THE ABDOMEN Sot 



from left to right round an axis, represented by the superior mesenteric artery. 

 Inasmuch as the mesentery of the U"lo<^P ^^ involved in this rotation, the 

 original right side of the mesentery becomes, after rotation, the left side, and 

 vice versa. 



Caecum and Vermiform Appendix. — The caecum appears about the fifth 

 week of embryonic Ufe as a bud in connection with the distal hmb of the 

 U-loop of intestine. It assumes the form of a bhnd diverticulum, which is 

 at first of uniform dimensions. It soon, however, becomes conical. The 

 proximal part undergoes enlargement, and represents the permanent caecum. 

 The distal part remains long and narrow, and represents the vermiform 

 appendix, whicn ultimately becomes connected with the left and posterior 

 part of the caecum. The various positions occupied by the caecum and its 

 migration to the right iliac fossa have just been referred to. Suffice it to say 

 that it lies, at first, on the left side of the median line, then it crosses to the 

 right side, being at this stage high up below the liver, and finally it sinks 

 into its usual permanent abode. The descent of the caecum, however, may 

 be arrested at any stage. 



For the development of the rectum and anal canal, see Index. 



Development of the Peritoneal Cavity and Peritoneum. — The peritoneal 

 cavity is one of the four divisions of the coelom or body-cavity. The other 

 three divisions — namely, the pericardial cavity and the two pleural cavities, 

 are contained within the thorax, and are separated from the peritoneal cavity 

 by the diaphragm. 



The parietal peritoneum is formed by the somatic mesoderm of the somato- 

 pleure of the body-wall, and the visceral peritoneum is derived from the 

 splanchnic mesoderm of the splanchnoplettre of the primitive intestinal tube. 

 The opposed surfaces of these two layers cire lined with mesothelium, the 

 flattened cells of which give rise to the lining endothelium of the peritoneal 

 cavity. 



In early life the entire alimentary tube is characterized by its shortness 

 and straightness. Its ventral and dorsal aspects are attached to the ventral 

 and dorsal body-walls by mesodermic folds, called primitive mesenteries. 

 These mesenteries are two in number — namely, ventral and dorsal — and they 

 establish a continuity between the splanchnic mesoderm of the alimentary 

 tube and the somatic mesoderm of the body-wall. 



The primitive ventral mesentery extends from the pharynx to the upper 

 part of the duodenum, and consists of splanchnic mesoderm. The cephalic 

 part is known as the cardiac portion. The heart and pericardium are de- 

 veloped within it, and it gives rise to the ventral and dorsal mesocardia. The 

 caudal or posterior part of the ventral mesentery- forms the ventral, or anterior, 

 gastro-diiodenal mesentery. The primitive dorsal mesentery is the fold which 

 attaches the dorsal aspect of the primitive alimentary' tube, over its entire 

 length, to the dorsal body-wall. Like the ventral mesentery, it consists of 

 splanchnic mesoderm. 



The primitive mesenteries are thus two in number — namely, ventral and 

 dorsal. 



The ventral gastro-duodenal mesentery extends from the ventral aspects 

 of the primitive stomach and upper part of the duodenum to the ventral body- 

 wall on the cephalic side of the umbilicus. As the liver, which is undergoing 

 development within the caudal or hepatic layer of the septum transversum, 

 descends, it separates from the middle or diaphragmatic layer of that septum', 

 which gives rise to the ventral portion of the diaphragm; and as it descends 

 it passes between the two layers of the gastro-duodenal mesentery. This 

 mesentery is now divided into two parts by the developing Uver. The part 

 between the liver and (i) the ventral portion of the diaphragm and {2) the 

 ventral wall of the abdomen as low as the umbilicus becomes the falciform 

 ligament of the liver; and the part between the liver (future portal fissure) 

 and (i) the stomach (future small curvature); and (2) the duodenum (future 

 first part for about an inch) becomes the small or gastro-hepatic omentum. 



The dorsal mesentery receives names corresponding to the parts of the 

 ahmentary tube with which it is connected. Thus, in the abdomen, there are 



51 



