1020 THE DIGESTIVE SYSTEM. | | 
diameter (‘5 to 2°0 mm.). They form an almost continuous layer as far as the opening 
of the bile duct ; beyond this they diminish progressively, and completely disappear near — 
the duodeno-jejunal flexure. 
The mucous membrane, which is thicker in the duodenum than in any other part of 
the small intestine, is covered throughout with broad short villi. Its other character- 
istics have been already fully described. 
Various Forms of Duodenum.—Three different types of duodenum have been described— 
(1) The annular, in which the curves separating the various portions are open, and the two 
extremities come fairly close to one another. (2) The U-shaped, in which the transverse part of 
the third portion is very long, and the ascending portion nearly vertical ; and (3) the V-shaped 
duodenum, in which the transverse part of the third portion is very short or absent. 
Duodenal Pouch. — A diverticulum of the duodenum, arising from its left side just above the 
opening of the bile duct, and running into the substance of the pancreas, is occasionally found. 
It is possibly connected with one of the outgrowths of the duodenum trom which the pancreas 
is developed in the embryo. 
Vessels and Nerves.—The duodenum receives its blood from the superior and interior 
pancreatico-duodenal arteries, branches of the gastro-duodenal and superior mesenteric arteries 
respectively. The blood is returned by the corresponding veins, the superior of which opens into 
the superior mesenteric, and the inferior into the beginning of the portal vein. 
The lymphatics pass to a set of glands placed along the pancreatico-duodenal arteries, and 
thence to the eceliae glands. 
The nerves come from the solar plexus of the sympathetic. 
Duodeno-jejunal Flexure.— When the ascending terminal portion of the duo- 
denum reaches the under surface of the pancreas, at a point opposite the left 
side of the first or second lumbar vertebra, it turns abruptly forwards, downwards, 
and to the left, and passes into the jejunum. This abrupt bend is known as the 
duodeno-jejunal flexure. Unlike the rest of the duodenum, which is subject to 
considerable variations in position, the duodeno-jejunal flexure is fixed by a thin 
band of unstriped muscle, which is attached above to the strong connective tissue 
around the cceliae axis, as well as to the left crus of the diaphragm, and below 
joins the muscular coat of the duodenum at the flexure. This band is known as 
the suspensory muscle of the duodenum (inusculus suspensorius duodeni—Treitz). 
The course taken by the gut at the duodeno-jejumal flexure is variable: the chief directions 
in their order of frequency are—(1) downwards, forwards, and to the left; (2) directly forwards 
and downwards ; (3) to the left, and then downwards ; (4) forwards and to the right (Harman). 
Some of the fibres of the suspensory muscle are said by Lockwood to pass into the mesen- 
tery, and he consequently calls it “the suspensory muscle of the duodenum and mesentery 
proper.” 
THE JEJUNUM AND ILEUM. 
The upper two-fifths, that is, about 8 feet of the small mtestine beyond the 
duodenum, are known as the jejunum (intestinum jejunum). The succeeding three- 
fifths, which usually measures about 12 feet, constitute the ileum. The ileum 
opens into the large intestine at the junction of the cecum and ascending colon, 
where its orifice is guarded by the ileo-ceecal valve. 
Both the jejunum and ileum are connected to the parietes by a large fold of 
peritoneum —the mesentery —which conveys vessels and nerves from the posterior 
abdominal wall to these divisions of the intestine. 
The part of the tube to which the mesentery is connected is known as the 
mesenteric or attached border, the opposite side is the free border. 
The mesentery (mesenterium) is a broad fan-shaped fold, composed of two 
layers of peritoneum, which connects the small intestine to the back of the abdomen. 
One border of the fold is wide and contains the intestine within it (Fig. 683). 
The other, known as the root of the mesentery (radix mesenteriil), is compara- 
tively narrow, being only 6 or 7 inches wide, but it is much thicker than the 
part near the gut, for it contains between its layers a considerable amount 
of fatty subperitoneal tissue, in addition to the large vascular trunks pass- 
ing to the intestine. The root is attached to the posterior abdominal wall 
along an oblique line, extending approximately from the left side of the 
second lumbar vertebra to the night iliac fossa (Fig. 691). In this course 
its line of attachment passes from the duodeno-jejunal flexure down the front of 
