Obfervations on the Structure of the Eyes of Birds; by Mr. Pierce Smith, Student of Phyfic; from the Philofophical Tranfactions.

HILE examining the eyes of

fingular ftructure, which I believe has not been hitherto noticed; and though not the object I had in view, in the examination, it will perhaps elucidate feveral remarkable circumftances in the natural hiftory of thefe animals, and may ultimately be applied to the eyes of other animals, and add one additional difcovery to thofe already made on this beautifully conftructed organ.

In March, 1792, I oblerved, while diflecting the eyes of birds, an irregular appearance of the fclerotica, in that part of it which immediately furrounds the cornea; and which in them is generally flat. On a more minute examination, it appeared to be fcales lying over each other, and which appeared capable of motion on each other. Thele appearances I fhewed to Dr. Fowler, of London, and likewife to Mr. Thomson, furgeon, Edinburgh. In June, this paper was copied out, at my request, by Mr. Irving, who refided in the fame houfe with me. On inveftigating this fingular structure, the fcales were found to be of bony hard

nefs, at least much more so than any other part of the sclerotica. On the infide of the fclerotic coat of the eye there was no appearance of thefe fcales, that part of it being fimilar to the reft of the sclerotica.

fpreading over the fcales, and terminating at laft in forming the four recti mufcles belonging to the eye; fo that, upon the contraction of these muscles, motion of the fcales would be produced. This imbricated appearance of part of the fclerotica, and the detection of the tendinous fibres fpreading over fcales, terminating at last in the four recti muscles, led me to confider the ufe of this structure, what would be the effect of motion of the fcales upon the vifion of birds, and how far this can be applied to other animals.

It is a fact fo well known to per fons acquainted with optics, that it is almost unneceffary to mention it, that the rays of light, paffing through a lens, will be refracted to a point or focus beyond the lens, and this focus will be lefs diftant, in proportion as the lens approaches, to a fphere in fhape. Now this principle is very naturally applied to the explanation of the ufe of this apparatus. These scales lying each partly over the next, fo as to allow of motion, will, on the con

traction

traction of the recti muscles, inferted into and covering them, move over each other, and thus the circle of the fclerotica will be diminished, and of course the cornea, which is immediately within the circle made by thefe fcales, will be prefled forwards, or, in other words, rendered more convex, and thus the focus of the eye becomes altered, its axis being elongated. This conftruction, and confequent convexity of the cornea, muft render fmall objects near the animal very diftinct. ·

On thefe mufcles relaxing, the elafticity of the fclerotic coat will reftore the cornea to its original flatnefs; it thus becomes fitted for viewing objects placed at a greater diftance from the eye, and this will be in proportion to the degree of relaxation.

There feems to exift in nature an economy of motion, to prevent fa tigue and exhauftion of the animal powers by continued voluntary mufcular action. If two oppofite actions of the fame frequency occur in two muscles, the one being antagonift to the other, the action of one ceafing, the action of the other muft take place previously to farther motion of the part; for inftance, on the biceps flexor of the arm acting, the arm will be bent, but on difcontinuing its action the arm will remain in the fame ftate, unlefs it was ftraightened by the action of the biceps exterior, its antagonist; but where one action in a part is required to take place almoft conftantly, and the oppolite action but feldom, to fave the animal from fatigue neceflarily induced by mafcular contraction, the gives an elaftic ligament, which from, its elafticity may be faid to be in continual action, without exhaufting the, animal, Thus, when

