Muscular layer of rectum

The muscular coat (tunica muscularis) consists of an external longitudinal, and an internal circular, layer of non-striped muscular fibers:

The longitudinal layer (longitudinal fibers) do not form a continuous layer over the whole surface of the large intestine. Around the rectum they spread out and form a layer, which completely encircles this portion of the gut, but is thicker on the anterior and posterior surfaces, where it forms two b ands, than on the lateral surfaces. In addition, two bands of plain muscular tissue arise from the second and third coccygeal vertebræ, and pass downward and forward to blend with the longitudinal muscular fibers on the posterior wall of the anal canal. These are known as the Rectococcygeal muscles.

The circular layer (circular fibers) form a thin layer over the cecum and colon, being especially accumulated in the intervals between the sacculi; in the rectum they form a thick layer, and in the anal canal they become numerous, and constitute the Sphincter ani internus.