THE INTESTINAL CANAL. 1011 



sides are covered with peritoneum. Above it is in contact with the right lobe of 

 the liver, leaving its "impression." Posteriorly there is no peritoneum, areolar 

 tissue connecting it with the kidney, vessels at its hilus. and vena cava. The 

 pancreatic and common bile-ducts open into its postero-internal wall below the 

 middle. The head of the pancreas is to its inner side. 



Now the duodenum changes its direction and passes more or less horizontally 

 from right to left in front of the great vessel-trunks and crura of the diaphragm, 

 moulding itself over the third or fourth lumbar vertebra. This is the transverse 

 or pre-aortii- portion and is two or three inches long. The head of the pancreas 

 is above it. It is crossed by the superior mesenteric vessels and mesentery. Its 

 anterior surface is covered by the peritoneum of the mesentery, but is separated 

 from it when the superior mesenteric vessels cross it from above. On the right 

 its posterior surface has no peritoneal covering, but on the left the posterior layer 

 of the mesentery may be prolonged behind it. In the middle line this part of 

 the duodenum is situated at the point of divergence of the two layers of the root 

 of the mesentery (Fig. 606). 



Thence the duodenum ascends along the left side of the vertebral column and 

 aorta, touches the left kidney, lies upon the left crus of the diaphragm, and ends 

 at the left side of the second or first lumbar vertebra. This part is about two 

 inches long and is called the fourth or ascending portion. 



It often turns abruptly forward to unite with the jejunum and form the duo- 

 deno-jejunal angle. This terminal portion, about 2 cm. long (less than one inch), 

 has been described as the fourth portion, but with the U-shaped duodenum it 

 makes the fifth portion. 



The duodenum begins with a short portion looking backward and ends with a 

 short portion looking forward. 



The five parts are 1. Superior hepatic curve, or pars superior horizontalis. 

 '2. Descending or vertical portion. 3. Pre-aortic or transverse portion. 4. Ascend- 

 ing portion. 5. Terminal portion to form (6) the duodeno-jejunal angle. 



When the above arrangement is complete, the duodenum has the form of the 

 letter U, considering the second, third, and fourth portions (Figs. 632 and 634). 

 When, however, the descending and ascending portions unite by a short curve 

 or angle, the transverse portion is practically lacking, and the duodenum is then 

 V-shaped (Fig. 633). The angle of the V is thrown to the right against the vena 

 cava, and the ascending portion crosses the abdominal aorta at a sharp angle. 

 The U-shaped duodenum usually descends to the fourth lumbar vertebra, and 

 seems to occur in foetal life when the ascending colon obstructs the way. The 

 V-shaped duodenum usually descends to the fifth lumbar vertebra, and* occurs 

 when there is plenty of room and no obstruction by the descent of the ascending 

 colon. 



The lengths of the parts vary in the two types thus (measured from fifteen 

 subjects) : 



Duodenum 

 In U- In V. 



Superior curve 4 cm. 4 cm. 



Descending portion 10.5 " 12 " 



Pre-aortic portion 9.5 " 2 " 



Ascending portion 7 " 13 " 



3l 31 



Peritoneal Relations of the Duodenum. The duodenum is included among the 

 extra-peritoneal viscera. In the foetus it is completely invested, but the two 

 layers of its mesentery have been separated or perhaps appropriated by the rapidly 

 growing kidneys, and its posterior surface has adhered to the posterior abdominal 

 wall. Its visceral layer has become parietal peritoneum. To get at this peri- 

 toneum it is necessary, on the cadaver, to practise certain manipulations : to 

 disclose the initial superior parts, draw the hepatic flexure of the colon down and 

 to the left, and lift up the anterior margin of the liver. The pylorus and supe- 



