This Structure of the Human Eye

The eye is one of the nicest pieces of mechanism which the human understanding can contemplate; but as it requires a knowledge of its anatomical structure, and of the principles of optics, to enable us to appreciate its admirable functions, I shall confine myself to a few general descriptions and remarks.

The eye is nearly of a globular form. It consists chiefly of three coats and three humors. The first or outer coat is termed sclerotica; it is everywhere white and opaque, and is joined, at its anterior edge, to another which has more convexity than any other part of the globe of the eye, and, being exceedingly transparent, is called the cornea. These two parts are perfectly different in their structure, and are supposed, by some anatomists, to be as distinct from each other as the glass of a watch is from the case into which it is affixed.—Next within this coat is that called the choroides, on account of its being furnished with a great number of vessels. It serves, as it were, for a lining to the other, and is joined with that part of the eye termed the iris. The iris is an opaque page 38 membrane like the choroides, but of different colors in different eyes, as gray, black, or hazel. It is composed of two sets of muscular fibers, the one of a circular form, which contracts the hole in the middle, called the pupil, when the light is too strong for the eye; and the other of radial fibers, tending everywhere from the circumference of the iris toward the middle of the pupil; which fibers, by their contractions, dilate and enlarge the pupil, when the light is weak, in order to let in more of its rays.—The third coat is called the retina, upon which are painted the images of all visible objects, by the rays of light which flow from them. It spreads like network all over the inside of the choroides, and is nothing more than a fine expansion of the optic nerve; by which nerve the impressions of visible objects are conveyed to the brain.

The inside of the globe of the eye, within these tunics or coats, is filled with three humors called the aqueous, the crystalline, and the vitreous. The aqueous humor lies at the forepart of the eye, and occupies all the space between the crystalline and the prominent cornea. It has the same specific gravity and refractive power as water, and seems chiefly of use to prevent the crystalline from being easily bruised by rubbing, or by a blow—and perhaps it serves for the crystalline humor to move forward in while we view near objects; and backward, for remoter objects; without which, or some other mechanism effecting the same purpose, we could not, according to the law of optics, perceive objects distinctly when placed at different distances.—Behind the aqueous lies the crystalline humor, which is shaped like a double convex glass, and is a little more convex on the back than on the forepart. This humor is transparent like crystal, is nearly of the consistence of hard jelly, and converges the rays which pass through it from visible objects, to its focus at the bottom or back part of the eye.—The vitreous humor lies behind the crystalline, and fills up the greater part of the orb of the eye, giving it a globular shape. It is nearly of the consistence of the white of an egg, and very transparent; its forepart is concave, for the crystalline humor to lodge in, and its back part being convex, the retina is spread over it. It serves as a medium to keep the crystalline humor and the retina at a due distance. From what has been now stated, it is obvious, that the images of external objects are depicted on the retina, in an inverted position, in the same manner as the images formed by a common convex lens; but how the mind, in this case, perceives objects erect, is a question, about which the learned have been divided in their opinions.^*

The ball of the eye, as now described, is situated in a bony cavity, called its orbit, composed by the junction of seven different bones, hollowed out at their edges. This cavity is, in all the vacant spaces, filled with a loose fat, which serves as a proper medium for the eye to rest in, and as a socket in which it may move. It is sheltered by the eyebrows, which are provided with hair, to prevent the descending sweat of the forehead from running down into it. As a still further protection to this delicate organ, it is furnished with the eyelid, which, like a curtain, is drawn over it with inconceivable swiftness, for its security on the approach of danger. It also serves to wipe from it superfluous moisture, and to cover it during sleep. In the upper part of its orbit it is furnished with a gland, to supply it with water sufficient to wash off dust, and to keep its outer surface moist, without which the cornea would be less transparent, and the rays of light would be disturbed in their passage; and the superfluous water is conveyed to the nose through a perforation in the bone.

