Internal anal sphincter is pa of
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. Ref - BDC 6e vol2 pg413-416
Internal longitudinal fibres of rectum
Puborectalis muscle
Deep perineal muscles
Internal circular muscle fibres of rectum
Abdomen and pelvis