Iris sphincter muscle

The iris sphincter muscle (pupillary sphincter, circular muscle of iris, circular fibers) is a muscle in the part of the eye called the iris.

It is found in vertebrates and some cephalopods.

In humans, it functions to constrict the pupil in bright light (pupillary reflex) or during accommodation. Its dimensions are about 0.75 mm wide by 0.15 mm thick.

It is controlled by parasympathetic fibers that originate from the Edinger-Westphal nucleus, travel along the oculomotor nerve (CN III), synapse in the ciliary ganglion, and then enter the eye via the short ciliary nerves.