Site author Richard Steane
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The iris reflex

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

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