Structure/Morphology
The muscular layer of the rectum is composed exclusively of smooth muscle. like the colon, it can be divided into inner and outer sublayers. The inner circular sublayer has its muscle fibers encircling the lumen.
Key Features/Anatomical Relations
The lining of the rectum has two types of horizontal folds. The first type can be identified on the external surface and contains the mucosa, submucosa, and both the circular and the longitudinal layers of smooth muscle. The second type does not contain longitudinal muscle fibers and, thus, is only noted on the internal surface (Standring, 2016).
Of the three, large, transverse rectal folds seen on the internal surface of the rectum (superior, middle, and inferior), it's within the middle fold that the circular muscles fibers are most conspicuous.
Function
The longitudinal and circular layers of the muscular layer of the rectum are responsible for producing peristaltic contractions of the gut, thus propelling the ingested substance through the lumen of the gut.
References
Standring, S. (2016) Gray's Anatomy: The Anatomical Basis of Clinical Practice. Gray's Anatomy Series 41st edn.: Elsevier Limited.
