The iris is responsible for monitoring the amount of light that enters the eye, with the aide of two pairs of smooth muscle structures. In bright light, the muscle contracts around the pupil, causing the pupil to contract. If this process didn't occur, the bright and intense light would overpower your photoreceptors, causing temporary blindness. When light is dim, the smooth muscle, positioned around the pupil, contracts, causing the pupil to open wide and dilate.

The muscles are controlled by nerves, which is the reason why a physician shines a light into an injured or unconscious person's eye, to assess the condition of his or her nerves. When light is shone into the eye, pupils that are functioning properly should contract and when the light is removed the pupils should respond by dilating. Fixed or dilated pupils are often an indication of an extensive malfunction of the nervous system and should be taken very seriously. Certain drugs can also make the pupils dilate.

The light that comes into the eye focuses on both the cornea and the lens. The cornea has a curved shape and is the structure that bends most of the light that comes into the eye. The curved shape of the cornea is immobile and cannot be adjusted. The regulating capacity responsible for bending the light and the ability to modify focus between distant and close objects is achieved by an adjustment of the curved shape of the lens, which is referred to as being 'accommodation'.

The ciliary muscle adjusts the curvature of the lens with contractions and as the inner radius of the muscle shrinks, the amount of tension on the fibres that attach to the lens is reduced. This helps the lens to protrude outwards, which is necessary when focusing on an object positioned at close proximity. When the ciliary muscle is in a relaxed mode, an increase of tension occurs in the ring of muscle, which works to flatten and stretch it, pulling distant objects into focus.

The flexibility and suppleness of the lens is integral to sight and good vision and the condition presbyopia is illustrative of the changes that occur when the lens becomes more rigid and stiff with age, causing blurred and hazy vision. This condition eventually affects everyone to varying degrees and sets in after the age of 40. The stiffening of the lens restricts the bulging or protruding action and even when the ciliary muscles are contracted to their maximum degree, the protruding shape cannot take effect.

If you notice that you have to hold reading material further away from your eyes in order to focus more clearly and you are over 40, you may be experiencing presbyopia and should schedule an appointment with your optician for a diagnosis.

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Anatomy of the Eye