FOOD AND DIGESTION. 



371 



These are continuous \vith eacli other, and communicate by means of 

 an opening guarded by a valve, the ileoccecal valve, which allows the 

 passage of the products of digestion from the small into the large bowel, 

 but not, under ordinary circumstances, in the opposite direction. 



The Small Intestine. The Small Intestine, the average length 

 of which in an adult is about twenty feet, has been divided, for conven- 

 ience of description, into three portions, viz., the duodenum, which ex- 

 tends 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 of 

 four principal coats, viz., the serous, muscular, sub-mucous, and mucous. 



Fig. 255. 



Fig. 256. 



Fig. 25& Horizontal section of a small fragment of the mucous membrane, including one 

 entire crynt of Lieberkuhn and parts of several others. 



Fig. 256. Piece of small intestine (previously distended and hardened by alcohol), laid open to 

 show tne normal position of the valvulae conniventes. 



(1.) The serous coat is formed by the visceral layer of the perito- 

 neum, and has the structure of serous membranes in general. 



(2.) The muscular coats consist of an internal circular and an ex- 

 ternal longitudinal layer: the former is usually considerably the thicker. 

 Both alike consist of bundles of unstriped muscle supported by con- 

 nective tissue. They are well provided with lymphatic vessels, which 

 form a set distinct from those of the mucous membrane. 



Between the two muscular coats is a nerve plexus (Auerbach's 

 plexus) (fig. 254), similar in structure to Meissner's (in the submucous 

 tissue), but with more numerous ganglia. 



(3.) Between the mucous and muscular coats is the submucous coat, 

 which consists of connective tissue, in which numerous blood-vessels 

 and lymphatics ramify. A fine plexus, consisting mainly of non-medul- 



