The iris controls the pupil size using two muscles that either dilate (open) or constrict (close) the pupil.
Iris Muscles Controlling Pupil Size
The iris, the colored part of your eye, acts like the diaphragm of a camera, controlling the amount of light that enters the eye through the pupil. It achieves this using two key muscles:
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Iris Sphincter: This muscle is a circular muscle located near the edge of the pupil. When it contracts, it reduces the size of the pupil (miosis or constriction), allowing less light to enter. Think of it like a drawstring that tightens to close an opening.
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Iris Dilator: This muscle extends radially from the pupil to the edge of the iris. When it contracts, it pulls the iris back, widening the pupil (mydriasis or dilation), allowing more light to enter. This muscle acts like spokes extending outward from the pupil's center.
Summary of Pupil Control
Here's a table summarizing the function of each muscle:
| Muscle | Action | Effect on Pupil Size | Light Conditions |
|---|---|---|---|
| Iris Sphincter | Contracts | Decreases (Constricts) | Bright Light |
| Iris Dilator | Contracts | Increases (Dilates) | Dim Light |
In essence, the interplay between the iris sphincter and iris dilator muscles dictates the diameter of the pupil, thereby regulating the amount of light reaching the retina.
