Wikipedia
Internal anal sphincter
The internal anal sphincter, IAS, (or sphincter ani internus) 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 external anal sphincter.
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.