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it gives form without rigidity. On this account the ball yields to pressure, but quickly regains its form. It also serves as the basis of attachment for the muscles. If we compare the eye to a globular watch, then the sclerotic represents the outer case.
2. The more protuberant part of the ball is covered with a thick, strong, but very transparent membrane, called the cornea (C, Fig. 4). It corresponds to the crystal of the watch. Its function is to admit the light, and at the same time to refract it, so as to assist in forming the image, as will be explained hereafter.
3. Running across from the circle of junction of the cornea with the sclerotic, and cutting off the more protuberant clear part from the main part of the ball, and thus corresponding in position to the face of the watch, there is an opaque, colored plate called the iris, 1. It is the colored part of the eye, black, brown, blue, or gray, in different individuals. This transverse plate is not perfectly flat, but protrudes a little in the middle. In its center is a round hole, called the pupil, corresponding in position with the hole in the watch face for attachment of the hands. The pupil seems to be jet black, because the observer looks through the pupil into the dark interior of the ball. The function of the pupil is to admit, and at the same time regulate the amount of, light.
4. Linings.—Thus much is visible to the naked eye without dissection. But if the ball be now carefully opened, the part behind the iris is found to be lined with two thin membranes. (a.) Immediately in contact with the sclerotic is the choroid, a thin, vascular membrane, the anterior layer of which is colored with black pigment, which gives it a deep-brown, velvety appearance. Its function is to quench the light as soon
as it has done its work of impressing the retina. The choroid extends forward as far as the cornea. The anterior or forward portion of the choroid, separated from the sclerotic, drawn together as a curtain, and thickened by muscular tissue, forms the iris already described. Just before separating from the sclerotic to form the iris, it splits into two layers: one, the anterior, goes to form the iris, as already said, while the other, the postərior, is gathered into a circular, plaited curtain, or series of converging folds, which
T surrounds the outer margin of the lens (to be presently described) like a dark, plaited collar. These plaits, or folds, seventy to seventy-two in number, are called the ciliary SECTION OF Eve.–a, sclerotic ; 8,
cornea; c, conjunctiva; d, iris; e, processes (Fig. 5, and e, Fig.
lens; f, ciliary muscle behind the 20, p. 38). Beneath the outer dark ciliary processes; g, retina;
h, optic nerve. (After Cleland.) portion of this dark, plaited collar, and therefore in contact with the sclerotic, is a muscular collar, with radiating and circular fibers, called the ciliary muscle (Fig. 5, f, and Fig. 20, d). (6.) Within the choroid, innermost and most important of all, is the retina (Fig. 4, R). This is, in fact, a concave expansion of the optic nerve (0, Fig. 4). This nerve, coming from the brain, enters the eye-socket near its point, penctrates the sclerotic and the choroid, then spreads out within as a thin semi-transparent concave membrane of nerve-tissue, covering the whole interior of the ball as far forward as the ciliary collar. Its function is to receive and respond to the impressions of light. Its wonderful structure and functions will be explained hereafter.
5. Contents. The ball thus described is not hollow and empty, but filled with refractive media, as transparent as finest glass. These are :
(a.) Crystalline, or Lens.- Immediately behind the iris, and in contact with it, is found the crystalline. It is a flattened ellipsoid, or double convex lens, as clear as finest glass, about one third of an inch in diameter and one sixth of an inch in thickness, firm enough to handle easily, but elastic and easily yielding to pressure. On section it is found to consist of layers increasing in density from surface to center, as shown in Fig. 5, e, and in Fig. 13, on page 31. The lens is invested with a very thin, transparent membrane, capsule of the lens, which not only invests it, but continues outward as a curtain, to be attached to the sclerotic near the junction of the cornea. The elastic rigidity of the sclerotic pulls gently on this curtain and makes it taut, and the taut membrane in its turn presses gently on the elastic compressible crystalline and slightly flattens it. We shall see the importance of this when we come to speak of the adjustment of the eye for distance.
The perfect transparency of the lens is obviously necessary
for distinct vision; cataract, a common cause of blindness, arises from its opacity.
The lens, with its continuing curtain, completely divides the interior of the ball into two compartments, an anterior and a posterior.
(6.) The anterior chamber is filled with a clear, watery liquor, called the aqueous humor (Figs. 4 and 5), a small portion of which is behind the iris, but by far the larger portion between the iris and the cornea. The two parts are in connection through the pupil. If the cornea be punctured, the aqueous humor runs out, the clear protuberant part of the eye collapses, and the sight is for the time ruined. If, however, the wound heals without scar, or if the scar be to one side of the direct line of sight, the cornea will fill again and the sight may be recovered.
(c.) The posterior and much larger chamber is filled with a transparent, glassy substance, about the consistence of soft jelly, called the vitreous humor. This humor is in direct contact with the lens and curtain in front, and with the retina over its whole globular surface.
SECTION II.-FORJATION OF THE IMAGE.
The eyeball, as thus described, may be regarded as consisting essentially of two distinct portions, viz. : 1. A nervous expansion, the retina, specialized for responding to light-vibrations ; 2. An optical instrument, the lens apparatus, placed in front of the retina, and specially arranged to make the impression of light strong and definite, by means of an image. These two are entirely different in their origin. In embryonic development, the one is an outgrowth from the brain, the other an ingrowth from the epidermis and cutaneous tissues. These afterward meet and unite to form this wonderful organ.
Now the sole object of this complex instrument is the formation of a perfect image on the retina. Without images we would perceive light, but not objects; and distinctness of objects is exactly proportioned to distinctness of retinal images. If the image of an object is distinct, the object will be distinct; if the image is blurred, the object, both in outline and in details of surface, will be blurred. If there is no image, no object will be visible. Therefore the image must be a fac. simile of the real object, for the apparent object will be a fac-simile of the image.
The entire distinctness of these two parts of the eye may be made plain by an illustration. Suppose, then, that the whole instrumental apparatus of lenses were removed, leaving only the concave retina. If this could be retained in a healthy condition (which of course is practically impossible), it would be easy to make a glass instrument which, put into the concavity, would produce sight as perfect as, perhaps more perfect than, the natural eye.
Conditions of a Perfect Image.- A serviceable image must be sufficiently bright, and perfectly sharp and distinct in outline and surface details. Brightness only requires a sufficient amount of light. In order to be perfectly distinct, it is necessary that rays from different points in the object, even the most contiguous, should not mix on the image, but all the rays from each point on the object must be carried to its own point on the image. Now, it is impossible that both of these conditions should be fulfilled, except by some such arrangement as we find in the eye.
For see : suppose the light to enter by a hole only, like the pupil; and, further, in order that there be light enough, let the hole be somewhat large; then the light, diverging from any point, b, Fig. 6, A, of the object a b c, and entering the hole of diaphragm d d, will form a diverging pencil, and spread out over the whole circle b', on the screen & 8. Similarly, the rays from a will spread out and form the circle a', and from c the circle d'. Thus it is seen that rays from widely differ