For the purpose of enabling the eye to move in its socket, six muscles are provided. These are admirably contrived to move it in every direction, upward or downward, to the right or to the left, or in whatever direction the occasion may require; and thus we are spared the trouble of turning our heads continually toward the objects we wish to inspect. If we want to look upward, one of these muscles lifts up the orb of the eye; if we would cast our eyes to the ground, another muscle pulls them down. A third muscle moves the globe outward toward the temples, and a fourth draws it toward the nose. A fifth, which slides within a cartilaginous ring, like a cord over a pulley, and is fastened to the globe of the eye in two points, makes it roll about at pleasure A sixth lies under the eye, and is designed to temper and restrain within proper bounds the action of the rest, to keep it steadily fixed on the object it beholds, and to prevent those frightful contortions which otherwise might take place.^* By these, and a multitude of other mechanical contrivances, all acting in harmonious combination, the eye, as a natural telescope and microscope, is made to advance, to recede, to move to the right, and to the left, and in every other direction; and to view near and distant objects with equal distinctness so that a single eye, by the variety of positions if may assume, performs the office of a thousand.^†

The utility of these several movements, and the pain and inconvenience which would be suffered were any of them wanting, can scarcely be conceived by any one whose eyes have always remained in a sound state. We are so much accustomed to the regular exercise of our visual organs, that we seldom reflect on the numerous delicate springs which must be set in action, before the functions of vision can with ease be performed. But were any one of the muscular organs, now described, to fail in its functions, we should soon experience so many inconveniences, as would throw a gloom on all the other comforts of life; and convince us how much we are indebted, every moment, to the provident care and goodness of our beneficent Creator, for thousands of enjoyments which we seldom think of, and for which we are never sufficiently grateful. “With much compassion, as well as astonishment, at the goodness of our loving Creator,” says Dr. Nieuwentyt, “have I considered the sad state of a certain gentleman, who, as to the rest, was in pretty good health, but only wanted the use of those two little muscles that serve to lift up the eyelid, and so had almost lost the use of his sight—being forced, as long as this defect lasted, to shove up his eyelids every moment with his own hands.”^‡

Such are only a few general outlines of the structure of the eye; for no notice has been taken of the numerous minute veins, arteries, nerves, lymphatics, glands, and many other particulars which are connected with this organ. But all this delicate and complicated apparatus in the structure of the eye would have been of no use whatever for the purpose of vision, had not a distinct substance been created to act upon it, exactly adapted to its nature and functions. In order that the eye might serve as the medium of our perceptions of visible objects, light was formed, and made to travel from its source at the rate of 192,000 miles in a second of time. This prodigious velocity of light is doubtless essential to the nature of vision; since it actually exists, and since we find that it radiates with the same swiftness from the most distant visible star, as from the sun which enlightens our system. To abate the force of the amazing velocity, its particles have been formed almost infinitely small—a circumstance which alone prevents this delightful visitant from becoming the most tremendous and destructive element in nature. Dr. Nieuwentyt has computed that, in one second of time, there flows 418,660,000,000,000,000,000,000,000,000,000,000,000,000,000^* particles of light out of a burning candle, which number contains at least 6,337,242,000,000 times the number of grains of sand in the whole earth, supposing every cubic inch of the earth to contain a million of grains. It has been justly remarked by Mr. Ferguson, and other authors, that “if the particles of light were so large, that a million of them were equal in bulk to an ordinary grain of sand, we durst no more open our eyes to the light, than suffer sand to be shot point-blank against them from the mouth of a cannon.” It may also be remarked, that the property which all bodies possess of reflecting light, is essential to the purpose of vision, without which the splendid and variegated scene of nature would be changed into a dreadful gloom; and were the rays of light of one uniform color, and not compounded of various hues, one object could not be distinguished from another, and the beautiful aspect of our globe would instantly disappear.

Thus we see that the eye is adapted to light, and light to the eye; and in this admirable adaptation the wisdom of the Creator is strikingly displayed. For light has no effect upon the ear, or upon any other organ of sensation, so as to produce a perception of visible objects; as, on the other hand, the undulations of the air have no effect upon the eye, so as to produce the sensation of sound. The eye did not produce the light, nor did the light form the eye; they are perfectly distinct from each other, yet so nicely adapted in every particular, that had any one quality or circumstance been wanting in either, the functions of vision could not have been performed in the manner in which they now operate; which strikingly demonstrates, that one and the same Intelligent Being, possessed of a wisdom beyond our comprehension, formed the curious structure of the eye, and indued the rays of light with those properties of color, motion, and minuteness, which are calculated, through the medium of this organ, to produce, in sentient beings, the ideas of visible objects. And, surely, he never intended that such exquisite skill and contrivance should be altogether overlooked by rational beings, for whose pleasure and enjoyment all this benevolent care is exercised.