The eye is a complex sensory organ that provides the sense of sight. In many ways, the eye is similar to a digital camera. Like the optical system of a camera, the cornea and lens of the eye capture and automatically focus light. The iris also automatically adjusts the eye to differences in illumination of visual fields. In many aspects, the optical system of the eye is far more elaborate and complex than a camera. For example, the eye has the ability to track moving objects with coordinated eye movements. The eye can also protect, maintain, self-repair, and clean its transparent optical system. The light detector in a digital camera, the charge-coupled device (CCD), consists of closely spaced photodiodes that capture, collect, and convert the light image into a series of electrical impulses. Similarly, the photoreceptor cells in the retina of the eye detect light intensity and color (wavelengths of visible light that are reflected by different objects) and encode these parameters into electrical impulses for transmission to the brain via the optic nerve. The retina has other capabilities beyond those of a CCD: It can extract and modify specific impulses from the visual image before sending them to the central nervous system (CNS).
Because the eyes are paired, two somewhat different and overlapping images (visual fields) are sent to the brain. The brain can be compared to a computer, which processes the slightly different images from each eye, separates them into layers, and projects them onto the primary visual cortex located in the occipital lobes. Complex neural mechanisms coordinate eye movements, enabling us to perceive depth and distance to achieve a three-dimensional image. Therefore, the way in which we perceive the world around us largely depends on impulses processed within the retina and analysis and interpretation of these impulses by the CNS.
The eye measures approximately 25 mm in diameter. It is suspended in the bony orbital socket by six extrinsic muscles that control its movement. A thick layer of adipose tissue partially surrounds and cushions the eye as it moves within the orbit. The extraocular muscles are coordinated so that the eyes move symmetrically around their own central axes.
Layers of the Eye
The wall of the eye consists of three concentric layers or coats.
The eyeball is composed of three structural layers (Fig. 24.1):  The corneoscleral coat, the outer or fibrous layer, includes the sclera, the white portion, and the cornea, the transparent portion.
FIGURE 24.1 • Schematic diagram of the layers of the eye. The wall of the eyeball is organized in three separate concentric layers: (a) an outer supporting layer, the corneoscleral coat (colorless and blue); (b) a middle vascular coat or uvea (pink); and (c) an inner photosensitive layer, the retina (yellow).
 The vascular coat, the middle layer, or uvea, includes the choroid and the stroma of the ciliary body and iris.  The retina, the inner layer, includes an outer pigment ep ithelium, the inner neural retina, and the epithelium of the ciliary body and iris. The neural retina is continuous with the central nervous system through the optic nerve.
The corneoscleral coat consists of the transparent cornea and the white opaque sclera.
The cornea covers the anterior one-sixth of the eye (Fig. 24.1a). In this windowlike 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. In children, it has a slightly blue tint because of its thinness; in elderly people, it is yellowish 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.