eye

The receptor organ for light. Some form of eye occurs very widely among animals, ranging from the simple ocellus to the complex eyes of arthropods (see compound eye), vertebrates, and cephalopod molluscs (see octopus eye). Eyes of the latter two groups, although remarkably similar, have evolved quite independently and thus offer a classic example of analogous biological structures. Eyes often occur in pairs, enabling the perception of depth through binocular vision, or in some animals (such as chameleons), the formation of two separate images.

The human and mammalian eye

Light enters the mammalian eye through the pupil, and is focused by the cornea and the lens onto the retina. The shape of the lens can be changed by the ciliary muscles so that the image always comes to a sharp focus at the retina. This image is inverted (upside-down). At the retina, the light rays are converted into electrical impulses which are then transmitted through the optic nerve to the brain, where the image is translated and perceived in an upright position.

Section of a human eye

Chambers of the eye

The eye has three internal cavities: the anterior and posterior chambers, and the vitreous cavity. The anterior chamber lies between the cornea and the anterior (front) part of the iris, whereas the posterior chamber is situated between the lens and the posterior (back) part of the iris. The anterior chamber is filled with aqueous humor, a runny substance similar to cerebrospinal fluid in composition. The aqueous humor supplies oxygen and nutrients to the iris and cornea. The vitreous cavity fills the space from the lens to the retina and contains vitreous humor, a jelly-like material that maintains internal pressure to support the eyeball and transports nutrients to the cornea.

Other components of the eye

See the table below for brief descriptions of the major components of the mammalian eye and their functions. Click on a link to find out more.

part of eye	description	function
cornea	Front part of the tough outer coat, the sclera. It is convex and transparent.	Protects front of eye and bends light to form an image on the retina.
conjunctiva	Membrane covering the exposed front part of the eye, and lining the eyelids. It is kept moist by antiseptic secretions from the tear glands.	Protects the cornea
sclera	The opaque 'white of the eye' – also called the sclerotic. It is a tough and fibrous outer layer covering the whole of the eye except the cornea.	Protection
iris	Pigmented (determines the color of your eyes) so light cannot pass through. Its muscles contract and relax to alter the size of its central hole or pupil.	Protects the photoreceptors in the retina from being damaged by too much light
pupil	A black hole in the center of the iris. It is the dark pigmented layer inside the eye – the choroid – which makes the pupil appear black.	Allows light to enter eye
lens	Transparent, bi-convex, flexible disk behind the iris attached by the suspensory ligaments to the ciliary muscles	Brings the light entering through the pupil to a focus on the retina. The ciliary muscles control the lens' thickness and curvature
ciliary muscle	Ring of muscle fibers around lens	Controls lens thickness and curvature
suspensory ligaments	Ligament between lens and ciliary muscle	Supports lens and connects it to the ciliary muscle
retina	The lining of the back of eye containing two types of photoreceptor cells – rods (sensitive to dim light and black and white) and cones (sensitive to color). A small area called the fovea in the middle of the retina has many more cones than rods.	Screen on which images are formed as a result of light being focused onto it by the cornea and lens. The fovea is the point of maximum visual sharpness.
optic nerve	Bundle of sensory neurons at back of eye	Carries signals from the photoreceptors of the retina to the brain. At the point where the sensory neurons leave the retina to form the optic nerve – the so-called blind spot – there are no rods and cones, and no image can therefore be seen.

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• ANATOMY AND PHYSIOLOGY

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