There are two types of muscles contained in the iris: (1) the iris sphincter (which constricts the pupil); and (2) iris dilator muscle (which dilates the pupil).
Again, the muscle in the iris which constricts the pupil is called the iris sphincter muscle. This muscle is located all around the circumference of the pupil and it helps control the amount of light entering the eye.
Again, the muscle in the iris which dilates the pupil is called the iris dilator muscle. This muscle is also located around the circumference of the pupil and consists of myoepithelial cells. These cells are in a "bicycle tire" or "spoke-like" formation around the iris. When these cells are stimulated, they contract and open the pupil.
One important clinical pearl: the iris sphincter muscle is generally much stronger than the iris dilator muscle. Thus to achieve good pupillary dilation, it is more important to weaken the iris sphincter muscle (e.g. with tropicamide 1%) in addition to stimulating the iris dilator muscle (e.g. with phenylephrine 2.5%).
