Circular muscular layer of rectum

The circular muscular layer of rectum forms the inner component of the muscularis externa of rectum and consists of a continuous layer of smooth muscle fibres arranged circumferentially around the rectal wall. Unlike the colon, where the circular muscle layer is relatively thin and particularly concentrated between the sacculations (haustra), the circular layer of the rectum forms a well-developed and uniform muscular coat that contributes to the storage and controlled evacuation of faecal contents.

Transverse rectal folds

Throughout the rectum, localized thickenings of the circular muscle layer, together with the overlying mucosa and submucosa, contribute to the formation of the transverse rectal folds (valves of Houston). These permanent folds project into the rectal lumen and help support the weight of faecal material, reducing its direct descent into the anal canal and thereby assisting continence.

Internal anal sphincter

Inferiorly, as the rectum continues into the anal canal, the circular muscle layer becomes markedly thickened to form the internal anal sphincter. This involuntary sphincter consists of smooth muscle under autonomic control and is responsible for maintaining most of the resting anal tone.

During defecation, contraction of the rectal circular muscle is accompanied by relaxation of the internal anal sphincter, allowing the passage of faeces.