DIGESTION IN THE INTESTINES. 



239 



of description, into three portions, viz., the duodenum, which 

 extends for eight or ten inches beyond the pylorus ; the jeju- 

 num, which occupies two-fifths, and the ileum, which occupies 

 three-fifths of the rest of the canal. 



The small intestine, like the stomach, is constructed of three 

 principal coats, viz., the serous, muscular, and mucous. The 

 serous coat, formed by the visceral layer of the peritoneum, 

 need not be here specially described. The fibres of the mus- 

 cular coat of the small intestine are arranged in two layers ; 

 those of the outer layer being disposed longitudinally ; those 

 of the inner layer transversely, or in portions of circles encom- 

 passing the canal. They are composed of the unstriped kind 

 of muscular fibre. 



Between the mucous and muscular coats, there is a layer 

 of submucous tissue, in which numerous bloodvessels and a 

 rich plexus of nerves and ganglia are imbedded (Meissner). 



The mucous membrane is the most important coat in relation 

 to the function of digestion. The following structures which 

 enter into the composition of the mucous membrane may be 

 now successively described : the valvulce conniventes ; the wlli ; 

 and the glands. The general structure of the mucous mem- 

 brane of the intestines resembles that of the stomach (p. 215), 

 and, like it, is lined on its inner surface by columnar epithe- 

 lium. Lymphoid or Retiform tissue (Fig. 72) enters largely 



FIG. 72. 



The figure represents a cross-section of a small fragment of the mucous mem- 

 brane, including one entire crypt of Lieberkiihn and parts of several others; a, 

 cavity of the tubular glands or crypts ; b, one of the lining epithelial cells ; c, the 

 lymphoid or retiforni spaces, of which some are empty, and others occupied by 

 lymph cells, as at d. 



into its construction ; and on its deep surface is a layer of the 

 muscular is mucosce (p. 216). 



