[1] Historically, the sphincter was described as consisting of three parts (deep, superficial, and subcutaneous).

Some sources still describe it in two layers, deep (or proximal) and superficial (or distal or subcutaneous).

[1] The muscle attaches anteriorly onto the perineal body, and posteriorly onto the anococcygeal ligament.

[1] The sphincter consists mostly of slow twitch fibers that allow extended continuous contraction.

[1] This article incorporates text in the public domain from page 425 of the 20th edition of Gray's Anatomy (1918)