Internal anal sphincter is pa of
Internal longitudinal fibres of rectum
Puborectalis muscle
Deep perineal muscles
Internal circular muscle fibres of rectum
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
Abdomen and pelvis