398 STRUCTURE OF THE SMALL INTESTINE. [BOOK II. 



composed of longitudinal and circular fibres, marks off the mucous 

 membrane proper from the underlying submucous tissue." 



" In the small intestine the outer longitudinal muscular layer is 

 evenly distributed over the whole circumference of the tube and is 

 everywhere much thinner than the inner circular layer, which is the 

 more important of the two. The individual fibre-cells of these 

 muscular layers of the intestine are large and well-developed. In 

 the thin sheet of connective tissue which separates indistinctly the 

 two layers, lies the plexus of Auerbach, a plexus of nerve-fibres, for 

 the most part non-medullated, at the nodes of which are gathered 

 groups of very small nerve-cells, the substance of each cell being 

 especially scanty. This plexus supplies the two muscular layers with 

 nerve-fibres. The submucous coat contains, besides blood-vessels 

 and lymphatics, a somewhat similar plexus of nerve-fibres, called the 

 plexus of Meissner ; from this plexus fine nerve-fibres proceed to the 

 blood-vessels, to the muscularis mucosse, and possibly to other 

 structures 1 ." 



The mucous "This is thrown into folds which are not, as in 

 membrane of the case of the stomach, temporary longitudinal folds, 

 the small in- ru qm but permanent transverse folds, the valvulce 

 testine. y : i i ir ,1 i c ^ 



conmventes, reaching hall-way or two-thirds 01 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 alternating, 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*." 



Description "The latter are very much smaller and are more 



of the glands numerous than the former, several crypts being placed 

 f Lieber n. .^ ^ e interval between two villi. Both are found 

 on the projecting valvulae as well as in the valleys between, and 

 both extend along the whole length of the intestine from the pylorus 

 to the ilio-caecal 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 



1 Dr M. Foster, A Text-Book of Physiology, Part n. p. 441, Macmillan and Co., 

 London and New York, 1889. 



2 Ibid. p. 442. 



