CH. XXVII. ] 



THE INTESTINES 



453 



the fibres of which are arranged in two layers : the outer longitudinal 

 and the inner circular. 



Valvulce Conniventes. The valvulse conniventes (fig. 381) com- 

 mence 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 thfe middle of the ileum. They are formed 

 by a doubling inwards of the mucous membrane; the crescentic, 

 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. 382, 383, and 384) are 

 confined exclusively to the mucous membrane of 

 the small intestine. They are minute vascular 

 processes, from -fa 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 same animal, and 

 differ according as the lymphatic vessels or 

 lacteals which they contain are empty or full; 

 being usually, in the former case, flat and pointed 

 at their summits, in the latter cylindrical. 



Each villus consists of a small projection of mucous membrane ; 

 its interior consists of fine adenoid tissue, which forms the frame- 

 work in which the other constituents are contained. 



The surface of the villus is clothed by columnar epithelium, which 

 rests on a fine basement membrane; while within this are found, 

 reckoning from without inwards, blood-vessels, fibres of the muscularis 

 mucosce, and a lymphatic or lacteal vessel sometimes looped or 

 branched (fig. 384). 



The epithelium is continuous with that lining the other parts of 

 the mucous membrane. The cells are arranged with their long axis 

 radiating from the surface of the villus (fig. 383), and their smaller 

 ends resting on the basement membrane. The free surface of the 

 epithelial cells of the villi, like that of the cells which cover the 



FIG. 381. Piece of small in- 

 testine (previously dis- 

 tended and hardened by 

 alcohol), laid open to 

 show the normal posi- 

 tion of the valvulae 

 conniventes. Natural 

 size. 



