GUIDE
Pelvis and Perineum: Rectum and Anal Canal
[Plate 372, Ischioanal Fossae]Rectum
Gross structure
Approximately 12 cm in length
Is a part of the large bowel
Commences at level S3 vertebral body and follows the curve of the sacrum and coccyx
Is a continuation of the sigmoid colon at rectosigmoid junction
Extends from rectosigmoid junction to anal canal (anorectal junction)
Anorectal junction
Lies at level of pelvic floor
Puborectalis muscle forms a U-shaped sling at this point
Perineal flexure annulates anorectal junction
Is palpated as anorectal ring on digital rectal examination
Three lateral curves/flexures in the rectum (upper and lower curves deviate to the right; the middle to the left)
Rectal ampulla
In region of middle and lower curves
Somewhat dilated
Very distensible
Internal structure
Smooth mucosa, changes from rugose mucosa of sigmoid colon
Transverse rectal folds
Three infoldings of mucosa and submucosa project into lumen
Superior, middle, and inferior
Called rectal valves of Houston
Result from three lateral flexures