Popular Education / For the use of Parents and Teachers, and for Young Persons of Both Sexes

by Ira Mayhew

The Sense of Sight.—The sense of sight, which is the most refined and admirable of all the senses, still remains to be considered. The senses generally serve as interpreters between the material universe without and the spirit within. But it is more especially by the sense of sight that we are enabled to hold converse with the external world. Without it we should be deprived of a large portion of the pleasures of life not only, but even of the means of maintaining our existence. It is through the sense of vision that the wisdom, power, and benevolence of the Deity are chiefly manifested to us.

I shall describe the apparatus of vision only so far as is necessary in order to subserve my leading object, which is the preservation and improvement of this sense, and the means of rendering it tributary to intellectual and moral culture. The eye, which is the organ of vision, is an optical instrument of the most perfect construction. It is surrounded by coats, which contain refracting mediums, called humors. There are three coats, called the sclerotic, the choroid, and the retina; and three humors, called the aqueous, the crystalline, and the vitreous.

The sclerotic or outer coat, called also the white of the eye, is an opaque, fibrous membrane. It has almost the firmness of leather, possesses little sensibility, and is rarely exposed to inflammation or other diseases. It invests the eye on every side except the front, and besides maintaining its globular form and preserving its internal and delicate structure, serves for the attachment of those muscles which move this organ. The opening in the fore part of this opaque coat is filled by the transparent cornea, which resembles a watch crystal in shape, and is received into a groove in the front part of the sclerotic coat in the same manner that a watch-glass is received into its case. But for this arrangement light could not gain admission to the eye.

The choroid coat, which constitutes the second investing membrane of the eye, is of a dark brown color upon its outer surface, and of a deep black within. The internal surface of this membrane secretes a dark substance resembling black paint, upon which the retina is spread out, and which is of great importance in the function of vision, as it seems to absorb the rays of light immediately after they have struck upon the sensible surface of the retina.

The retina, which is the third and innermost membrane of the eye, is the expansion of the optic nerve, and constitutes the immediate seat of vision. Such is the arrangement of the humors of the eye, and so perfectly are they adapted to the functions they are called upon to perform, that in the healthy state of this organ, the light entering the pupil is so refracted as to paint upon the retina an exact image of the objects from which it proceeds. The optic nerve, whose expansion forms the retina, receives this image and transmits it to the mind.

Arnott has well remarked, that "a whole printed sheet of a newspaper may be represented on the retina on less surface than that of a finger nail; and yet not only shall every word and letter be separately perceivable, but even any imperfection of a single letter. Or, more wonderful still, when at night an eye is turned up to the blue vault of heaven, there is portrayed on the little concave of the retina the boundless concave of the sky, with every object in its just proportions. There a moon in beautiful miniature may be sailing among her white-edged clouds, and surrounded by a thousand twinkling stars, so that to an animalcule supposed to be within and near the pupil, the retina might appear another starry firmament with all its glory."

Besides these three coats, and the cornea which constitutes about one fifth of the anterior portion of the outer coat, it is necessary to notice the iris, so called from its variety of color in different persons, and upon which alone the color of the eye depends. The iris is a circular membrane situated just behind the cornea, and is attached to one of the coats at its circumference. In its center is a small round hole, called the pupil; and sometimes spoken of familiarly as the sight of the eye, as no light can enter the eye except through it. The iris possesses the power of dilating and contracting, so as to admit more or less light, as it may be needed. This change in the size of the pupil is effected by two sets of muscular fibers. The first set converge from the circumference of the iris to the circular margin of the pupil, and constitute the radiated muscle. The outer ends of these fibers are attached to the sclerotic coat, which is unyielding; hence, when they contract, the pupil enlarges to receive more light. The other set is composed of circular fibers, which go round in the iris from the border to the pupil, and constitute the orbicular muscle, the contraction of which diminishes the size of the pupil. When too much light enters the eye, the excited and sensitive retina immediately gives warning of the danger, and the nerves, which are plentifully distributed to the iris, stimulate the orbicular muscle to contract, and the radiated one to relax, by which the size of the pupil is lessened. But when the light which enters the pupil is insufficient to transmit a distinct image of objects to the brain, the orbicular muscle relaxes, and the radiated one contracts, so as to enlarge the pupil. The contraction of the pupil is readily seen when a person passes from a darkened room into a bright sunlight, or when a light is first brought into a room in the twilight of evening. Any person may notice this contraction in his own eye by beholding himself in a glass immediately after passing from a dark to a well-lighted room. So, also, when a person looks at an object near the eye, the pupil contracts, but when he looks at an object more remote, it dilates. The muscles of the iris are somewhat under the control of the will; for most persons can contract or dilate the pupil, in some degree, at pleasure. Some persons possess this faculty to a great extent.

The three humors of the eye have been compared to the glasses of a telescope, and the coats to the tube, which keeps them in their places. The aqueous humor is situated in the fore part of the eye, and is divided by the iris into what are called the anterior and posterior chambers of the eye. The crystalline humor, or lens, is situated immediately behind the aqueous humor, a short distance back of the pupil, and is a perfectly transparent double convex lens, closely resembling in shape the common burning glass. This resemblance does not stop here; for this lens, like the burning glass, possesses the property of converging the rays of light which fall upon it, and bringing them to a focus. When this lens becomes so opaque as to obstruct the passage of light, either partially or entirely, a person is said to have a cataract. This can be cured only by a surgical operation. The vitreous humor, situated back of the other two, forms the principal part of the globe of the eye. It differs from the aqueous in one important particular. When that is discharged in extracting the crystalline lens for cataract or otherwise, it will be restored again in a few hours, and the eye will continue to perform its function. But if this be discharged by accident, the eye is irrecoverably lost. This, however, does not often occur; for, as we shall presently see, the eye is admirably fortified.

The eye is a perfect optical instrument, infinitely surpassing all specimens of human skill. This is true, view it in what light we may. It not only possesses the power of so adjusting its parts as to adapt it to the examination of objects at different distances, and in light of different degrees of intensity, but we are enabled to direct it at will to objects above, beneath, or around us.

The various motions of the eye are produced by six little muscles. These are attached at one extremity to the immovable bones of the orbit, while at the other extremity they are inserted into the sclerotic coat, four of them near its junction with the cornea, by broad, thin tendons, which give to the white of the eye its pearly appearance. These muscles are so arranged by the matchless skill of the Architect as to enable the beholder to direct the eye to any object he chooses, and to hold it there for any length of time that is compatible with the laws by which muscular exercise should be regulated. By the slight or intense action of four of these, called the straight muscles, the eye is less or more compressed, and the relative positions of its humors are by this means so nicely adjusted as to enable us to view objects near by or at a distance. The other two are called oblique muscles, one of which, with its long tendon passing through a cartilaginous loop, acts upon the principle of the fixed pulley, and turns the eye in a direction contrary to its own action. When the external muscle becomes too short, the eye turns out; but if the internal muscle is unduly contracted, the eye turns inward, toward the nose. One eye is sometimes turned up or down, but this is of less frequent occurrence.