DIGESTION. 



281 



deep surface is the musculans mucoscz (m m, Fig. 191), the fibres of 

 which are arranged in two layers: the outer longitudinal and the inner 

 circular. 



ValvulcB Conniventes. The valvulae conniventes(Fig. 192) commence 

 in the duodenum, about one or two inches beyond the pylorus, and 

 becoming larger and more numerous immediately beyond the entrance 

 of the bile duct, continue thickly arranged and well developed through- 

 out the jejunum; then, gradually diminishing in size and number, they 

 cease near the middle of the ileum. They are formed by a doubling 

 inwards of the mucous membrane; the crescentic, nearly circular, folds 

 thus formed being arranged transversely to the axis of the intestine, and 



FIG. 190. 



FIG. 191. 



FIG. 190. Horizontal section of a small fragment of the mucous membrane, including one en- 

 tire crypt of Lieberkiihn and parts of several others; a, cavity of the tubular glands or crypts; 6, 

 one of the lining epithelial cells; c, the lymphoid or retiform spaces, o: which some are empty, and 

 others occupied by lymph cells, as at d. 



FIG. 191. Vertical section through portion of small intestine of dog. v, two villi showing e, 

 epithelium; g, goblet cells. The free surface is seen to be formed by the v ' striated basilar border." 

 while inside the villus the adenoid tissue and unstriped muscle-cells are seen; If. Lieberkuhn's folli- 

 cles; m m, muscularis mucosse, sending up fibres between the follicles into the villi; sm, submucous 

 tissue; containing (grri), ganglion cells of Meissner's plexus. (Schofield.) 



each individual fold seldom extending around more than ^ or f of the 

 bowel's circumference. Unlike the ruga? in the oesophagus and stomach, 

 they do not disappear on distention of the canal. Only an imperfect 

 notion of their natural position and function can be obtained by looking 

 at them after the intestine has been laid open in the usual manner. To 

 understand them aright, a piece of gut should be distended either with 

 air or alcohol, and not opened until the tissues have become hardened. 



