Muscular layer of large intestine

The muscular layer of large intestine (tunica muscularis or muscularis externa) is composed of an inner circular and an outer longitudinal layer of smooth muscle fibres. Its arrangement differs significantly from that of the small intestine and contributes to the characteristic sacculated appearance of the colon.

Outer Longitudinal Layer

The longitudinal muscle fibres do not form a continuous outer layer throughout the cecum and colon. Instead, they are concentrated into three longitudinal bands known as the taeniae coli, each approximately 12 mm wide.

The taeniae coli are shorter than the intestinal wall, producing puckering of the colon and creating the characteristic haustra (sacculations) of the cecum and colon. If the taeniae are dissected away, the bowel lengthens and the sacculated appearance disappears.

---- Transition from sigmoid to rectosigmoid junction to rectum

In the sigmoid colon, the longitudinal fibres gradually become more dispersed. At the rectosigmoid junction, the three taeniae spread out to form a continuous longitudinal muscle layer around the rectum. This layer is thicker anteriorly and posteriorly than laterally.

---- Rectococcygeal muscles

Additional smooth muscle bundles, known as the rectococcygeal muscles, arise from the second and third coccygeal vertebrae and pass downward and forward to blend with the longitudinal muscle layer on the posterior wall of the anal canal. These muscles help maintain the anorectal angle and support rectal function.

Inner Circular Layer

The circular muscle fibres form a relatively thin layer in the cecum and colon, but are particularly concentrated between the haustra.

In the rectum, the circular layer becomes considerably thicker and more uniform. Within the anal canal, it is markedly thickened and forms the internal anal sphincter, an involuntary smooth muscle sphincter responsible for maintaining continence at rest.

Summary point for muscularis externa of the large intestine

• Outer longitudinal layer: condensed into three taeniae coli in the cecum and colon.

• Inner circular layer: continuous throughout the large intestine.

• Taeniae coli: taenia libera, taenia omentalis, and taenia mesocolica.

• Haustra: produced because the taeniae are shorter than the bowel wall.

• Rectum: taeniae spread out to form a continuous longitudinal layer.

• Anal canal: circular layer thickens to form the internal anal sphincter.

• Rectococcygeal muscles: connect coccyx to the longitudinal muscle layer of the anal canal and rectum.