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On the left iliac fossa, the sigmoid colon is supported by a mesentery called the mesosigmoid. The sigmoid colon is an intraperitoneal organ because it is supported by a mesentery that descends into the pelvis. In the pelvic cavity at the level of S2 vertebra, the sigmoid colon becomes continuous with the rectum. The rectum is the third part of the large intestine and has no supporting mesentery.

Links and References:
Grant's: 2.71
Netter: 266
Rohen/Yokochi: 287, 298