Observations on the Structure of the eyes of Birds.

By Mr. Pierce Smith, Student of Physic. From Philosophical Transactions of the Royal Society of London, vol. 85 (1795), pp. 263ff

In March, 1792, 1 observed, while dissecting the eyes of birds, an irregular appearance of the sclerotica, in that part of it which immediately surrounds the cornea, and which in them is generally flat. On a more minute examination, it appeared to be scales lying over each other, and which appeared capable of motion on each other. These appearances I showed to Dr. Fowler of London, and to Mr. Thomson, surgeon, Edinburgh. In June, this paper was copied out at my request, by Mr. Irving, who resided in the same house with me. On investigating this singular structure, the scales were found to be of bony hardness, at least much more so than any other part of the sclerotica. On the inside of the sclerotic coat of the eye there was no appearance of these scales, that part of it being similar to the rest of the sclerotica. Tendinous fibres were detected., spreading over the scales, and terminating at last in forming the 4 recti muscles belonging to the eye; so that on the contraction of these muscles) motion of the scales would be produced. This imbricated appearance, and the detection of the tendinous fibres spreading over scales terminating at last in the 4 recti muscles, led me to consider the use of this structure, what would be the effect of motion of the scales on the vision of birds, and how far this can be applied to other animals.

It is a fact so well known to persons acquainted with optics, that it is almost unnecessary to mention it, that the rays of light passing through a lens, will be refracted to a point or focus beyond the lens and this focus will be less distant in proportion as the lens approaches a sphere in shape. Now this principle is very naturally applied to the explanation of the use of this apparatus. These scales lying each partly over the next, so as to allow of motion, will on the contraction of the recti muscles inserted into and covering them, move over each other, and thus the circle of the sclerotica, will be diminished, and of course the cornea which is immediately within the circle made by these scales will lie pressed forwards, or in other words rendered more convex, and thus the focus of the eye becomes altered, its axis being elongated. This construction and consequent convexity of the cornea, must render small objects near the animal very distinct. On the muscles relaxing, the elasticity of the sclerotic coat will restore the cornea to its original flatness; it thus becomes fitted for viewing objects placed at a greater distance from the eye, and this will be in proportion to the degree of relaxation.

There seems to exist in nature an economy of motion, to prevent fatigue and exhaustion of the animal powers, by continued voluntary muscular motion. If 2 opposite actions of the same frequency occur in 2 muscles, the one being antagonist to the other, the action of one ceasing, the action of the other must take place previously to further motion of the part; for instance, on the biceps flexor of the arm acting, the arm will be bent, but on discontinuing its action the arm will remain in the same state, unless it was straightened by the action of the biceps extensor its antagonist; but where one action in a part is required to take place almost constantly, and the opposite action but seldom, to save the animal from fatigue, necessarily induced by muscular contraction, she gives an elastic ligament, which from its elasticity may be said to be in continual motion without exhausting the animal. Thus when the opposite action which is of less frequent occurrence is required, it is performed by overcoming the resistance, or elasticity of this elastic ligament, which on the muscle giving over its action again, resumes its former state. The elastic cartilages of the ribs performing in some degree the function of a muscle, are of use in respiration; likewise the elastic ligaments which support the claws of all the feline genus, keeping them from friction against the ground. These claws at the volition of the animal, by muscles appropriated for that purpose, are brought into action or extended. From the above-mentioned structure, the same thing appears to take place in the eyes of animals. When an animal is desirous of seeing minute objects, the recti muscles act, and thus, by rendering the eye more convex enlarge the angle under which the object is seen. How necessary is this structure to these animals in particular; for without it a bird would be continually exposed to have its head dashed against a tree when flying in a thick forest, its motions being too rapid for the common structure of the eye. The eagle, when soaring high in the air, observes small objects on the earth below him, inconceivable to us, and darts upon them instantaneously. Here we must allow that there must be an extraordinary alteration in the focus of this eye in almost an instant of time. How could this be performed unless the animal had this apparatus? The eyes of quadrupeds, as I shall afterwards show, can perform this alteration; though not in the same degree, as it is not necessary, their modes of life being different. A swallow sailing through the air pursues a gnat or small fly to almost certain deatruction. This apparatus is very distinct in all these birds. Whenever we find the subsistence or safety of an animal entrusted to, or depending more particularly on 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 3 coats, 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 sinaller 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, among which these animals live.

As no description, however accurate, can give an idea of the structure of any part of the animal body, I have caused small sketches to be made explaining all the different circumstances mentioned in the paper.

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 resembled 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 involved in much difficulty, as the eyes of quadrupeds appeared on examination not to have these imbricated scales, which are so obvious in birds; but all this difficulty vanished on taking hold of one of the 4 recti muscles of the eye of a sheep; 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 circumstances, 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.

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