Longitudinal muscular layer of rectum

The longitudinal muscular layer of rectum forms the outer component of the muscularis externa of the rectum and consists of smooth muscle fibres continuous with those of the sigmoid colon. In the colon, the longitudinal muscle is arranged into three distinct bands known as the taeniae coli. At the rectosigmoid junction, these taeniae spread out and fuse to form a continuous longitudinal muscular layer that completely surrounds the rectum, distinguishing it from the rest of the colon.

The longitudinal muscle layer is not of uniform thickness. It is generally thicker on the anterior and posterior surfaces of the rectum and thinner on the lateral surfaces. Inferiorly, the longitudinal fibres continue into the anal canal, where they contribute to the structural support of the anorectal region.

Posteriorly, the longitudinal muscular layer of rectum receives additional smooth muscle fibres known as the rectococcygeal muscles. These fibres arise from the anterior surfaces of the second and third coccygeal vertebrae and pass downward and forward to blend with the longitudinal muscle fibres on the posterior wall of the anal canal. The rectococcygeal muscles help anchor the anorectal junction and contribute to the maintenance of its position within the pelvis.

The longitudinal muscular layer shortens the rectum during defecation and works in coordination with the circular muscle layer to propel faecal contents towards the anal canal.