The Eye: Structure and Function

The eye is the principal sense organ of vision, and its wall is built of three concentric layers surrounding an internal system of a lens and fluid-filled chambers that together focus light onto a light-sensitive inner layer.

The outer (fibrous) layer is made up of the sclera, a tough, white, protective coat that covers most of the eyeball and gives it its shape, and, at the front of the eye, the transparent cornea, through which light first enters and which contributes the greatest part of the eye's total focusing (refractive) power. The middle (vascular) layer, sometimes called the uvea, consists of the choroid, a pigmented and richly vascular layer that nourishes the outer part of the retina and absorbs stray light to reduce internal scattering; continuing forward, this layer becomes the ciliary body, which contains the ciliary muscles that control the shape of the lens, and finally the iris, a pigmented, muscular diaphragm with a central opening, the pupil, whose diameter the iris adjusts to control how much light enters the eye. The inner (nervous) layer is the retina, which contains the light-sensitive photoreceptor cells — rod cells, which are extremely sensitive and function in dim light but do not distinguish colour, and cone cells, which require brighter light but provide colour vision and finer detail. The retina has a small central region called the fovea (within the macula) where cone cells are most densely packed and vision is sharpest, and a separate point called the optic disc, where the nerve fibres of the retina gather to leave the eye as the optic nerve; because this point contains no photoreceptors at all, it is known as the blind spot.

The internal chambers and lens complete the optical system: a biconvex, transparent, elastic lens sits just behind the iris, held in place by suspensory ligaments attached to the ciliary body, and it can be made more or less curved by the action of the ciliary muscles — this ability to change shape and adjust focus for near or distant objects is called accommodation. In front of the lens, the space between the cornea and the lens (the anterior chamber) is filled with a watery fluid, the aqueous humour, which helps maintain the shape of the front of the eye and nourishes the cornea and lens; behind the lens, the much larger posterior chamber is filled with a thicker, gel-like fluid, the vitreous humour, which maintains the eyeball's overall spherical shape. …