the oppofite action, which is of left frequent occurrence, is required, it is performed by overcoming the refiftence, or elafticity of this elaftic ligament, which, on the muscle giving over its action again, refumes its former ftate. The claffic cartilages of the ribs performing, in fome degree, the functions of a mulcle, are of ufe in refpiration; likewife the elaftic ligaments which fupport the claws of all the feline genus, keeping them from friction against the ground. Thefe claws, at the volition of the animal, by mufcles арpropriated for that purpose, are brought into action or extended. From the abovementioned ftructure, the fame thing appears to take place in the eyes of animals. When an animal is defirous of feeing minate objects, the recti mufcles act, and thus, by rendering the eye more conves, enlarge the angle under which the object is feen. How neceffary is this ftructure to thefe animals in particular; for without it a bird would be continually expofed to have its head dafhed against a tree when flying in a thick foreft, its motions being too rapid for the common structure of the eve. eagle, when foaring high in the air, obferves fmall objects on the earth below him, inconceivable to us, and darts upon them inftantaneouily. Here we must allow that there muft be an extraordinary alteration in the focus in this eye, in almoft an ́inftant of time. How could this be performed unless th animal had this apparatas? The eyes of quadrupeds, as I thall afterwards fhew, can perform this alteration though not in the fame degice. as it is not neceffary, their modes e: life being different. A fwallow, failing through the air, purties a

The

great

ever we find the fubfiftence or fafety of an animal intrufted to, or depending more particularly upon, one fenfe than the reft, we are fure to find that fenfe proportionably perfect; as in quadrupeds, the organ of fmelling is remarkably perfect, and leads them to their prey; fo the eyes of birds are proportionably perfect, being the means, not only of their fupport, but from them they receive the firft intimation of approaching danger.

The eyes of Birds, like thofe of other animals, confift of three coats, the felerotica, choroides, and retina. The human eve, as well as thofe of quadrupeds, is nearly fpherical; in birds the fphere is more oblate, the fclerotica, as it approaches the cornea, becoming fuddenly flat. The cornea though fmall, when com pared with the fize of the whole eye is more convex, as it forms the fegment of a fmaller circle added to the larger formed by the fclerotica. The realon or advantage of this flatnefs is not very evident. It prevents them, perhaps, from projecting fo far as to expofe them to danger from the trees and grafs amongst which thefe animals live.

After having examined the eyes of birds, and teeing this curious apparatus, I was next led to the examination of the eyes of quadrupeds, that I might fee in what manner

they resembled the eyes of birds, ing able to accommodate their eyes to objects at different distances.

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

reci mufcles of the eye of a fheep; and by tearing and diffecting, 1 found that it terminated in, and with the other parts compofed, the cornea; fo that on the firft volition of the mind, the recti mufcles on contracting will have the power of fixing the eye, and keeping it fieady, and at the fame time by contracting more or lefs, will adapt the focus of the eye to the dittance of the object, but in a leís degree than in birds. On thefe mufcles giving over acting, the eye will be rettored to its former ftate by the elafticity of the fclerotic coat.

From a knowledge of thefe circumftances, we may, from rational principles, explain, why people, by being long accuftomed to view fmall objects, obtain, in time, a fort of microfcopic power, if it may be fo called; that is, the mufcles which contract the cornea, will, by cuttom, increase their power of action, and grow ftronger, like the other mufcles of the body; other phenomena of vifion on these principles may be explained.

roneous, but has also made known feveral important facts, in addition to those which had been discovered by that author. As he appears, however, from these letters, to have fallen into fome mistakes himself, and has certainly not exhaufted the fubject which he has treated in them, I fhall venture to communi. eate to this learned body a few obfervations I have made refpecting it, which may contribute both to correct his errors, and to increafe our knowledge of the cause of thofe motions, which have been attributed by Mr. Galvani and others to an animal electricity. Thefe obfervations will be fo arranged, as to furnish answers, more or lefs fatisfactory, to the following questions: does the incitement of the influence which, in Mr. Galvani's experiments, occafions the muscles of animals to contract, either wholly, or in part, depend upon any peculiar property of living bodies? What are the conditions neceflary 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 furface and the nerve which penetrates it, Mr. Galvani, contends that, previously to this effect, the inner and outer parts of the mufcle contain different quantities of the electric fluid; that the nerve is confequently in the fame ftate, with respect to that fluid, as the internal fubftance of the muscle; and that upon the application of one or more metals between its outer furface and the nerve, an electrical difcharge takes place, which is the caufe of the contrac tion of the mufcle. In fhort, he fuppofes a complete fimilarity to

exist between a muscle, in a proper condition to exhibit this appearance, and a charged Leyden phial; the nerve of the former anfwering, as far as his experiments are concerned, the fame purpose as the wire, which is connected with the internal furface of the latter.

