Anatomy Pelvis and Perineum 3 13ef9fae Internal anal sphincter is a pa of A Internal longitudinal fibers of rectum B Internal circular muscle fibers of rectum C Puborectalis muscle D Deep perineal muscles High-Yield Explanation Internal anal sphincter is formed by thickened circular muscle coat of the anal canal (which is continuous with rectum) and surrounds the upper two-third of the canal.