734 ORGAXS OF DIGESTION. 



bone. In front, it is covered by the descending layer of the transverse meso- 

 colon, and crossed by the superior mesenteric vessels ; behind, it rests upon the 

 aorta, the vena cava, and the crura of the Diaphragm ; above it, is the lower 

 border of the pancreas, the superior mesenteric vessels passing forwards be- 

 tween the two. 



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 gastro- 

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



The jejunum (jejunus, empty), so called from being usually found empty after 

 death, includes the upper two-fifths of the rest of the small intestine. It com- 

 mences at the duodenum on the left side of the second lumbar vertebra, and 

 terminates in the ileum ; its convolutions being chiefly confined to the umbilical 

 and left iliac regions. The jejunum is wider, its coats thicker, more vascular, 

 and of a deeper color than those of the ileum ; but there is no characteristic 

 mark to distinguish the termination of the one and the commencement of the 

 other. 



The ileum (*ix>, to twist], so called from its numerous coils or convolutions, 

 includes the remaining three-fifths of the small intestine. It occupies chiefly 

 the umbilical, hypogastric, right iliac, and occasionally the pelvic regions, and 

 terminates in the right iliac fossa by opening into the inner side of the com- 

 mencement of the large intestine. The ileurn is narrower, its coats thinner and 

 less vascular than those of the jejunum ; a given length of it weighing less 

 than the same length of jejunum. 



Structure. The wall of the small intestine is composed of four coats : serous, 

 muscular, cellular, and mucous. 



The serous coat is derived from the peritoneum. The first, or ascending 

 portion of the duodenum, is almost completely surrounded by this membrane ; 

 the second, or descending portion, is covered by it only in front; and the third, 

 or transverse portion lies behind the descending layer of the transverse meso- 

 colon, by which it is covered in front. The remaining portion of the small 

 intestine is surrounded by the peritoneum, excepting along its attached or 

 mesenteric border ; here a space is left for the vessels and nerves to pass to 

 the gut. 



The muscular coat consists of two layers of fibres, an external or longitudi- 

 nal, and an internal or circular layer. The longitudinal fibres are thinly scat- 

 tered over the surface of the intestine, and are most distinct along its free border. 

 The circular fibres form a thick, uniform layer; they surround the cylinder of 

 the intestine in the greater part of its circumference, but do not form complete 

 rings. The muscular coat is thicker at the upper than at the lower part of the 

 small intestine. 



The cellular, or sulmucous coat, connects together the mucous and muscular 

 layers. It consists of a loose, filamentous, arcolar tissue, which forms a nidus 

 for the subdivision of the nutrient vessels, previous to their distribution to the 

 mucous surface. 



The mucous membrane is thick and highly vascular at the upper part of the 

 small intestine, but somewhat paler and thinner below. It presents for exami- 

 nation the following constituents: 



Epithelium. Simple follicles. 



Valvulso conniventes. ( Duodenal glands. 



Villi. Glands. ^ Solitary glands. 



( Agminate or Peyer's glands. 



The epithelium, covering the mucous membrane of the small intestine, is of 

 the columnar variety. 



Tim rti/t'/tlx connivenles (valves of Kerkring) are reduplications or foldings 

 of the mucous membrane and submucous tissue, containing no muscular fibres. 



