Tags: Anatomical Structure, Muscle.
The sphincter ani internus (or internal anal sphincter IAS) is a muscular ring that surrounds about 2.5-4.0 cm of the anal canal; its inferior border is in contact with but quite separate from the sphincter ani externus.It is about 5 mm thick and is formed by an aggregation of the involuntary circular fibers of the rectum. Its lower border is about 6 mm from the orifice of the anus.
