Recalling the development of the jejuno-ileum it will not be difficult to recognize in the numerous coils of small intestine which succeed to the duodeno-colic isthmus the results of the increase in length of the descending or efferent limb of the human embryonal umbilical loop. Tracing these coils it will be found that the terminal portions of the ileum correspond to the apex and to the proximal part of the ascending or recurrent limb of the primitive loop, while the remainder of this limb furnishes the cæcum and the next succeeding segment of the large intestine. Following the tube up to this point the colic boundary of the duodeno-colic isthmus will be reached; from here the short large intestine of the carnivore descends straight into the pelvis, attached to the ventral surface of the vertebral column by a mesocolon which corresponds to the distal part of the original primitive dorsal mesentery.
Now with the parts still in this position examine carefully the arrangement of the mesentery and of the intestinal blood vessels. Starting with the duodenum it will be seen that the primitive sagittal mesentery of this portion of the intestine has followed the gut in its turn to the right, so that the original right layer of the sagittal membrane is now directed dorsad and lies in contact with the parietal peritoneum which invests the background of the abdominal cavity in the right lumbar region below the liver and covers the ventral surface of the right kidney. Beneath this parietal peritoneum the inferior vena cava is seen, receiving the right renal vein and ascending to enter the dorso-caudal aspect of the right lobe of the liver. If now we assume that in the cat the opposed serous surfaces of the original right leaf of the mesoduodenum, now directed dorsad, and of the parietal peritoneum adhere to each other, and that the visceral peritoneum covering the dorsal surface of the descending duodenum likewise becomes obliterated by adhesion to the subjacent parietal peritoneum, we will obtain the arrangement found in the adult human subject, in which the descending duodenum is fixed by adhesion below the right lobe of the liver and ventrad of the medial portion of right kidney, right renal vein and inferior vena cava. During this process of anchoring the head of the pancreas, which is found between the two layers of the free mesoduodenum of the cat, would also become fixed to the abdominal background by adhesion of the original right leaf of the mesoduodenum, investing what has now become the dorsal surface of the pancreas, to the parietal peritoneum. The original left layer of the primitive mesoduodenum would then appear as secondary parietal peritoneum covering what has now become the ventral surface of the transversely disposed head of the gland. The stages may be represented schematically in [Figs. 138]-[140].
Figs. 138 and 139 shows the arrangement in the cat where a free duodenum and mesoduodenum exists, with the pancreas included between its layers.[2]
It will be noticed that the duodenum in the cat can be carried over to the median line (Fig. 138) exposing the entire ventral aspect of the right kidney and the inferior vena cava beneath the primary lumbar parietal peritoneum. This manipulation will also expose the dorsal surface of the head of the pancreas, covered by what originally was the right leaf of the mesoduodenum.
Fig. 140 indicates the results of adhesion of the duodenum, pancreas and mesoduodenum to the parietal peritoneum as it normally occurs in the human subject. It will be seen that the primary parietal peritoneum can be traced mesad over the ventral surface of the right kidney as far as the point X, and that from here on to the median line the peritoneum is secondary parietal peritoneum, consisting of the visceral peritoneal investment of the ventral surface of the duodenum and of the original left leaf of the mesoduodenum, beneath which the ventral surface of the pancreas is seen. Pancreas and duodenum occupy in the adult secondarily a “retro-peritoneal” position, i. e., the peritoneum now covering the ventral surface of these viscera appears as a continuation of the parietal peritoneum, the transition between primary and secondary parietal peritoneum occurring along the line marked X in Fig. 140. The opposed peritoneal surfaces indicated by the dotted lines have become adherent and converted into loose connective tissue in which the pancreas and duodenum lie imbedded. In the human embryo this process of adhesion begins in the eighth week, starting at the duodeno-jejunal flexure and ascending gradually toward the pylorus. At the end of the fourth month the union is complete.
Proceeding caudad it will next be observed that the peritoneum of the mesentery occupies the narrow neck of the duodeno-colic isthmus, and that large vessels (the superior mesenteric) pass between its two layers at this point to supply the segments of the intestine forming the loop. In conformity with the greatly increased length of the intestine it will be found that the mesentery expands from the narrow pedicle at the neck in a fan-shaped manner in order to develop a sufficiently long margin for attachment to the intestine. The following points should be carefully borne in mind in studying the mesentery with the intestines in this position:
1. The mesentery presents two free surfaces, right and left. With the coils of the small intestine turned over to the right, the left leaf of the mesentery is turned toward the observer.
