscle, and that the power of no compound conductor can begreater than that of the worst conducting substance, which constitutes a part of it.

It may be said, however, that, although there is no proof that any influence naturally resides in the nerves or muscles, capable of producing the effects mentioned by Mr. Galvani, these substances may still, by some power independent of the properties they possess in common with dead matter, contribute to the excitement of the influence, which is so well known to exist in them, after a certain application of metals. Before I enter upon the discussion of this supposition, I must observe, that there are two cases of such an application of metals: the first is, when we employ only one metal; the second, when we employ two or more. With respect to the first case, a late author, Dr. Fowler, who seems to have made many experiments relative to this point, positively asserts, that he never saw a fair instance of motion being produced by the mere application of a single metal to a muscle and its nerve. I shall, therefore, defer treating this case, till I speak of the conditions which are necessary for the excitement of the influence. Nor will the present subject suffer from this delay; for if it be shewn, as I

expect it will, that, when two or more metals are used, the muscle and its nerve do not furnish any thing but what every other moist substance is equally capable of doing, it will, I think, be readily granted, that they can give nothing more, when only one metal is applied to them.

In regard to the second case, Mr. Volta has affirmed, or has said at least, what I regard as equivalent to affirming, that, when two metals are employed, the influence in question is excited by their action upon the mere moisture of the parts which they touch. The proofs, however, of this assertion were reserved for some future communication. But as more than two years have now elapsed since they were promised, and none have been given to this society, or have appeared, as far as I can learn, in any other way, I hope I shall not be thought precipitate, if, at this distance of time, I offer one of the same point, which seems to me both plain and decisive.

It is known, that, if a muscle and its nerve be covered with two pieces of the same metal, no motion will take place upon connecting those pieces, by means of one or more different metals. After making this experiment one day, I accidentally applied the metal I had used as the connector, and which I still held in one hand, to the coating of the muscle only, while with the other hand I touched the similar coating of the nerve, and was surprised to find that the muscle was immediately thrown into contraction. Having produced motions in this way sufficiently often to place the fact beyond doubt, I next began to consider its relations to other facts formerly known. I very soon perceived, that the

of exciting the influence, which in those experiments occasions the muscles to contract. But it ap pears rather extraordinary, that none of those, who contend for the identity of this influence and the electric fluid, have ever suspected, that the only very good dry conductor of the latter, which weknow, beside the metals, possesses like them the property of exciting the former. I confess, however, that it was not this consideration but accident, which led me to discover that charcoal is endowed with this property, and in such a degree that, along with zinc, it excites at least as strongly as gold with zinc, the most powerful combination, I believe, which can in this way be formed of the metals. But, to prevent disappointments, I must mention, that Call charcoal is not equally fit for this purpose, and that long keeping seems to diminish its power.

It being shewn that charcoal is also to be ranked among the exciters of this influence, I shall now (speak of the circumstances, in which both it and the metals must be 14 placed, to fit them for the exercise of their power. With respect to metals, Mr. Volta maintains, that, to this end, it is only necessary that two different species be applied to any other body which is a good conductor of electricity, and that a communication be established between the two metalic coatings. But charcoal is a much better conductor of electricity than water, and yet metals in contact with it alone will not excite. Again, Mr. Volta says, that the simple application of two metals to two parts of an ani. = mal, disturbs the equilibrium of the electric fluid, and disposes it to pass from one of the parts to the other, VOL, XXXVII:

which passage actually takes place, as soon as a conductor is applied between the metals. But what should prevent the passage of the fluid before the application of a new conductor, since the metals were already connected by means of the moisture of the animal? farther, a consequence of this opinion is, that, if the under surfaces of two different metals be placed in moisture, and their upper surfaces be afterwards connected by means of a nerve, still attached to its muscle, contractions ought then to be produced; since the whole quantity of the electric fluid, necessary to restore the equilibrium, which has been disturbed by the action of the metals, must pass through the nerve. This experiment I have made, and as I did not find the muscle to contract, I must hold Mr. Volta's opinion, on this point, to be likewise ill-founded. The fact is, that as far as the contraction of muscles is a test, whether the influence exists or not, and we have no other, it is never excited when two metals, or one metal and charcoal, are necessary for this purpose, unless these substances touch each other, and are also in contact with some of the fluids formerly mentioned.

But there is still another requisite for the excitement of the influence, which is a communication by means of some good conductor of electricity, between the two quantities of fluid, to which the dry exciters are applied, beside that which takes place between the same quantities of fluid, when the dry exciters are brought into contact with each other. As from this last circumstance, a complete circle of con. nection is formed among the different substances employed, it has been [*E] imagined