Now, if this were just, fuch a muscle ought to contract, whenever a communication is formed between its internal furface and the nerve, by means of any conductor of electricity; and accordingly Mr. Volte, who to a certain extent adopts Mr. Galvani's theory, afferts this to be the cafe, as often as the experiment is made upon an animal which has been newly killed. But I am inclined to believe that he refts this affertion upon fome 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 mufcle to which the nerve belonged, bat never faw contractions by this means excited. I have alfo frequently taken hold of a nerve of an ani mal, which was recently killed, with a non-conductor of electricity, and have in this way applied its loofe end to the external furface of the muscle which it entered, without ever obferving motion to follow. I think, therefore, I am entitled to conclude, not only that the theory advanced by Mr. Galvani, refpeding the caufe of the mufcular notions in his experiments, is erro neous; but also, that the influence, whatever its nature may be, by which they are excited, does not exift in a difengaged state in the mufcles and nerves, previously to the application of metals. Should it be

urged

urged against this conclufion, that, fince metals are much better conductors of electricity than moift fubftances, the charge of a mufcle may be too weak to force its way through the latter, though it may be able to pafs 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 furfaces of the mufcle, and that the power of no compound conductor can be greater than that of the worst conducting fubftance, which conftitutes a part of it.

It may be faid, however, that, although there is no proof that any influence naturally refides in the nerves or muscles, capable of producing the effects mentioned by Mr. Galvani, thefe fubftances may ftill, by fome power independent of the properties they poffefs in common with dead matter, contribute to the excitement of the influence, which is fo well known to exift in them, after a certain application of metals. Before I enter upon the difcuffion of this fuppofition, I must obferve, that there are two cafes of fuch an application of metals: the firft is, when we employ only one metal; the fecond, when we employ two or more. With refpect to the firft cafe, a late author, Dr. Fowler, who feems to have made many experiments relative to this point, pofitively afferts, that he never faw a fair instance of motion being produced by the mere application of a fingle metal to a muscle and its nerve. I fhall, therefore, defer treating this cafe, till I fpeak of the conditions which are neceffary for the excitement of the influence, Nor will the prefent fubject fuffer from this delay; for if it be fhewn, as I expect it will, that, when two

or more metals are used, the mufcle and its nerve do not furnish any thing but what every other moist fubftance 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 cafe, Mr. Volta has affirmed, or has faid at leaft, what I regard as equivalent to affirming, that, when two metals are employed, the influence in queftion is excited by their action upon the mere moisture of the parts which they touch. The proofs, however, of this affertion were referved for fome future communication. But as more than two years have now elapfed fince they were promifed, and none have been given to this fociety, or have appeared, as far as I can learn, in any other way, I hope I fhall not be thought preci pitate, if, at this diftance of time, I offer one of the fame point, which feems to me both plain and decifive.

It is known, that, if a mufcle and its nerve be covered with two pieces of the fame metal, no motion will take place upon connecting thofe pieces, by means of one or more different metals. After making this experiment one day, I accidentally applied the metal I had ufed as the connector, and which I ftill held in one hand, to the coating of the mufcle only, while with the other hand I touched the fimilar coating of the nerve, and was furprised to find that the mufcle was immediately thrown into contraction. Having produced motions in this way fufficient ly often to place the fact beyond doubt, I next began to confider its relations to other facts formerly known. I very foon perceived, that

the