VAN 121: Veterinary Gross Anatomy-II (2+2)

Refractive media of eye

REFRACTIVE MEDIA OF THE EYE

• The refractive media include the cornea, the aqueous humor in the anterior and in posterior chambers, the lens and the vitreous humor.
• The cornea is described earlier in the fibrous tunic.
• The aqueous humor is a clear fluid of an alkaline reaction, consisting chiefly of water with a trace of albumin and chloride or sodium. It fills the anterior and posterior chambers of the eyeball. The anterior chamber is the space in front by the cornea and behind by the anterior face of the iris and the central part of the lens. The posterior chamber is a narrow space bound by the posterior surface of the iris infront, and the circumference of the lens, its ligaments and ciliary process behind. The two chambers communicate freely with each other through the pupil.
• The crystalline lens is a biconvex transparent body that is situated in front of the vitreous body and in partial contact with the posterior face of the iris. The periphery or equator of the lens is almost circular and is surrounded by the ciliary processes. The anterior face is convex and the posterior face is much more convex and rests in the fossa hyaloidea of the vitreous body. Three structural components make up the lens are capsule, anterior epithelium, lens substance. It is enclosed in a capsule (lens capsule) that is attached by the suspensory ligament to the ciliary processes. The anterior epithelium is a single layer of cuboidal cells on the anterior surface under the capsule. There is no posterior epithelium as these cells have given rise to the primitive lens fibres during development. At the equator, the cells are arranged meridionally in rows. This is the region where new lens fibres are formed constantly. The central points of the surfaces of the lens are anterior and posterior poles and the line that connects them is the axis of the lens. The lens substance consists of concentric lamellae of lens fibres that are colorless and transparent. It is enclosed by a structureless highly elastic membrane, the capsule that consists of a softer cortical substance and a dense central part, the nucleus of the lens. The lens substance when hardened is seen to consist of concentric lamellae of lens fibres united by an amorphous cement substance. The lens is devoid of vessels and has no nerves.
• The vitreous humor is the transparent jelly situated behind the lens and between it and the retina. In front, it presents a deep fossa -fossa hyaloidea for the lens and is enclosed by the hyaloid membrane a condensation of the stroma. It composed of jelly like connective tissue, the meshes of which are filled by fluid -vitreous humor.