=Fossa Intersigmoidea.=--A second peritoneal pocket or fossa is encountered in the region of the sigmoid flexure and its mesocolon. The formation of this fossa is closely associated with the adult disposition of the sigmoid mesocolon as part of the original primitive vertical dorsal mesentery. In the typical arrangement of the parts the sigmoid or omega loop of the large intestine has a free mesocolon. The adhesion of the descending mesocolon to the parietal peritoneum usually ceases along a line drawn horizontally from the lateral margin of the left psoas at a level with the crest of the ilium to the medial side of the iliac vessels. This line, along which the mesocolon ceases to be adherent to the parietal peritoneum, joins the attachment of the distal portion of the sigmoid mesocolon, which partially retains its primitive vertical origin to the dorsal midline, at a right angle. This angle is the site of the _intersigmoid fossa_, the entrance into which is seen usually as a round opening of variable size on elevating the sigmoid flexure and putting its mesocolon on the stretch. Fig. 159 shows the area of adhesion between the primitive descending mesocolon and the parietal peritoneum (from C mesad) which results in the formation of a free mesocolon for the sigmoid flexure. Frequently in the angle formed by the horizontal and vertical line of attachment of the sigmoid mesocolon a non-adherent strip of the primitive mesocolon roofs in a more or less extensive intersigmoid fossa, whose fundus is directed upwards and inwards. =Caecum, Appendix and Ileo-colic Junction.=--Several peritoneal fossae and folds are found in the ileo-colic region in connection with the caecum, appendix and termination of the ileum. The practical importance of this portion of the intestinal tract and the great morphological interest which attaches to the same make it worth while to consider its anatomy in a separate chapter. PART II. ANATOMY OF THE PERITONEUM IN THE SUPRA-COLIC COMPARTMENT OF THE ABDOMEN.