Layers of the

The wall of the eye consists of three concentric layers or coats

The eyeball is composed of three structural layers (Fig. 23.1):

• Corneoscleral coat, the outer or fibrous layer, which includes the sclera, the white portion, and the cornea, the transparent portion

• Vascular coat, the middle layer or uvea, which includes the choroid and the stroma of the ciliaiy body and iris

• Retina, the inner layer, which includes an outer pigment epithelium, the inner neural retina, and the epithelium of the ciliary body and iris

On the anterior aspect of the eye the layers are modified to admit and regulate the passage of light. The neural retina is continuous with the central nervous system through the optic nerve. The internal cavity of the eye is filled with a transparent gel, the vitreous body, which helps to maintain shape.

The corneoscleral coat consists of the transparent cornea and the white opaque sclera

The cornea covers the anterior one sixth of the eye (Fig. 23.1a). In this window-like region, the surface of the eye has a prominence or convexity. The cornea is continuous with the sclera (Gr. skleros, hard). The sclera is composed of dense fibrous connective tissue that provides attachment for the extrinsic muscles of the eye. The sclera constitutes the "white" of the eye but has a slightly blue tint in children because of its thinness and is yellow in the elderly because of the accumulation of lipofuscin in its stromal cells. The corneoscleral coat encloses the inner two layers except where it is penetrated by the optic nerve.

The uvea consists principally of the choroid, the vascular layer that provides nutrients to the retina

Blood vessels and melanin pigment give the choroid an intense dark-brown color. The pigment absorbs scattered and reflected light to minimize glare within the eye. The choroid contains many venous plexuses and layers of capillaries and is firmly attached to the retina (Fig. 23.1 b). The anterior rim of the uveal layer continues forward, where it forms the stroma of the ciliaiy body and iris.

The ciliaiy body is a ring-like thickening that extends inward just posterior to the level of the corneoscleral junction. Within the ciliary body is the ciliaiy muscle, a smooth muscle that is responsible for lens accommodation. Contraction of the ciliary muscle changes the shape of the lens, which enables it to bring light rays from different distances to focus on the retina.

The iris is a contractile diaphragm that extends over the anterior surface of the lens. It also contains smooth muscle and melanin-containing pigment cells scattered in the connective tissue. The pupil is the central circular aperture of photosensitive photosensitive

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