THE CONTENTS OF THE ABDOMEN. 135 



of the entrance into the bursa omentalis. The peritoneum passes from the ligament to 

 the anterior surface of the pars superior; the posterior surface of this portion, directed 

 toward the bursa omentalis, is also covered by peritoneum. The remaining portions 

 of the duodenum, on the contrary, are covered only anteriorly by a peritoneal reflection, 

 but even this is completely wanting in two narrow places, namely, at the crossing of 

 the transverse mesocolon and at the crossing of the superior mesenteric vessels (Plate 15). 

 When the stomach is empty, the superior portion of the duodenum runs in a frontal direction, 

 but when the stomach is full, it lies more in a sagittal plane. It is situated just beneath the 

 quadrate lobe of the liver. The pars descendens (Plates 15 and 18 and Figs. 61 and 67 ) lies at the 

 right side of the second lumbar vertebra and is crossed at right angles by the transverse colon. 

 If one desires to see the interior portion of the pars descendens, which disappears at the upper 

 margin of the transverse colon, the transverse colon, together with the great omentum, must 

 be reflected upward. This renders visible the duodenum, upon the posterior abdominal wall, 

 and the region of the flexura inferior duodeni, i. e., the transition of the descending into the 

 ascending portion. The pars descendens produces the impressio duodenalis upon the right 

 lobe of the liver (see page 131); the external convex margin of this portion of the 

 intestine borders upon the internal margin of the right kidney (Plate 15 and Figs. 61 and 

 67), to which the peritoneal covering of the duodenum is reflected; the interior border is 

 in relation with the inferior vena cava. The pars ascendens passes to the left, and after crossing 

 the inferior vena cava, at the level of the third lumbar vertebra, and the abdominal aorta (Fig. 

 61 and Plate 18) becomes continuous with the duodenojejunal flexure at the left of the vertebral 

 column. The root of the mesentery, running in an approximately vertical direction, crosses the 

 duodenojejunal flexure (Plate 15) and contains the superior mesenteric artery and vein, the 

 vein being situated to the right of the artery (see also Plate 61). 



The upper portion of the duodenum is the seat of predilection for duodenal ulcers. Since 

 this part is covered anteriorly and posteriorly by peritoneum, ulcers situated in the posterior, 

 as well as those located in the anterior wall of the duodenum may perforate into the peritoneal 

 cavity. (Perforating ulcers of the posterior duodenal wall open into the bursa omentalis.) 

 Such a perforation into the peritoneal cavity may be prevented by preceding adhesions between 

 the duodenum and the liver, the gall-bladder, or the posterior abdominal wall. 



The Small Intestine. The small intestine is from six to eight meters (twenty to twenty- 

 five feet) long (jejunum about three-fifths, ileum about two-fifths of the total length) [ac- 

 cording to English and American authors, this is reversed, the jejunum comprising two-fifths, 

 the ileum three-fifths. ED.], and its numerous coils fill the space below the transverse colon 

 and the transverse mesocolon, overlie more or less the ascending and descending colon, and 

 rest upon the iliac fossas, as well as upon and between the pelvic viscera (Plates 1 1 and 16). The 

 great omentum (Plate 16; it has been removed in Plate n) hangs down from the transverse 

 colon, and, according to the degree of its development, separates the intestines more or less com- 

 pletely from the anterior abdominal wall. There is a certain regularity in the course pursued 

 by the intestinal coils, but it is subject to great variation. 



The superior mesenteric artery, the second of the single visceral branches of the abdominal 

 aorta, enters the root of the mesentery at the lower margin of the pancreas (Plate 15 and Fig. 



