1056 THE DIGESTIVE SYSTEM. —" 
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. an outgrowth of the canal appears, which 
represents the future cecum, 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. 711) and the superior mesenteric artery, running down between the 
layers of its mesentery. 
At the same time the two 
Ventral mesentery extremities of the coil 
Ventral mesentery 
GEsophagus, | Liver 
8 
Stomach Bie oust ‘ approach one another, 
a sf, OMAG = 
Spleen and form a narrow neck 
: 4DViaphragm 5 ae a es 
Uspiest to the _loop, as shown 
ts Line crosses in Fig. 711. There now 
41very 
— mesogastrium ta kes place By change 
. ; ‘ oye 
4 Hanon which entirely modifies 
uperior mesen- Mts o 
teric artery the position of the parts 
Pancreas aS 
Small intestine 
‘ “ rt te Duodenu : g 8 
Se aC real riage —this is a rotation of 
Colon the whole loop, with its 
Small mesentery, around the 
TESTING Inferior mesen- 2 
s artery superior mesenteric 
Qo « . ae 1 q\5 - 
Vitelline__ artery as an axis (Fig. 
TAL). The result of 
this rotation is that the 
duet 
Cecum \ : : 
Inferior mesenteric artery _// Rectum original right side of the 
Rectum | hem esentery loop of gut and mesen- 
Aorta tery becomes the left 
“ de ; and the beginning 
Fic, 711.—Two DIAGRAMS TO ILLUSTRATE THE DEVELOPMENT OF THE si de ; ; and the be 
INTESTINAL CANAL. of the large intestine is 
: ’ carried across the duo- 
The figure to the right shows the rotation of the intestinal loop round the ; ; 
superior mesenteric artery. In both figures the parts are supposed to be denum (Fig. 712), thus 
viewed from the left side. explaining the passage 
of the duodenum behind 
the transverse colon in the adult. At the same time the cecum comes to lie near 
the middle of the abdomen below the liver, a position in which it will be found during 
the third month. Subsequently, it passes further 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. The development of the cecum and appendix is described 
at page 1027. 
Peritoneum. — At first the primitive alimentary canal is suspended from the 
dorsal wall of the embryo, along the middle line, by a simple dorsal mesentery, which 
extends along the whole length of the tube, and is common to all its divisions—a condition 
found in the adult stage of many reptiles. There is also present, in the upper part of the 
cavity, after the stomach and liver descend into the abdomen, a ventral mesentery (Fig. 
711), which connects the stomach and duodenum to the back of the liver, and, passing on, 
connects the front of the liver to the anterior abdominal wall. The portion of this 
ventral mesentery, between the stomach and liver, becomes the lesser omentum ; its 
anterior portion, between the liver and the abdominal wall, forms the falciform ligament 
(Fig. 711); and, in its lower margin, the umbilical vein runs from the umbilicus to the 
liver, 
The portion of the dorsal mesentery lying behind the stomach is known as the 
mesogastrium. At first it is relatively short ; but with the growth of the posterior wall 
of the stomach, and the turning of that organ over on its right side, the mesogastrium 
becomes elongated, and is folded on itself, forming more or leas of a pouch, directed down- 
wards and to the left. The wall of this pouch becomes in part the great omentum, and 
within it is developed the small sac of the peritoneum. In the rotation of the stomach 
and the accompanying passage of the lesser omentum from an antero-posterior to a more or 
Jess transverse direction, a portion of the cavity of the abdomen is, as it were, caught in 
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