In the lower animals the descending mesocolon is always an extensive and conspicuous membrane. It is well developed in all monkeys and the anthropoidea, as the remains of the primary vertical fold of the dorsal mesentery, while the ascending mesocolon is a secondary production, acquired during the development of the bowel by rotation.
In most of the lower monkeys the ascending mesocolon is also largely or entirely free. The descending mesocolon can always in these animals be reflected to the median line (cf. Fig. 155).
The line of attachment in man of the descending mesocolon is usually along the lateral border of the left kidney and vertical, while the line of attachment of the ascending mesocolon is usually less vertical, crossing the caudal end of the right kidney obliquely from right to left and with an upward direction (Fig. 156).
Fig. 156.—Schema of visceral and peritoneal relations of ventral surface of right kidney.
In like manner when both the ascending and descending mesocola are absent as free membranes the left or descending colon is adherent along the lateral border of the kidney to the abdominal parietes, while the ascending colon is fixed at the hepatic flexure a little obliquely across the ventral surface of the caudal end of the corresponding gland ascending toward the medial margin.
Treves found in the cases of persistent ascending mesocolon in the adult that the membrane varied in breadth from 1″ to 2″ while the persistent fold on the left side varied between 2″ and 3″ in breadth.
In the fœtus, up to 5″–6″ in length, the descending mesocolon is usually an extensive fold. Its attachment is vertical, but nearer to the median line than in the adult, usually along the medial border of the left kidney. It is at times found attached along this line in the adult.
An ascending mesocolon is rare even in the fœtus. The cæcum and beginning of the ascending colon are completely invested by peritoneum, but above the parts so invested the colon is usually adherent along an oblique line to the ventral and medial aspect of the right kidney.
In the fœtus at full term, if the cæcum is still undescended and in contact with the liver, it is not uncommon to find the cephalic portion of the descending colon provided with a mesocolon, while the caudal part of the descending colon is fixed by adhesion to the ventral surface and lateral border of the left kidney. This free membrane is then really a part of the transverse mesocolon. Where the cæcum descends to the iliac fossa the portion of the fœtal descending colon so invested is drawn over to the right and incorporated in the transverse colon.