914 THE ORGANS OF DIGESTION. 



The third or transverse portion (pre-aortic portion) varies much in length; 

 when the duodenum assumes the ordinary U-shaped form, it measures from two to 

 three inches ; but when it presents the rarer V-shaped form, it is practically want- 

 in or very much reduced in length. It commences at the right side of the fourth 

 lumbar vertebra and passes from right to left, with a slight inclination upward, in 

 front of the great vessels and crura of the Diaphragm, and ends in the fourth por- 

 tion just to the left of the abdominal aorta. It is crossed by the superior mesen- 

 teric vessels and mesentery. Its front surface is covered by the anterior layer of 

 the mesentery, but near the middle line it is separated from this layer of the 

 mesentery by the superior mesenteric vessels as they cross this portion of the 

 duodenum. Its posterior surface is uncovered by peritoneum, except toward its 

 left extremity, where the posterior layer of the mesentery may sometimes be found 

 covering it to a variable extent. This surface rests upon the aorta, the vena cava 

 inferior, and the crura of the Diaphragm. By its upper surface this portion of the 

 duodenum is in relation with the head of the pancreas. 



The fourth or ascending portion of the duodenum is about two inches long. 

 It ascends on the left side of the vertebral column and aorta, as far as the level 

 of the upper border of the second lumbar vertebra, where it turns abruptly forward 

 to become the jejunum, forming the duodeno-jejunal flexure. It is covered 

 entirely in front and partly at the sides by peritoneum, derived from the left 

 portion of the mesentery. It touches the left kidney, slightly overlapping its 

 inner margin, and rests upon the left crus of the Diaphragm. 



The first part of the duodenum, as stated above, is somewhat movable, but the 

 rest is practically fixed and is bound down to neighboring viscera and the posterior 

 abdominal wall by the peritoneum. In addition to this, the fourth part of the 

 duodenum and the duodeno-jejunal flexure is further bound down and fixed by a 

 structure to which the name of musculus suspensorius duodeni has been given. 

 This structure commences in the connective tissue around the coeliac axis and left 

 crus of the Diaphragm, and passes downward to be inserted into the superior 

 border of the duodeno-jejunal curve and a part of the ascending duodenum, and 

 from this it is continued into the mesentery. It possesses, according to Treitz, 

 plain muscular fibres mixed with the fibrous tissue, of which it is principally made 

 up. It is of little importance as a muscle, but acts as a suspensory ligament. 



Vessels and Nerves. The arteries supplying the duodenum are the pyloric 

 and pancreatico-duodenal branches of the hepatic, and the inferior pancreatico- 

 duodenal branch of the superior mesenteric. The veins terminate in the splenic 

 and superior mesenteric. The nerves are derived from the solar plexus. 



Jejunum and Ileum. The remainder of the small intestine from the termination 

 of the duodenum is named jejunum and ileum; the former term being given to the 

 upper two-fifths and the latter to the remaining three-fifths. There is no morpho- 

 logical line of distinction between the two, and the division is arbitrary ; but at 

 the same time it must be noted that the character of the intestine gradually under- 

 goes a change from the commencement of the jejunum to the termination of the 

 ileum, so that a portion of the bowel taken from these two situations would present 

 characteristic and marked differences. These are briefly as follows : 



The jejunum, which derives its name from the Latin word jejunus (empty), 

 because it was formerly supposed to be empty after death, is wider, its diameter 

 being about one inch and a half, and is thicker, more vascular, and of a deeper 

 color than the ileum, so that a given length weighs more. Its valvular conniventes 

 are large and thickly set and its villi are larger than in the ileum. The glands 

 of Peyer are almost absent in the upper part of the jejunum, and in the lower 

 part are less frequently found than in the ileum, and are smaller and tend to 

 assume a circular form. Brunner's glands are only found in the upper part of 

 the jejunum. By grasping the jejunum between the finger and thumb the val- 

 vulae conniventes can be felt through the walls of the gut ; these being absent in 

 the lower part of the ileum, it is possible in this way to distinguish the upper from 

 the lower part of the small intestine. 



