244 



MESENTERY 



Development of the mesentery and lesser sac of the peritoneum. 



A. ventral mesentery is not developed behind the umbilicus. The fold so called 

 in front of it is largely taken up by the liver-parenchyma (fig. 292, p. 238), and 

 in the adult is represented by the gastro-hepatic omentum and falciform ligament. 

 The dorsal mesentery extends along the whole length of the gut. It becomes drawn 

 out into an extensive sheet when the vitelline loop is formed. As the gut 

 increases in length the neck of the mesentery (which lies within the body while 

 the rest of the sheet is enclosed in the coelom of the umbilical cord) becomes 

 narrowed until the end of the vitelline loop (the future mid-point of the transverse 

 colon) comes into close relationship on the dorsal wall of the abdomen with the end 

 of the duodenum. This relationship persists through all the later stages, and 

 when the loop of the colon is formed, a twisting of the neck of the mesentery 

 necessarily takes place in such fashion that the small gut with its mesentery, i& 

 carried to the left under the distal (colonic) part of the vitelline loop with its 

 mesentery (fig. 301). When this rotation is complete, and the colonic loop is 



FlG. 299. DlAGBAM OF THE MESENTEBY, STOMACH, 

 AND INTESTINE OF A HUMAN EMBBYO OF SIX 

 WEEKS. (Toldt.) 



sf, stomach ; g.c., greater curvature ; I.e., smaller 

 curvature ; mg, mesogastrium ; spl, spleen ; p, pan- 

 creas; c, ceecum ; r, rectum; me, mesentery; 

 ao, aorta ; cl, cceliac axis ; s.mes.a., i.mes.a., supe- 

 rior and inferior mesenteric arteries. 



FlG. 300. DlAGBAM OF A SECTION ACBOSS THE 

 ABDOMEN OF A HUMAN EMBBYO OF THE 

 THIBD MONTH. (Toldt.) 



I, I, liver; k, kidneys ; g.o., great omentum ; 

 g'.o'., omental sac ; s.o., small omentum. The 

 other letters as in fig. 299. 



carried to the right, the meso-colon is necessarily stretched into a transverse plane. 

 At the same time the portion of the gut distal to the vitelline loop being carried 

 to the left, as the small intestine develops, the mesentery proper to it assumes a 

 transverse position and becomes the left half of the transverse mesocolon. The 

 ascending and the descending colon have at first a free mesentery, but they become 

 fixed by the disappearance of the posterior layer on each side, and the adult 

 conditions are realised. 



The lesser sac of the peritoneum l is very early r oreshadowed by the 

 formation of the diverticulum in the mesenteric septum mentioned above. Some 

 observers attribute its formation to the downward growth of the mesolateral fold, 

 while others regard it as an actual inpushing of the ccelomic space to form a pocket. 

 The opening into the diverticulum (foramen of Winslow) at first lies between the 

 rudiment of the right lung and the stomach, but later, by the extension of the 

 liver-trabeculse into the dorsal pillar of the mesolateral fold and the formation 

 of the caval lobe of the liver, it comes to lie between the liver and duodenum. 



1 A review of the literature of the lesser sac of the peritoneum, by Broman, will be found in 

 Ergebnisse der Anatomic und Entwickelungsgeschichte, 1905. 



