The Anatomy of the Human Peritoneum and Abdominal Cavity / Considered from the Standpoint of Development and Comparative Anatomy

by George S. Huntington

The course of the vessels in the ascending and descending mesocola is not altered by the secondary adhesions. These vessels are in the adult situated behind the secondary parietal peritoneum derived from the mesocola.

The origin of the transverse mesocolon obtains by the fixation of the hepatic and splenic flexures high up in the abdomen a transverse course, and the transverse growth of the abdomen holds the membrane in this position cephalad of the duodeno-jejunal flexure, so that on elevating the transverse colon the mesocolon appears as separating the upper from the lower abdominal compartment. This posterior line of attachment or so-called “root of the transverse mesocolon,” is nothing more than the upper limit of the area of adhesion between the primitive parietal peritoneum and the opposed surfaces of the ascending and descending mesocola. Reference to the abdominal cavity of the cat after complete rotation ([Fig. 146]) will show the original continuity of the three mesocola very clearly. A secondary connection is established along the lateral border of ascending and descending colon (Fig. 158), between the primitive parietal peritoneum and the ventral visceral peritoneal investment of the large intestine. Both of the vertical segments of the colon now appear fixed. Their dorsal surfaces are uncovered by peritoneum and can be reached in the lumbar region, laterad of the kidney, without opening the peritoneal cavity (lumbar colotomy).

The caudal portions of both kidneys are covered, beneath the secondary parietal peritoneum, by a layer of loose connective tissue representing the result of obliteration by adhesion of the first and second of the original three layers of prerenal peritoneum, viz., the primitive parietal (1) and the two layers of the mesocola (2 and 3).

Line of Attachment of the Mesentery of the Jejuno-ileum.—Examination of the caudal surface of the transverse mesocolon in the cat, with the parts in the above outlined position, will show how and why in the adult human abdomen the duodeno-jejunal angle appears to dip out from beneath the transverse mesocolon, becoming gradually more and more free until complete transition to the mobile jejunum is obtained. From this point, situated to the left of the second lumbar vertebra, the dorsal attachment of the adult human mesentery of the jejuno-ileum extends somewhat obliquely caudad and to the right to terminate in the right iliac fossa at the ileo-colic junction.

Returning to the conditions presented by the cat’s intestines to obtain an explanation of this line of fixation we must recall the fact that in the peritoneum included within the limits of the umbilical loop, after differentiation of small and large intestine, but before rotation, we have both the elements of the mesentery of the small intestines and of the ascending and transverse mesocolon combined ([Fig. 141]). For it will be seen that this membrane carries at this time vessels both to the jejuno-ileum and to the segments of the large intestine (cæcum, ascending and transverse colon). This fact will be at once recognized if the cat’s intestines are arranged to correspond to the primitive condition ([Fig. 136]) and the mesentery examined.

After rotation and differentiation of the colic segments and after the adhesion of the ascending and descending colon in man, the course of the main trunk of the superior mesenteric artery passes, after crossing the third portion of the duodenum, down and to the right to terminate near the ileo-colic junction by anastomosis with its ileo-colic branch. The adhesion of the right and left mesocola to the dorsal parietal peritoneum proceeds mesad as far as this line, leaving free the mesentery of the small intestines, which contains the vasa intestini tenuis derived from the left side of the main vessel. The secondary line of attachment of the mesentery to the abdominal background is therefore along this line. To obtain a clear idea of these processes of development in man assume that in the cat, after rotation and establishment of the three divisions of the colon, the two vertical (ascending and descending) mesocola become adherent to the dorsal parietal peritoneum, leaving the mesentery of the small intestine free.

Fig. 159.—Schematic figure to show lines of mesocolic adhesion, formation of root of transverse mesocolon and root of mesentery of jejuno-ileum in human subject.

Fig. 159 illustrates schematically the area of mesocolic adhesion in the human subject after complete rotation, and the line of the mesentery of jejuno-ileum.

Fixation of the ascending and descending cola and of their mesocola proceeds cephalad as far as the line AB, which thereby constitutes the root of the free transverse mesocolon.