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 illustrates schematically the area of mesocolic adhesion in the human subject after complete rotation, and the line of the mesentery of jejuno-ileum.
[Illustration: 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.]
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.