THE PERITONEUM 



1249 



velopment of the heart and lungs, while the abdominal portion serves as a matrix 

 for the developing liver, and ultimately becoming the falciform ligament and gastro- 

 hepatic omentum. The mesoderm lining the body cavity as well as the free 

 surfaces of the mesenteries soon assumes the character of a serous membrane, 

 and is then called the peritoneum. Topographically, this membrane may be sub- 

 divided into: (a) Parietal peritoneum, investing the inner surface of the abdominal 

 cavity; (6) visceral peritoneum, investing the alimentary tube and its derivatives; 

 (c) mesenteric peritoneum, connecting the two former, as a suspensory support 

 for the alimentary tract, and carrying the vessels and nerves to it. 



The pharynx, oesophagus, stomach, and part of the duodenum are developed 

 from the foregut; the descending and sigmoid parts of the colon, the rectum, and 

 the tubular diverticulum of the allantois are developed from the hindgut; the 

 midgut gives origin to the remainder of the alimentary tube." 



Duodenum. 



Small 

 intestine. 



Yolk stalk. 



Rectum. 



Duodenum. 



Mesocolon. 



Cecum. 

 Vermiform 

 appendix. 

 Mesentery. 



Yolk stalk. 



Rectum. 



FIGS. 968 and 969. Illustrating two stages in the development of the human alimentary canal and its 

 mesentery. The arrow indicates the entrance to the bursa omentalis. (Hertwig.) 



The upper part of the foregut becomes dilated in the form of branchial pouches 

 to form the pharynx; the succeeding part remains tubular 1 and with the descent 

 of the stomach becomes elongated to form the ossophagus. About the fifth week 

 a fusiform dilatation, the future stomach, makes its appearance. The stomach, 

 together with a succeeding part of the duodenum, has a ventral mesentery, called 

 the ventral mesogastrium, in addition to its dorsal attachment, the dorsal meso- 

 gastrium. In the base of the dorsal mesogastrium courses the aorta which sup- 

 plies a series of branches, embedded in the mesoderm, to the alimentary tube. 

 The stomach undergoes a further dilatation, and its two curvatures can be recog- 

 nized, the greater directed dorsad and the lesser ventrad, while its two surfaces 

 look to the right and left respectively. The midgut undergoes great elongation 

 and forms a loop which projects downward and forward; from the arch of the loop 

 the vitelline duct passes to the umbilicus. For a time a part of the loop extends 

 beyond the abdominal cavity into the umbilical cord, but is withdrawn into the 

 cavity by the end of the third month. 



About the sixth week a lateral diverticulum makes its appearance on the caudal 

 part of the loop a short distance from the vitelline duct, and indicates the future 

 cecum and appendix. The part of the loop on the distal side of the cecal diver- 

 ticulum increases in diameter and forms the future ascending and transverse 

 portions of the large intestine. The cecal diverticulum shares only partially in 

 this increase in calibre, its pendant portion remaining rudimentary and forming 

 the appendix. 



1 The epithelium of the oesophagus and certain other portions of the gut increases so greatly in thickness by 

 active proliferation of the cells that the lumen is nearly or wholly closed during the second month, to be 

 restored about one month later (Congenital stenosis). 



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