The intensity of light through our eyes is controlled by pupil and iris.The pupil is a hole located in the centre of the iris of the eye that allows light to strike the retina. It appears black because light rays entering the pupil are either absorbed by the tissues inside the eye directly, or absorbed after diffuse reflections within the eye that mostly miss exiting the narrow pupil.iris is a contractile structure, consisting mainly of smooth muscle, surrounding the pupil. Light enters the eye through the pupil, and the iris regulates the amount of light by controlling the size of the pupil.
The iris contains two groups of smooth muscles; a circular group called the sphincter pupillae, and a radial group called the dilator pupillae. When the sphincter pupillae contract, the iris decreases or constricts the size of the pupil. The dilator pupillae, innervated by sympathetic nerves from the superior cervical ganglion, cause the pupil to dilate when they contract. These muscles are sometimes referred to as intrinsic eye muscles.
- Sphincter muscles constrict the pupil like a drawstring.
- Dilator muscles that radiate out from the center of the iris help to dilate the pupil.
Behind the scenes, there is a long nerve that helps the eye react to changing conditions. This nerve takes a surprising route through the body, starting in the brain then traveling down the spinal cord, passing above the lungs, up through the neck, back through part of the brain and then, finally, to the pupil. Because of that strange route, problems with the brain, upper lungs, and some eye health issues may be discovered when there is an unexpected change in the pupils.