448 



THE ALIMENTARY CANAL. 



[CH. xxvu. 



vessels and lymphatics ramify. A fine plexus, consisting mainly 

 of non-medullated nerve-fibres, Meissner's plexus, with ganglion 

 cells at its nodes, occurs in the submucous tissue from the stomach 

 to the anus. 



(4.) The mucous membrane is the most important coat in 

 relation to the function of digestion. The following structures, 

 which enter into its composition, may now be successively 

 described : the valvulce conniventes ; the villi ; and the glands, 

 The general structure of the mucous membrane of the intestines 

 resembles that of the stomach, and, 

 like it, is lined on its inner surface by 

 columnar epithelium. Adenoid tissue 

 (fig. 378) enters largely into its construc- 

 tion ; and on its deep surface is the mus- 

 cularis mucosce (m, fig. 381), the fibres 

 of which are arranged in two layers : the 

 outer longitudinal and the inner circular. 

 Valvulce Conniventes. The valvulse 

 conniventes (fig. 380) 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 

 throughout 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 cres- 

 centic, nearly circular, folds thus formed 

 are arranged transversely to the axis of the intestine, but each 

 individual fold seldom extends around more than ^ or f of 

 the bowel's circumference. Unlike the rugae in the oesophagus 

 and stomach, they do not disappear on distension of the canal. 

 Their function is to afford a largely increased surface for secretion 

 and absorption. They are covered with villi. 



Villi. The Villi (figs. 381, 382, and 383) are confined exclu- 

 sively to the mucous membrane of the small intestine. They 

 are minute vascular processes, from -^ to ^ of an inch ('5 to 

 3 mm.) in length, covering the surface of the mucous membrane, 

 and giving it a peculiar velvety, fleecy appearance. Krause 

 estimates them at fifty to ninety in number in a square line at 

 the upper part of the small intestine, and at forty to seventy in 

 the same area at the lower part. They vary in form even in the 



Fig. 380. Piece of small in- 

 testine (previously dis- 

 tended and hardened by 

 alcohol ) ,laid open to show 

 the normal position of 

 the valvulse conniventes. 

 Natural size. 



