Function of human vision

Vision begins as light enters the eye through the cornea, the transparent outer layer of tissue on the eye. The light then passes through the pupil, the round opening in the iris, which is the coloured part of the eye. The pupil changes size depending on how much light enters the eye. The light continues on through the crystalline lens which changes shape to refract the light reflected off the object in view: this is how the eye focuses. Finally the light reaches the retina and its energy excites the retinal cells, causing a nerve impulse that travels via the optic nerve to the brain for interpretation.
Components of the eye
Cornea

Transparent avascular membrane located at the front of the eye. Optically the most important part of the eye. Contact lenses are placed directly on the cornea.
Aqueous humour

Clear, transparent liquid between the cornea and the front of the crystalline lens.
Iris

Circular pigmented fibrovascular tissue, perforated in the centre to form the pupil. The iris is located between the cornea and the crystalline lens. (Iris = colour of eye)
Pupil

Opening through which light penetrates the eye. The diameter of the pupil varies to control the amount of light that can enter the eye.

Crystalline lens

The lens of the eye, which adjusts to focus light on the retina. This process is called accommodation.

Retina

The retina contains two types of photoreceptor cells: cones, which function in bright light and see colours, and rods, which are responsible for our night vision. The optic nerve, which transmits photochemical energy to the brain, is an extension of the retina.
Fovea

The region of the retina that provides the best visual acuity (vision). The light entering the eye undergoes changes as it travels through the various parts of the eye. The light from an object more than six metres away from the eye arrives as parallel rays. The light crosses through the cornea, which refracts it slightly. Then it crosses the anterior chamber and the pupil to reach the crystalline lens, which refracts it further. Then the light crosses the vitreous body and reaches the retina in the area called the fovea (by now, the image is inverted). When the light strikes the retina (a tissue composed of nerve cells), nerve impulses are sent to the brain along the optic nerve. The brain decodes the nerve impulses and reorients the image, so we can interpret what we are seeing.

Vitreous body

A colourless, transparent, gelatinous mass enclosed in a fine membrane called the hyaloid membrane. 98-99% of it is made up of vitreous humour. The vitreous and aqueous humours both refract light and exert pressure to help maintain the shape of the eyeball. The vitreous body is in direct contact with the retina, which is an extension of the brain.

Sclera

A 1-mm thick membrane that covers the eyeball, serving as a protective coating. The sclera is covered by a transparent mucous membrane called the conjunctiva. The opaque, fibrous sclera forms the white of the eye. In front of the iris there is a port-hole in the sclera about 11 mm across. This part of the eye is more convex, and is covered by the cornea, a perfectly clear membrane.

The choroid is another layer that lies between the sclera and the retina, comprised mainly of blood vessels that nourish the eye's third tunic, the retina.
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