266 ANATOMY FOR NURSES [Chap. XIV 



in front of the kidney ; it again turns in a right angle direction 

 to the left and passes horizontally across the front of the ver- 

 tebral column. This third portion of the duodenum lies retro- 

 peritoneally, so that only its anterior aspect is covered by peri- 

 toneum. The small intestine now passes forward so as to leave 

 the posterior abdominal wall, and becomes completely invested 

 by peritoneum and has a true mesentery. The point at which 

 it becomes completely invested by peritoneum marks the ter- 

 mination of the duodenum and the beginning of the jejunum. 



The jejunum. — The jejunum, or empty intestine, so called 

 because it is always found empty after death, constitutes about 

 two-fifths of the remainder, or seven and a half feet (2.2 m.), of 

 the small intestine, and extends from the duodenum to the ileum. 



The ileum. — The ileum, or twisted intestine, so called from its 

 numerous coils, constitutes the remainder of the small intestine, 

 and extends from the jejunum to the large intestine, which it joins 

 at a right angle. 



There is no definite landmark to determine the point at which 

 the jejunum ceases and the ileum begins, although the mucous 

 membrane of the one differs somewhat from the mucous mem- 

 brane of the other; the change is a gradual transition, and one 

 structure shades off into the other. The lengths in feet as given 

 are arbitrary, but those usually accepted. 



Coats of the small intestine. — The small intestine has four 

 coats, which correspond in character and arrangement with 

 those of the stomach. 



(1) The serous coat furnished by the peritoneum forms an al- 

 most complete covering for the whole tube except for part of the 

 duodenum. 



(2) The muscular coat of the small intestine has only two 

 layers: an outer, thiimer and longitudinal; and an inner, thicker 

 and circular. This arrangement is necessary for the peristaltic 

 action of the intestine. 



(3) The submucous, or areolar coat, carries blood-vessels, 

 lymphatics, and nerves. 



(4) The mucous coat is thick and very vascular. 



ValvulcB conniventes. — About two inches beyond the pylorus 

 the mucous and submucous coats of the small intestine are ar- 

 ranged in circular folds called valvulae conniventes. Each of these 



