CHAPTER IX.

THE EYE-ITS AMAZING STRUCTURE-INSTRUCTIVE FABLE.

THE human eye is a most admirable piece of mechanism. It occupies a commanding situation, being placed in the upper part of the face, and, like a sentinel posted on a hill, it perceives at a glance every thing around. The purposes to which this exquisite instrument is adapted, defy enumeration.

The

eye

is nearly spherical. Its first or outer coat is the white part, called sclerotica, marked on the figure

a a. It is every where opaque, except the small circular part in front called the cornea, b b, which is transparent, and fixed in the sclerotica like a glass in a watch-case; while to increase the power of vision, the anterior part of the eye projects and forms a large segment of a small circle.

The iris is the coloured circle which surrounds the pupil, varying in different persons, so that it is sometimes hazel, and in others black or gray. It is said to be the most acutely sensible part of the body, and is an opaque curtain in the inner chamber of the eye. The pupil in the centre of the iris, may be so enlarged or contracted, as to admit a larger or smaller stream, according to the intensity of light. How admirable the provision that its diameter should be lessened when a very luminous object is presented to the eye; and that it should be increased in a faint light, in order that a greater number of rays may be admitted! The pain sometimes experienced when we suddenly come from darkness into a strong light, arises from the pupil being burdened by an undue quantity of rays before it has time to contract. In like manner, when we go from a strong light into obscurity, we at first imagine ourselves in total darkness, because sufficient rays cannot enter the contracted pupil to enable us to distinguish objects ;

but, in a few minutes, the pupil dilates, and objects before invisible are clearly perceived.

e

Within the inner coverings of the eye are three humours. First appears the aqueous humour c c, of a watery consistence and perfectly fluid, that the iris may move in it with the utmost freedom. Immediately behind the pupil is the crystalline humour d; it collects the rays of light like a double convex lens, that they may thus be concentrated and a perfect image formed. At the back of this lens is the vitreous humour e, distending the ball of the eye into its spherical form, so that it may easily move in its orbit. Each of these humours has its distinct character, and according to their density they have different powers of refracting the rays of light, but all serving, in their united operation, to give the most correct image of the object, and in its truest colours.

This appears exquisitely minute and distinct indeed, on the retina f, an expansion of the optic nerve g, and the immediate seat of sensation. It is a membrane of the most delicate texture of any in the human frame; according to Sir Charles Bell, it is transparent in a recent state, and so soft that it will tear with its own weight.

Most astonishing is the sensibility with which it is endowed, as proved by the great degree of light which the eye can bear, and the low degree of light at which objects are visible. Thus the proportion between the degrees of light illuminating one by the sun and by the moon, at any equal altitude, is calculated at 90,000 to 1. It has also been shown that if we see the sail of a windmill six feet in diameter, at the distance of 4000 toises or fathoms, the eye being supposed to be an inch in diameter, the picture of this sail at the bottom of the eye will be one-eight thousandth part of an inch, which is the six hundred and sixty-sixth part of a line, or about the sixty-sixth part of a common hair!

Such, then, is a general description of the beautiful and admirable optical instrument which the Great Artificer has placed in its socket, or orbit, formed of seven bones; and in which is deposited a loose fat, so that the eye may move as in a soft and warm bed. So

remarkably smooth is it, that though the eye performs thousands of movements every day, for seventy or eighty years, yet not the least injury is discoverable. But even this tender care is not enough.

On the skin that covers

these bones are hairs, which preserve the eye from dust and other inconveniences. As the cornea is constantly exposed to the action of the air, it is liable to become dry and parched, which would greatly injure its transparency, and consequently render the eye useless. Provision is therefore made for this necessity by the eyelids, which also afford a covering for defence, and for the exclusion of the rays of light. Other parts of this remarkable structure are no less admirable.

Thus the fore-part of the globe of the eye is connected with the surrounding integuments by a membrane, abundantly supplied with nerves, for the purpose of giving it that exquisite sensibility which is required to warn the eye of danger from the intrusion of foreign bodies. On arriving at the base of the eyelids, this membrane is folded forwards, so as to line their inner surfaces, and to be continuous with the skin that covers their outer sides. The surface of this membrane, called the conjunctiva, and also that of the cornea, are kept constantly moist by the tears, which are as constantly secreted by the lachrymal glands.