What is the function of the sphincter muscles?

Sphincter muscles contract to close various orifices in the body. These muscles are ring-shaped and can be found in the stomach, anus and even the eyes.

The sphincter found in the stomach, also called the "sphincter pylori," keeps food from passing into the small intestine until it is properly mixed with digestive juices. The most commonly known, the sphincter ani externus, keeps the anus closed until waste is ready to be excreted. The ring of muscle fibers in the iris responsible for contracting the aperture when exposed to bright light is also a sphincter. It is known as the "sphincter pupillae."