CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 467 



tudinal folds, rugce, but permanent transverse folds, the valvulce 

 conniventes, reaching half-way or two-thirds of the way round the 

 tube. Each fold is a fold of the whole mucous membrane carrying 

 with it a part of the submucous tissue, the latter thus forming a 

 middle sheet between the mucous membrane on the upper surface 

 and that on the lower surface of the fold. The folds, which vary in 

 size, large and small frequently alternately, begin to appear at a 

 little distance from the pylorus; they are especially well developed 

 just below the opening of the bile and pancreatic ducts, and are 

 continued down to about the middle of the ileum, where, becoming 

 smaller and irregular, they gradually disappear. They serve to 

 increase the inner surface of the intestine and present an obstacle 

 to the too rapid transit of material along the tube. 



Over and above the coarser inequalities of surface caused by 

 these folds, the level of the mucous membrane is broken on the 

 one hand by tongue-like projections, the villi, and on the other 

 hand by tubular depressions, the glands or crypts of Lieberkuhn. 

 The latter are very much smaller and are more numerous than the 

 former, several crypts being placed in the interval between two 

 villi. Both are found on the projecting valvulse as well as in the 

 valleys between, and both extend along the whole length of the 

 intestine from the pylorus to the ileocsecal valve ; but while the 

 villi vary a good deal, being short and few immediately next to 

 the pylorus, very numerous and large in the duodenum and upper 

 part of the intestine, less numerous, smaller, and more irregular in 

 the lower part, the crypts have nearly the same characters and are 

 uniformly distributed throughout. Very much as in the case of 

 the stomach, the muscularis mucosse runs in an even line (except 

 for the sweeps of the valvulse conniventes) at a little distance from 

 the bases of the closely packed crypts, and at a greater distance 

 (viz. the length of the crypts) from the bases of the villi; as we 

 shall see, however, the muscularis mucosse sends up muscular fibres 

 into each villus. 



259. Before proceeding to describe the villi and crypts it 

 will be convenient to study the characters of the peculiar connective 

 tissue lying between the epithelium above and the muscularis 

 mucosse below. The upper surface of this tissue is defined by 

 what may be spoken of as a basement membrane, which however 

 appears not to be here (at least over the villi) as in the stomach 

 a continuous sheet composed of flat connective-tissue corpuscles 

 fused together, but to have a structure which we shall presently 

 describe. The muscularis mucosse consists of an outer longitu- 

 dinal and an inner circular sheet of plain muscular fibres, in 

 some places the one, and in other places the other being pre- 

 dominant; each sheet consists in most cases of a single layer of 

 fibres, the constituent fibres being cemented into flat bundles and 

 the bundles united by fine connective tissue. Between the flat 

 bundles vessels pass to and from the submucous tissue below and 



