ABSTRACT
Anatomy The rectum extends from the pelvic inlet to the anus, and is approximately 30 cm in length in the adult horse.1 Because there is no apparent line of demarcation between the distal small colon and the proximal rectum, the plane of the pelvic inlet is used as the division between the small colon and rectum.1 The rectum can be divided into two segments. The cranial (peritoneal) segment is proximal to the peritoneal reflection, and the caudal (retroperitoneal) segment is caudal to the peritoneal reflection and forms a flask-shaped dilatation called the rectal ampulla. The peritoneal reflection is approximately 15 to 20 cm proximal to the anus in an adult 1000-lb (450 kg) horse.1-3 The peritoneal segment of the rectum is suspended by the mesorectum, a continuation of the mesocolon. As the rectum passes retroperitoneally, it is attached by connective tissue to surrounding structures in the pelvic canal.1 The outer muscular layer of the ampulla is thickened where the longitudinal muscle of the taenia fans out to form thick, loosely bound bundles of muscle tissue.2 The rectococcygeus muscle, which forms a large band on each side, originates from the longitudinal muscle and travels dorsocaudally, inserting onto the ventral surface of fourth and fifth caudal vertebrae.1
