THE ALIMENTARY APPARATUS. 233 



THE SMALL INTESTINE. 



The small intestine is a convoluted tube about twenty to 

 twenty-five feet in length, for the chylification of the food, occu- 

 pying the lower arid central portions of the abdominal and pelvic 

 cavities, and held in position to the spinal column by the mesen- 

 tery. It is divided into three portions the duodenum, jejunum, 

 and ileum. 



THE DUODENUM, so called from being about twelve fingers' 

 breadth in length, is about eight to ten inches in length. It 

 consists of four portions, from the position of its course ascend- 

 ing, descending, transverse, and terminal ascending: 



The -first or ascending portion is about two inches in length, 

 and ascends to the neck of the gall-bladder. It is completely 

 invested by peritoneum for about an inch. 



The second or descending portion, about three inches in 

 length, descends in front of the right kidney as far as the third 

 or fourth lumbar vertebra, and is overlapped in front by the 

 head of the pancreas, and into its posterior aspect the duct of 

 the pancreas and the common biliary duct open by a common 

 orifice. 



The third or transverse portion is attached to the crura of 

 the diaphragm and the vessels in front of the vertebral column, 

 passes behind the transverse mesocolon, and has the pancreas 

 above it, and the superior rnesenteric blood-vessels cross from 

 beneath the latter between the two or over the duodenum. 



The fourth or terminal ascending portion runs upward and 

 forward to the duodeno-jejunal flexure. 



The arteries are derived from the inferior pancreatico- 

 duodenal branch of the superior mesenteric and the superior 

 pancreatico-duodenal branch of the gastro-duodenal, a branch 

 of the hepatic. The veins terminate in the superior mesenteric 

 and splenic veins. The nerves are from the solar plexus. 



THE JEJUNUM, named from jejunus, empty, includes the 

 upper two-fifths of the small intestine, is continuous above with 

 the duodenum and below with the ileum. It occupies chiefly 

 the left iliac and umbilical region. 



THE ILEUM, so called from its twisted condition, includes 

 the remaining three-fifths, is continuous above with the jejunum 

 and below with the caput caecum of the large intestine. It occu- 

 pies the right iliac, hypogastric, and umbilical regions. 



The structure of the small intestine consists of four coats 

 the serous, muscular, fibrous or areolar, and mucous : 



The serous coat is the peritoneal covering; 



The 'muscular coat consists of two sets a longitudinal and 

 a circular ; 



