Differences in the rate of growth between the length of the body wall and the length of the mesocolon may play an important part in the production of peritoneal fossæ, small pouches which in some regions of the abdomen may assume considerable proportions. Such fossæ are found around the duodeno-jejuneal angle, the cæcum and appendix, and the sigmoid flexure. They will be considered more in detail with these respective regions, especially in reference to their relation to retro-peritoneal hernia.
In a certain proportion of cases adhesion between the parietal peritoneum and the ascending and descending mesocolon is incomplete or entirely wanting, resulting in the formation of a more or less completely free ascending and descending mesocolon. Treves, in an examination of 100 bodies, obtained the following figures:
In 52 subjects there was neither an ascending nor a descending mesocolon, the intestine being fixed in the manner which is regarded as normal.
In 22 there was a descending, but no trace of an ascending mesocolon.
In 14 a mesocolon was found in both the ascending and descending segments of the large intestine.
In 12 there was an ascending mesocolon, but no corresponding fold on the left side. Hence from this series a mesocolon may be expected on the left side in 36 per cent., on the right side in 26 per cent.
Fig. 155.—Abdominal viscera of Macacus cynomolgus, Kra monkey, hardened in situ. (Columbia University Museum, No. 1801.)
Both development and comparative anatomy would lead us to expect that the descending mesocolon would be found more frequently than the ascending.