PERICARDIUM, PLEUROPERITONEUM, DIAPHRAGM AND MESENTERIES. 383 



a lower (or caudal) which contains the rest of the digestive tube except the 

 duodenum. The mesentery of the duodenum and pancreas changes from a 

 serous membrane into subserous connective tissue, and these two organs as- 

 sume the retroperitoneal position characteristic of the adult (Fig. 339). 



The mesentery of the descending colon, or the descending mesocolon, lies in 

 the left side of the abdominal cavity, in contact with the peritoneum of the body 

 wall (see Fig. 342). It usually fuses with the peritoneum, and the descending 



Dors, mesogastrium 



Lesser omentum 

 (hep.-gast. lig.) 



Bile duct 



Mesoduodenum 



Transv. colon 



Spleen 



Duo.-jej. flexure 



Desc. colon 

 Desc. mesocolon 



Appendix 



Yolk stalk 



Medial line 



FIG. 342. Gastrointestinal tract and mesenteries in a human embryo. The arrow 

 points into the bursa omentalis. Kollmann. 



colon thus becomes fixed. After the ascending colon is formed, the ascending 

 mesocolon usually fuses with the peritoneum on the right side (see Fig. 342). In 

 a large percentage (possibly 25 per cent.) of individuals, the fusion between the 

 peritoneum and the ascending and descending mesocolon is incomplete or 

 wanting. 



The sigmoid mesocolon bends to the left to reach the sigmoid colon, but 

 forms no secondary attachments. It is continuous with the mesorectum which 

 maintains its original sagittal position. A sheet of tissue the mesoappendix 

 continuous with and resembling the mesentery, is attached to the caecum and 

 vermiform appendix (Fig. 342) . It probably represents a drawn out portion of 

 25 



