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Eyeball, the spheroid that contains the sense organs (and their accessories) of light discrimination and vision; it is present in all vertebrates and is constructed like a simple camera. It has opaque walls and a transparent front, the cornea. The eye is filled with transparent media. The space immediately behind the cornea is the anterior chamber, which is separated from the ring-shaped posterior chamber by a pigmented membrane, the iris. Both chambers are filled with a watery liquid called the aqueous humour. Behind the anterior structures is the spacious vitreal cavity, filled with gelatinous vitreous humour, called the vitreous body.
At the centre of the iris is a circular opening, the pupil. Behind the pupil is the crystalline lens, a transparent, elastic biconvex body suspended by delicate, tendinous threads, the suspensory ligament, or Zinn’s zonule. The zonule, in turn, is attached to the epithelium (covering) of the ciliary body. When the ciliary muscle relaxes the zonule fibres pull on the equator of the lens and flatten it. When the ciliary muscle contracts, the zonule is less taut and the lens surfaces becomes more curved. Changes of lens shape enable the eye to focus on objects at various distances.
The core of the lens is firmer than its rind, whose relative softness permits the lens to change shape under slight force. The forward surface of the lens is more curved than the posterior surface. The lens becomes harder and less transparent with age.
Rays of light coming from the exterior first are collected and refracted by the cornea; they then proceed to the lens, where additional refraction takes place. Most refraction is done by the cornea in terrestrial vertebrates and by the lens alone in aquatic animals.
The lens focusses the refracted rays of light on the retina, to form a real image that is much smaller than the object and is inverted.
The outer shell of the eyeball is made up of three coats: the fibrous tunic, the uveal tunic, and the retina. The outermost is the tough fibrous tunic; its larger, posterior segment, called the sclera, is opaque; the smaller, anterior segment, more curved than the sclera, is the transparent cornea. The scleral fibres are collagenous, elastic, and arranged to minimize the effects of changes in intraocular pressure and external forces. (Collagen is the main supportive protein in connective tissue.) The portion of the sclera close to the anterior chamber is encircled by Schlemm’s canal, by which aqueous humour is drained from the anterior chamber. The zone between the cornea and the sclera, called the limbus, is a semitransparent zone that blends with the conjunctiva, the mucous membrane that lines the eyelids and covers the anterior portion of the sclera. The cornea has five layers, of which the stroma, or substantia propria, is the thickest. The regular arrangement of the stromal fibres is essential to maintain the normal corneal transparency. The coat beneath the fibrous tunic is the uveal tunic. Its thinner and larger posterior segment, the choroid, functions to supply the photosensitive rods and cones of the retina with nutritive blood and to form a dark lining on the inside of the eyeball. The choroid is attached to the sciera only by delicate fibres; for this reason the choroid is easily detached from the sclera. The forward part of the choroid, the ciliary body, is a series of minute, radially arranged, ridgelike projecor serrated margin. (See also retina.)
