Rectum

Introduction

Exists dorsal to the reproductive organs, bladder and urethra.
The cranial portion of the rectum is attached to the dorsal body wall by a short mesorectum which is a continuation of the mesocolon.
The mesorectum is reflected to continue with the parietal peritoneum of the pelvic cavity and to cover the urogenital organs ventrally.
- This forms the rectogenital pouch.
- Therefore the most distal part of the rectum is retroperitoneal.
  - This distal, retroperitoneal part is directly attached to the vagina in the female and to the urethra in the male.
  - The retroperitoneal space is filled with soft tissue rich in fat.
The anal canal joins the bowel to the exterior.
- This is a short passage derived from the proctodeum (formed by invagination of the surface ectoderm).
- Before joining the anal canal, the rectum becomes dilated to form the rectal ampulla.
At the rectoanal junction, the lumen is constricted by longitudinal folds in the mucosa.
- These are normally pressed together to occlude the lumen.
There are two anal sphincters:
- Internal anal sphincter, formed by thickening of the circular smooth muscle of the gut and under autonomic control.
- External anal sphincter, formed from striated skeletal muscle and under voluntary control.

Termination of the lamina muscularis and longitudinal layer of the tunica muscularis.
The circular layer of the tunica muscularis forms the internal anal sphincter.
The external anal sphincter is formed from skeletal muscle.