ABDOMINAL CAVITY 499 



usually about from three to four inches in length, and takes 

 a downward course from the under surface of the liver. 

 Placed at its commencement in the epigastric region, it 

 descends into the umbilical region, lying close to the medial 

 side of the right lateral plane. Reaching the level of the 

 third lumbar vertebra, it turns towards the spine and enters 

 upon the third stage of its course. This second stage of the 

 duodenum is immovably fixed in its position. It is covered 

 by peritoneum only on its anterior and lateral surfaces, and 

 is crossed by the commencement of the transverse colon, 

 which, in this part of its course, does not possess a mesentery 

 (Fig. 170). Behind, it rests upon the right border of the 

 inferior vena cava and presents a variable relation to the renal 

 vessels and the anterior surface of the right kidney in the 

 neighbourhood of the hilum. To the right is the hepatic 

 flexure of the colon ; and to the left is the head of the 

 pancreas, which is moulded upon the medial side of this part 

 of the duodenum. 



The common bile-duct and the pancreatic duct open into 

 the second part of the duodenum a little below its middle, 

 at the junction of its medial and posterior aspects. 



Pars Horizontalis. The third part of the duodenum 

 crosses the posterior wall of the abdomen at the level of the 

 third lumbar vertebra, its direction being from right to left 

 and slightly upwards. To the right of the median plane it 

 lies in the upper part of the umbilical region, but near its 

 termination it rises above the subcostal plane into the 

 epigastric region. Its anterior and inferior surfaces are covered 

 by peritoneum. It lies behind the transverse colon, and it is 

 crossed by the upper part of the root of the mesentery of the 

 small intestine containing the superior mesenteric artery and 

 vein. Posteriorly it rests against the right ureter, the right 

 psoas magnus muscle, the right internal spermatic artery, the 

 inferior vena cava, and the abdominal part of the aorta. Its 

 upper border is in relation with the head of the pancreas and 

 the inferior pancreatico-duodenal artery, and its lower border 

 is in relation with coils of the jejunum. 



Pars Ascendens. The fourth part of the duodenum passes 

 upwards from the level of the upper part of the third lumbar 

 vertebra to the duodeno-jejunal flexure. In ' front and on 

 the left it is covered by peritoneum, and is in relation with 

 the upper part of the jejunum. To the right it is in 



132 a 



