724 A MANUAL OF ANATOMY 



is produced by a fold of peritoneum which contains the sigmoid artery. It 

 opens downwards and towards the left side. 



In early life the alimentary tube is very short and straight. Its ventral and 

 dorsal aspects are attached to the ventral and dorsal body-walls by meso- 

 dermic folds, which constitute the primitive ventral and dorsal mesenterjes. 



The primitive ventral mesentery extends from the pharynx to the duodenum. 

 Its cephalic or cardiac portion gives rise to the ventral and dorsal cardiac 

 mesenteries or mesocardia. Its caudal or posterior part forms the ventral or 

 anterior gastro-duodenal mesentery. 



The primitive dorsal mesentery connects the dorsal aspect of the entire 

 primitive alimentary tube to the dorsal body-wall. 



The ventral gastro-duodenal mesentery extends from the ventral aspect 

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

 wall on the cephalic side of the umbilicus. It is closely associated with the 

 lower layer of the septum transversum, within which the liver undergoes 

 development. As the liver descends, it carries with it the ventral gastro- 

 duodenal mesentery, which it divides into two parts. 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, forms the falciform ligament of the 

 liver ; and the part between the liver (portal fissure) and stomach (small 

 curvature) forms the small or gastro-hepatic omentum. 



The primitive dorsal mesentery receives names corresponding to the parts 

 of the alimentary tube with which it is connected. Thus, in the abdomen, 

 there are the mesogastrium, mesoduodenum, mesojejunum, meso-ileum, meso- 

 appendix, mesocolon (ascending, descending, iliac and pelvic), and meso-rectum. 



As development proceeds, these mesenteric folds undergo important changes. 



Mesogastrium. — This is the primitive posterior mesentery of the stomach, 

 and, as stated, is a portion of the primitive dorsal mesentery. It consists of 

 two layers of peritoneum, which extend between the great curvature of the 

 stomach (at this period directed backwards) and the dorsal body-wall, where 

 the two layers enclose the pancreas. 



As the stomach changes its position, the pyloric end being tilted forwards, 

 and the whole organ along with the duodenum turning round upon its right 

 side, the great curvature, which originally looked backwards, is now directed 

 downwards and towards the left side. During this change the mesogastrium 

 becomes stretched. Thereafter a gradual extension of the mesogastrium 

 takes place in a downward direction from the great curvature of the stomach 

 over the transverse colon, jejunum, and ileum; and in this manier the great 

 omentum is formed. 



At first the two posterior or ascending layers of the great omentun^ pass over 

 the front and upper part of the transverse colon without separating to enclose 

 it, and then pass backwards to the posterior abdominal wall, where they 

 enclose the pancreas. The lower layer is now reflected ventralwards from the 

 posterior abdominal wall until it meets with the transverse colon. Having 

 invested this part of the gut, it retraces its steps to the posterior abdominal 

 wall, thus forming the two layers which constitute the primitive transverse 

 meso-COlon. On referring to Fig. 310, it will be seen that, in this region, there 

 are four layers of peritoneum at this stage. The upper two layers are contin- 

 uous with the two posterior or ascending layers of the great omentum, and 

 represent the original mesogastrium. The lower two layers belong to the 

 primitive transverse mesocolon. Subsequently the lower of the upper two 

 layers and the upper of the lower two layers unite and disappear. There are 

 thus left only two layers of peritoneum, which constitute the transverse meso- 

 colon of the adult, the lower layer of which is part of the primitive transverse 

 mesocolon, whilst the upper layer is part of the great omentum. In fact, 

 both layers are ultimately derived from the two posterior or ascending 

 layers of the great omentum. As the result of these changes, the pancreas 

 comes eventually to lie behind the peritoneum, whereas it was originally 

 contained between the two layers of the mesogastrium. 



The remaining portions of the primitive dorsal mesentery are disposed as 

 follows : 



