Depending on the intensity of surrounding light, the iris makes an "automatic" adjustment to ensure that the retina receives the correct amount of light. This is another example of a protective reflex; too much light could damage the eye or give an excessively bright image, and too little light would result in an indistinct image.
The iris is a circular disc containing the pigment melanin, which prevents light from passing through, in the middle of which is a hole called the pupil. It contains 2 sorts of muscles:
Radial muscles are attached to the eye at the outside edge of the iris, radiating out like spokes of a wheel.
Circular muscles act like a series of rings around the pupil.
These 2 types of muscles are antagonistic, i.e. only one works at any one time:
Contraction of the radial muscles opens the pupil, allowing more light through, and
contraction of the circular muscles closes the pupil, allowing less light through.
Why do doctors/football team physios shine light in your eye?
>to check that there is no damage to the tissues, especially nerves, behind the eye
This topic has connections with other units on this site:-