Gross Morphology

The colon is a continuous structure originating at the ileocecal valve and extending to the anus. The cecum is the first part of the colon, which lies in a posterior position at the right iliac fossa and has an ovoid-like shape. This cavity is more generous in proportion than other compartments of the colon. The appendix (a blind-ending out pouching) originates in the cecum and its opening is usually visible during colonoscopy.

The ascending colon runs cephalad and anteriorly from the cecum to just inferior to the liver, to the hepatic flexure, emerging into the peritoneum. The transverse colon continues from the hepatic flexure to the splenic flexure, from which it travels distally and once again posteriorly to the sigmoid colon, an S-shaped, tortuous, narrow peritoneal structure. At the peritoneal reflection, the rectum arises and, closely following the sacral curve, leads to the anal canal. The rectum is a vault-like structure that can distend in order to accommodate fecal load. The anal canal bears two sphincters, an internal and an external anal sphincter. The internal sphincter is composed of inner circular smooth muscle fibers and a distal external fiber on the other side of a muscular pelvic diaphragm. The fibers of the external sphincter are intertwined with those of the levator ani, tethered anteriorly and posteriorly to the perineal body and the coccyx, respectively.

With respect to colonic mobility within the abdominal, peritoneal, and pelvic cavities, the cecum and flexures are less mobile, with the sigmoid colon being the most mobile. The transverse colon supports the greater omentum and has a variable degree of mobility.

Cross-sectionally, the colon has an external longitudinal muscle and an inner layer of circular musculature, the former of which has coalescence of fibers forming band-like structures known as taeniaecoli. These taeniae are particular to the large intestine, are located at one-third of the circumference from each other, and run continuously from one end of the colon to the other. Haustra are hemilunar-like outpouchings present between taeniae. The more proximal rectal taenial fibers surround the rectum; the inner fibers form the internal anal sphincter. The external fibers are intertwined with those of the levator ani and sandwiched between fibers running anterior to posterior from the peroneal body to the coccyx, forming the external sphincter.

Food Allergies

Food Allergies

Peanuts can leave you breathless. Cat dander can lead to itchy eyes, a stuffy nose, coughing and sneezing. And most of us have suffered through those seasonal allergies with horrible pollen counts. Learn more...

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