8o2 A MANUAL OF ANATOMY 



the mesogastrium, mesoduodenum, mesojejunum, meso-ileum, mesocolon, 

 and mesorectum. 



In the course of development these various parts of the primitive dorsal 

 mesentery undergo marked changes. 



I. Mesogastrium. — The mesogastrium is that portion of the primitive 

 dorsal mesentery which extends vertically between the great curvature of the 

 stomach, which at this period is directed dorsalwards, and the dorsal body- 

 wall. It is at first comparatively short, but it undergoes lengthening as the 

 stomach turns round on its right side. During this partial rotation of the 

 stomach the mesogastrium is somewhat doubled upon itself, and forms a 

 pouch, which is directed downwards and to the left. This pouch is the initial 

 stage in the formation of the small sac of the peritoneum. The pouch is 

 bounded ventrally by (i) the small or gastro-hepatic omentum, and (2) the 

 dorsal surface of the stomach (originally the right surface) ; dorsally by the 

 dorsal body-wall; and caudalwards by the somewhat doubled mesogastrium. 

 During the rotatory movement of the stomach, the small or gastro-hepatic 

 omentum, the direction of which was originally sagittal — that is, dorso- 

 ventral — now assumes a coronal or transverse position. In this manner, 

 combined with a slight doubling of the mesogastrium, a small part of the 

 peritoneal cavity is incompletely partitioned off, and this is the portion which 

 constitutes the upper part of the small sac, lying, as just stated, dorsal to the 

 small omentum and dorsal surface of the stomach. This upper part of the 

 small sac communicates with the general peritoneal cavity (from which it 

 has been partitioned off) by an opening, called the foramen of Winslow, which 

 is situated behind the right or free border of the small or gastro-hepatic 

 omentum. This opening is at first large, but, as the liver grows, it becomes 

 considerably curtailed. 



The great, or gastro-collc, omentum is developed from the mesogastrium, 

 which increases slowly in size downwards. When fully developed, it forms 

 the extensive sheet which descends from the great curvature of the stomach, 

 and lies ventral to the transverse colon and the coils of the jejunum and 

 ileum. As the great omentum grows downwards, the pouch, which represents 

 the upper part of the small sac, increases in size and also grows downwards. 

 This downward extension of the small sac occupies the interval between the 

 two anterior, or descending and the two posterior, or ascending, layers of the 

 great omentum, this part of the small sac being known as the bag or sac of 

 the great omentum. 



For the changes which this curtain undergoes, prior to assuming its con- 

 dition in the adult, see the Peritoneum in the Foetus. 



The gastro-splenic omentum and lieno-renal ligament are associated with 

 the development of the spleen (lien) in the mesogastrium. The portion of 

 the mesogastrium between the stomach and spleen gives rise to the gastro- 

 splenic omentum, whilst the portion between the spleen and the left kidney 

 forms the lieno-renal ligament. 



It is to be noted that the great or gastro-colic omentum is developed from 

 the mesogastrium, which is a portion of the primitive dorsal mesentery, whilst 

 the small or gastro-hepatic omentum is formed, as stated, by that portion of 

 the ventral gastro-duodenal mesentery which extends between the liver (the 

 future portal fissure) and the stomach (the future small curvature). 



As the development of the abdominal portion of the alimentary tube 

 proceeds, it undergoes considerable changes, which impress modifications 

 upon the peritoneum, and impart to it a remarkably complicated character. 



The stomach appears at first as a spindle-shaped dilatation of the alimentary 

 tube, which, together with the duodenal portion of the intestinal tube, is at 

 first straight. For the subsequent changes in form and position of the stomach 

 and intestinal canal, see their development. It is sufficient in this place to 

 state that the stomach becomes curved, and turns round on its right side, 

 carrying the duodenum along with it, whilst the intestinal tube undergoes 

 elongation and becomes very tortuous. 



The primitive ventral gastro-duodenal mesentery, as stated, gives rise to — 

 (1) the falciform ligament of the liver, and (2) the sirtall or gastro-hepatic 

 omentum. 



