1250 



THE DIGESTIVE SYSTEM. 



the abdomen. Already, at the fifth or sixth week, the adult "form of the stomach is 

 clearly indicated. 



This rotation of the stomach around its long axis, which is accompanied by a rotation 

 of the lower end of the oesophagus, explains the asymmetrical position of the two vagi. 

 In the adult the left nerve is found on the front of the stomach, which was originally the 

 left side of the organ ; similarly, the right nerve lies on the back, which was originally 

 the right side. 



Intestines. At first there is no separation into large and small intestines; the 

 primitive canal simply forms a slender tube, with a convexity towards the umbilical 

 orifice, through which the vitelline duct passes to the yolk sac. Later, the tube increases 

 in length, and in embryos of 11 or 12 mm. and about five weeks old an outgrowth of 

 the canal appears, which represents the future caecum, and indicates the separation into 

 large and small intestines. Growing longer, the intestine forms a large loop with the 

 vitelline duct springing from its apex (Fig. 973), and the superior mesenteric artery 

 running down between the layers of its mesentery. At the same time the two extremities 

 of the coil approach one another, and form a narrow neck to the loop, as shown in 

 Fig. 973. There now takes place a change which entirely modifies the position of the 

 parts this is a rotation of the whole loop, with its mesentery, around the superior 

 mesenteric artery as an axis (Fig. 973). The result of this rotation is that the original 

 right side of the loop of gut and mesentery becomes the left side ; and the beginning 

 of the large intestine is carried across the duodenum (Fig. 973), thus explaining the 

 passage of the transverse colon in front of the second part of the duodenum in the 

 adult. At the same time the caecum comes to lie near the middle of the abdomen 

 below the liver, a position in which it is found during the third month. Subsequently, 

 it passes farther to the right ; and finally, descending, comes to occupy its adult position. 



The small intestine continues to grow in length, and, as a result, is thrown into coils, 

 which become more and more complex as the length increases, until the adult condition 

 is attained. The terminal portion of the large bowel retains its position on the left side, 

 and passes down to the anus. 



Formation of Gastric and Intestinal Glands, etc. The epithelial lining of the intestinal 

 tube is composed, at first, of a single layer of cells, and the inner surface is smooth. In 

 the second month the epithelium increases rapidly, and as a result its surface is thrown 

 into folds and furrows, arranged irregularly. Mesenchymal tissue passes into the interior 

 of the folds, and also blood capillaries. The folds appear first in the stomach, especially 



Ventral mesentery in the regions of the 



Liver curvatures, and later 



Ventral mesentery in the duodenum and 



.Biie-duct sma ji intestine, and 



then in the large in- 

 testine, where they are 

 formed first in the 

 rectum and last in the 

 In 



the stomach the folds 

 are arranged so as to 

 surround small isolated 



Inferior mesen- depressions, which 



teric artery afterwards become the 

 foveolae gastricae. In 

 the small intestine 

 isolated elevations are 

 found, in place of con- 

 tinuous folds, and at a 

 later stage new eleva- 

 tions are formed be- 

 tween the primary ones. 



CEsophagus 



Stomach 

 Spleen 



./Stomach 



Diaphragm 

 Spleen 

 Line crosses 

 mesogastrium 

 -Pancreas 



Superior mesen- vermiform process. 



Superior mesen- 

 teric artery 



teric artery 

 Duodenum 



Colon 



Csecur 



Inferior mesenteric artery 



Rectum 



Rectum 

 \ 

 The mesentery 



Aorta 



. FIG. 972. Two DIAGRAMS TO ILLUSTRATE THE DEVELOPMENT OF THE 

 INTESTINAL CANAL. 



The figure to the right shows the rotation of the intestinal loop around the These papillary "eleva- 

 superior mesenteric artery. In both figures the parts are supposed to be + * +/ 



viewed from the left side. 



In the large intestine, 



the arrangement resembles that in the stomach. The glands of the stomach a 

 intestine, viz., the gastric and duodenal glands, and the intestinal glands in the sm 

 intestine are formed by an active proliferation of the epithelium at the bottom of 

 furrows, and at first the cells forming them are everywhere of a similar character, a 



