446 



THE ALIMENTARY CANAL. 



[CH. XXVII. 



length of which in an adult is about twenty feet, has been 

 divided, for convenience of description, into three portions, viz., 

 the duodenum, which extends for eight or ten inches beyond the 

 pylorus ; the jejunum, which forms two-fifths, and the ileum, 

 which forms three-fifths of the rest of the canal. 



Structure. The small intestine, like the stomach, is constructed 



Fig. 377. 



of four coats, viz , the serous, muscular, sub-mucous, and 

 mucous. 



(i.) The serous coat is formed by the visceral layer of the 

 peritoneum, and has the structure of serous membranes in 

 general. 



(2.) The muscular coat consists of an internal circular and an 

 external longitudinal layer : the former is usually considerably 

 the thicker. Both alike consist of bundles of unstriped muscle 



