Gross Anatomy
The anatomy of the large intestine includes the cecum (along with appendix) and the colon; in some descriptions (and the author agrees), it also includes the anorectum (rectum and anal canal).
The large intestine, which is the terminal part of gastrointestinal (GI) tract, is so called because its lumen (diameter) is larger, not because its length is greater, than that of the small intestine (duodenum, jejunum, ileum); in fact, small intestine is longer than the large intestine. The large intestine develops partly from the midgut (from cecum to distal transverse colon), the hindgut (from distal transverse colon to dentate line in anorectum), and proctodeum (below the dentate line).
The cecum, which is the proximal blind end (pouch) of the ascending (right) colon, is a blind cul-de-sac below the level of the ileocecal junction that lies in the right iliac fossa. The terminal ileum opens into the cecum at its medial wall, and the opening is guarded by an ileocecal valve.
The appendix (an appendage of the cecum), also called vermiform process or vermiform appendix, is a tubular structure with a blind end attached to the cecum. The base of the appendix lies on the posteromedial wall of the cecum 1-2 cm below the ileocecal junction. The tip of the appendix, however, floats in the peritoneal cavity and may be pelvic, preileal, postileal, or even retrocecal in position.
The cecum (the widest part of large intestine) leads to the ascending (right) colon, which ascends vertically from right iliac fossa through the right lumbar region into right hypochondrium under the liver. It then takes a right turn and continues as the horizontally placed transverse colon, which reaches across the epigastrium to the left hypochondrium under the spleen. It then takes a right turn again and descends vertically through the left lumbar region to the left iliac fossa as the descending (left) colon. The descending colon is followed by the inverted V-shaped sigmoid colon (the narrowest part of colon), which becomes the rectum at S3 level. The cecum and colon have 3 longitudinal muscular bands called tenia and multiple sacculations called haustra.
The rectum lies in the concavity of the sacrococcygeal hollow and changes to the anal canal at the anorectal angle created by the puborectal sling formed by the innermost fibers of the levator ani muscle. The rectum has a dilated middle part called the ampulla, which contains 3 semilunar transverse mucosal folds called “valves of Houston.” The rectum is related anteriorly to the urinary bladder (see the image below), prostate, seminal vesicles, and urethra in males and to the uterus, cervix, and vagina in females. Anterior to the rectum is the rectovesical pouch in males and the rectouterine pouch in females. The rectum changes to anal canal at the level of the coccyx. The anal canal is related to the perineal body in front and the anococcygeal body behind; both of these are fibromuscular structures.
Rectum with urinary bladder in front in a male patient
The greater omentum (gastrocolic ligament) is like an apron with 4 layers of peritoneum (often fused). Two layers go down from stomach and then run upward to be attached to the transverse colon.
The appendix, transverse colon, and sigmoid colon have a mesentery (called mesoappendix, transverse mesocolon and sigmoid mesocolon, respectively), but the ascending colon and descending colon and the rectum and anal canal are retroperitoneal; the cecum does not have its own mesentery but is covered in all aspects by peritoneum.
There are several peritoneal fossae (eg, superior and inferior ileocecal, subcecal, retrocecal) around the cecum, which can be sites of internal herniation of the small bowel. Transverse mesocolon, containing the middle colic vessels, is attached to the anterior surface of the pancreas. Sigmoid mesocolon, containing superior rectal vessels, has an inverted-V – shaped attachment to the posterior abdominal wall — the tip of the V lying on the pelvic brim over the left ureter. The rectum does not have a proper mesentery, but the soft tissue around the rectum is often referred to as “mesorectum” by surgeons.
The posterior surface of entire rectum is retroperitoneal (extraperitoneal). Its upper third is covered by peritoneum on the front and sides, the middle third is covered by peritoneum on the front only, and the lower third is completely retroperitoneal (extraperitoneal).
The large intestine, especially the colon, is covered with numerous omental appendages (appendices epiploicae)—appendages of fat, each containing a vessel of its own from the colonic wall. These are visible during surgery.