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Besides supplying nutrition to the non-vascular central parts of the eyeball, the choroid is useful in vision by preventing the reflection of the light from the background of the eye in such a way as would cause irregularity of its distribution, and thus dazzle and interfere with the distinctness of the image. The choroid is elastic, and can move over the neighboring sclerotic; it can be drawn forward by the contraction of the radiating ciliary muscle, which acts as a tensor of the choroid membrane.

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Showing the course of the fibres of the optic nerve, N, as they pass along the inner sur.

face of the retina, R, to meet the ganglion cells, g, whence special communications pass outward to the layer of rods and cones in the pigment layer, p, next the choroid, c, within the sclerotic, S.

The iris has a special power of motion, by means of which the opening in it can be made smaller, so as to regulate the amount of light admitted to the eye, and cut off more or less of the rays which would pass through the margin of the dioptric media. The importance of this will be better understood further on.

Within the choroid coat, and in immediate contact with it, is

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the nervous coat, or retina, formed by the expansion of the optic nerve, which passes toward the sclerotic obliquely, and enters it somewhat to the nasal side of the axis of the eye. The retina lines all the back part of the eyeball, and stretching forward, becomes fused with the ciliary processes, where, however, the nervous elements of the coat are wanting. The fibrils of the optic nerve reach the inner surface of the coats of the eye, and lie in immediate relation to the transparent medium, which occupies the greater part of the bulb. The fibres then lie internally to their terminals, which turn outward and are set against the choroid coat. The ultimate nerve endings are situated in pigmented protoplasmic cells, which form the outer layer of the retina.

THE DIOPTRIC MEDIA OF THE EYEBALL. The transparent substances which fill the eyeball are, together with the cornea, called the dioptric media. The aqueous humor lies in contact with the posterior surface of the cornea, and just fills the prominence which is formed by this part of the It is in this fluid that the movable iris is stretched and separates the aqueous department of the eye into anterior and posterior chambers. The vitreous humor occupies much the larger share of the eyeball. It lies in apposition to the retina, being separated

. from it only by a thin, transparent structure called the hyaloit membrane, which encloses the clear, gelatinous vitreous humor, and is fused with the ciliary part of the retina and choroid. The vitreous humor is developed from the young connective tissue of the mesoblast, and we find in the adult that mucus is the most abundant chemical substance in its texture, though the branched cells of the original mucous tissue have nearly all disappeared.

The most important of the dioptric media is the crystalline lens. It is placed between the aqueous and the vitreous humors, just behind the iris, which lies in contact with its anterior surface. It is like a strong magnifying glass, biconvex in shape, the posterior surface being more convex than the anterior. The lens is much harder than the vitreous humor, but its outer layers are little denser than a stiff jelly. It is enclosed by a firm, elastic capsule, which is drawn tightly over the anterior surface, and influences its shape. The lens is held in its position by the suspensory ligament, a thickened part of the hyaloid membrane, which is continued forward and attached to the anterior surface of the capsule, near its margin. The lens and its capsules, together

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Diagram of lens viewed from the side at different periods of life. 4, At birth; 6, Adult;

c, Old age. (Allen Thomson.)

with the vitreous humor, may be said to be enclosed in the hyaloid membrane, which, in front, is thickened and attached to the ciliary part of the choroid and the capsule. Thus, any tension exercised by the suspensory ligament tends to tighten the ante

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Showing early stages of the development of the lens. c, Epithelial tissue about to form the

lens ; 0, Optic cup; a, Epidermis. '(Cadiat.)

rior part of the capsule and flatten the anterior surface of the lens.

The shape of the lens varies at different times of life, being nearly spherical in the infant and tending to become less convex in old age (Fig. 218). The lens is developed from the outer layer of the embryo by the gradual thickening and growing inward of the epithelium, which meets the optic cup, and after a time is cut off from the parent tissue. The stages of its development may be followed in the preceding wood-cuts (Fig. 219).

The lens is composed of a number of peculiar band-like cells, derived from the epithelium. These are cemented together in

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A further stage of the development of the lens. (Cadiat.) a, Elongating epithelial cells forming lens ; b, Capsules ; c, Cutaneous tissue becoming conjunctiva; d, e, Two layers of optic cup forming retina ; J, Cell of mucous tissue of the vitreous humor; g, Intercellular substance; h, Developing optic nerve.

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parallel rows, eccentrically arranged in These bands are hexagonal in transverse section, and in the younger periods of . life may be seen to contain nuclei.

In the living state the lens is perfectly transparent, but after death it becomes slightly opaque. The nutriment for the adult lens is derived from the vessels of the choroid, which, however,

do not come into direct communication with its texture. On this account the nutrition of the lens is not so perfect as that of many other tissues, and is but imperfectly repaired after injury,

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FIG. 221.

6 Fragment of lens teazed out to show the separate fibres. (Cadiat.)-a, b, and c show

fibres with different sized nuclei.

which always leaves more or less opacity. Even without injury, opacity, giving rise to cataract, sometimes occurs during life.

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