Anatomy of the Large Intestine

Distinguishing the Small Intestine from the Large Intestine:

The large intestine is different from the small intestine due to the presence of three longitudinal muscle bands:

• Taenia mesocolica (inner band): attaches to the mesentery of the colon

• Taenia omentalis: attaches to the posterior peritoneum

• Taenia libera (free band): located anteriorly

These muscle bands form haustra, which are pouches between them. Additionally, the large intestine has appendices epiploicae, which are small peritoneal sacs containing fat.

Cecum and Appendix:

The cecum and appendix are two related structures that are located in the right iliac fossa, anterior to the iliopsoas muscle and fascia.

• The location of the appendix varies greatly, it can be found within the cecum or outside the small bowel loop.

Position of the Right Colic Flexure:

Located in the right hypochondrium, posterior to the ninth costal cartilage.

Hepatic and Pelvic Flexures of the Rectum:

• Hepatic flexure: Concave forward at the junction of the rectum with the sigmoid colon.

• Pelvic flexure: Concave backward at the junction of the anal canal with the rectum.

Peritoneum of the Cecum and Appendix:

• The cecum is completely covered by peritoneum, with both sides of the cecum adhering to the posterior abdominal peritoneum, forming the retrocecal recess.

• Mesentery of the appendix: connects the appendix to the ileum, with a free edge containing the appendicular artery.

Limits of the Ascending Mesocolon:

• Superiorly: the course of the superior mesenteric artery to the descending duodenum.

• Medially: the course of the superior mesenteric artery.

• Inferiorly: the oblique line from the bifurcation of the abdominal aorta to the base of the cecum.

Descending Mesocolon:

• Superiorly: the origin of the inferior mesenteric artery to the left colic flexure.

• Inferiorly: from the left para-median prominence along the inner border of the left psoas muscle.

Transverse Mesocolon:

• The lower limit of the transverse mesocolon is the lower limit of the mesocolon, and the upper limit of the right and left colic attachments.

Mesocolon of the Sigmoid Colon:

• Has two roots:

• Primitive root: from the root of the primitive mesentery of the gut, projecting to the front of the third sacral vertebra.

• Secondary root: the lower limit of the left colic attachment.

Muscle Layers of the Large Intestine:

• Inner: circular muscle.

• Outer: longitudinal muscle, mostly concentrated in bands, most prominent in the cecum and ascending colon, dispersed in the descending colon and absent in the sigmoid.

Ileocecal Valve:

• Has two superior and inferior leaflets, the inferior leaflet is longer, lip-shaped, with the ileocecal orifice in the middle, and the two ends of the valve are extended to create the ileocecal valve fold.

Anal Columns:

• These are longitudinal folds, each containing a terminal branch of the superior rectal artery, with the base of the column being the anal valves (crescent-shaped folds).

• Between the valves are small recesses called anal sinuses.

Anal Glands:

• There are 6 anal glands that drain into the small crevices within the anal valves.

• Lined by stratified columnar epithelium.

• These glands expand through the internal sphincter and even the external sphincter.

Hilton’s White Line:

• The intersphincteric groove, a depression at the lower limit of the internal sphincter.

Anal Canal below the Intersphincteric Groove:

• Lined by stratified squamous epithelium, with hair follicles, continuous with the skin around the anus.

• The submucosa contains a rich network of blood vessels, with a lot of connective tissue.

Mechanism of Fecal Continence:

• The transitional zone submucosa (area between the smooth and striated muscles) detects fecal matter in the lower rectum when the upper segment expands for stool passage.

Vascular Supply of the Right Colon:

• Superior mesenteric artery, with branches supplying:

• Ileocolic

• Ascending colon

• Anterior cecal

• Posterior cecal

• Appendicular

• Right colic

• Middle colic

Inferior Mesenteric Artery:

• Origin: abdominal aorta, above the bifurcation of the aorta, corresponding to vertebra L3.

Branches of the Inferior Mesenteric Artery:

• Left colic artery: connects with the middle colic artery and sigmoid arteries.

• Sigmoid arteries.

• Superior rectal artery: connects the sigmoid, middle rectum.

Inferior Rectal Veins:

• The venous plexus within the intestinal wall drains into the sigmoid veins and left colic veins, then into the superior portion of the inferior mesenteric vein and drains into the portal vein.