大肠

The large intestine begins at the ileocecal valve, which separates the ileum of the small intestine from the cecum. This valve controls material flow from the small intestine into the large intestine. The ileocecal sphincter, a mucous membrane fold, guards this opening and permits passage of digested material into the large intestine.

The colon is divided into four regions: ascending, transverse, descending, and sigmoid. The ascending colon rises on the right side and forms the right colic flexure near the liver. The transverse colon crosses horizontally, curves at the left colic flexure near the spleen, then descends as the descending colon. The sigmoid colon is S-shaped and terminates at the rectum.

The appendix, also called the vermiform appendix, is a coiled, twisted tube approximately 6 centimeters long attached to the posteromedial surface of the cecum. This small pouch extends from the junction where the small intestine meets the large intestine. Its exact function remains a subject of ongoing research in digestive system anatomy.

The anus is guarded by two sphincters: an internal anal sphincter composed of smooth muscle and an external anal sphincter composed of skeletal muscle. These sphincters typically keep the anus closed, opening only during elimination of feces. The anal canal's mucous membrane contains longitudinal folds called anal columns that house networks of arteries and veins.

The rectum is approximately 15 centimeters long and positioned anterior to the sacrum and coccyx. It begins where the sigmoid colon ends at the third sacral vertebra level and leads to the anal canal, the terminal end of the large intestine. The rectum serves as the final storage region before material is eliminated from the body.

The large intestine surrounds the small intestine on three sides, extending from the ileocecal valve to the anus. The ascending colon rises on the right side, the transverse colon crosses horizontally above, and the descending colon descends on the left side. This arrangement frames the small intestine within the abdominal cavity.

The ascending and descending colon are retroperitoneal, meaning they lie behind the peritoneum against the abdominal wall. The transverse and sigmoid colon are intraperitoneal, suspended by mesentery within the peritoneal cavity. This anatomical distinction affects their mobility and blood supply within the abdominal cavity.