The eye functions in a similar way to a sophisticated camera. Light from an object will first pass through the cornea (the transparent dome at the front of the eye), which is the major focusing element of the eye and begins the process of bending (or refracting) the light rays. It then enters the front of the eye, which is filled with clear fluid called the aqueous humour; passes through the pupil, the round opening in the middle of the coloured iris until it reaches the lens, the fine focusing element of the eye. The lens completes the work started by the cornea by bending (refracting) the light rays so they focus at a single point on the retina. Behind the lens and in front of the retina is the main cavity of the eyeball, filled with a clear gel called the vitreous humour. The retina is made up of nerve tissue and is fed by a network of blood vessels (the choroid) supplying it with a constant source of oxygen and nutrients. Light falling on the retina causes impulses to be transmitted along the optic nerve and so that the brain can assemble a "clear picture". However, this will only occur if all the various components of the eye are in working order.
The lens is encased within a bag known as the capsule, which is suspended by delicate fibres called zonules. These zonules are the ligaments attaching the lens to the circular muscle ring, the ciliary body. Contraction of this muscle varies the tension on the lens capsule and allows the lens to become fatter so that the eye can change its focus for near vision. This process is called accommodation and allows the eye to see for reading. With advancing age the lens becomes harder and the ciliary muscle weaker and the ability of the eye to change its focus for near vision reduces. This is called presbyopia and is the reason that most people will require reading glasses after the age of 50.
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