The iris sphincter muscle, also known as the pupillary sphincter or sphincter pupillae, is a muscle located in the colored part of the eye called the iris. The sphincter muscle fibers are located near the pupillary margin and are slightly anterior to the pigmented epithelium of the iris.

How many muscles are in an iris?

It is bathed in front and behind by a fluid known as the aqueous humour. The iris consists of two sheets of smooth muscle with contrary actions: dilation (expansion) and contraction (constriction). These muscles control the size of the pupil and thus determine how much light reaches the sensory tissue of the retina.

What do the muscles in your iris do?

The iris is the colored part of the eye. … Muscles in the iris cause the pupil to constrict in bright light and to dilate in dim light. The change in pupil size regulates the amount of light that reaches the posterior (back) part of the eye.