364 



THE DIGESTIVE SYSTEM 



a diameter of 27 millimeters (a little over 1 inch) is attained, at the 

 end of the ileum. The three portions differ also in the shape and 

 number of villi, and in other histologic details which will be described 

 below. The inner surface is modeled by a succession of tall circular 



folds, the valvulcB conni- 

 ventcs or plicce circularcs, 

 involving the submucous 

 layer. These plica3 be- 

 come less closely-spaced 

 throughout the lower 

 portion of the ileum and 

 generally disappear 

 toward its end. They 

 serve to increase the ab- 

 sorbent surface of the 

 intestinal mucosa, and 

 unlike the villi, which 

 are scattered over their 

 surface, they are not sub- 

 ject to variations depen- 

 dent upon altering de- 

 grees of distention. 



The structure of the 

 serous coat of the small 

 intestine is identical with 

 that of the stomach. The 

 muscular coat consists of 

 an inner and an outer 

 layer of unstriped muscle 

 fibers which are sep- 

 arated by a thin connec- 

 tive tissue septum. The inner circular layer is much thicker than the 

 outer longitudinal. 



The regular disposition of the muscle fibers as an outer longitudinal 

 and an inner circular layer serves as a guide to the recognition of the 

 direction in which a given microscopical section has been cut. In transec- 

 tions of the intestine the muscle fibers of the outer layer of the mus- 

 cular coat are transversely cut; in longitudinal sections o'f the organ 

 the same fibers are seen in longitudinal section. 



The Submucosa. The submucosa of areolar connective tissue is 



FIG. 340. SECTION THROUGH THE COMMENCEMENT 

 OF THE DUODENUM AT THE PYLORUS. (Klein.) 



v, villi; b, apex of a lymphoid nodule; c, crypts of 

 Lieberkiihn; m, muscularis mucosae; s, secreting 

 tubes of Brunner's glands; d, ducts of pyloric glands 

 of stomach; g, tubes of these glands cut across in 

 mucous membrane; t, deep-lying tubes situated in 

 submucous tissue, and corresponding with Brun- 

 ner's glands of the intestine. (From Quain's "Anat- 

 omy.") 



