THE DIGESTIVE ORGANS. 139 



Between the attached ends of the villi, and opening upon the sur- 

 face of the mucous membrane, are tubular depressions extending 

 nearly to the muscularis mucosse. These are the " crypts of Lieber- 

 kiihn," and have the appearance of simple tubular glands ; but it is 

 doubtful if they elaborate any peculiar secretion. These crypts are 

 present, not only in the whole extent of the small intestine, but 

 also throughout that of the colon. 



The crypts of Lieberkuhn are lined with columnar epithelium, 

 which also covers the surface of the mucous membrane and the villi 

 springing from it. The cells composing this epithelium multiply in 

 the crypts, and, as they mature, are gradually moved toward their 

 orifies, whence they replace those that have been destroyed upon the 

 surfaces of the villi. The cells possess a granular cytoplasm, which 

 becomes infiltrated with fat during digestion ; an oval, vesicular 

 nucleus ; and a delicate cell-membrane. The free ends of the cells 

 are formed by a well-marked cuticle, which may be either homo- 

 geneous in appearance, or present very fine vertical striations (Fig. 

 37). Many of the cells are mucigenous and contain globules of mucus 

 near their free ends, or appear as goblet-cells after the discharge of 

 that secretion. These cells are more abundant on the villi, where 

 they are older, than in the crypts lined with less mature cells. 



The epithelium rests upon a basement-membrane, which contains 

 nuclei, and is therefore composed, in part at least, of cells. Beneath 

 this basement-membrane is a layer of reticular and areolar tissues, 

 containing a variable number of lymphoid cells and numerous 

 capillary bloodvessels. The rest of the mucous membrane, down 

 to the muscularis mucosaB, and the axes of the villi are occupied by 

 areolar fibrous tissue. 



The thin muscularis mucosaB, which forms the deepest layer of 

 the mucous membrane, is made up of two layers of smooth muscular 

 tissue : an internal layer, in which the fibres run transversely to the 

 axis of the intestine, and an external longitudinal layer. From the 

 upper surface of this muscular layer of the mucous membrane 

 muscular fibres extend into the villi, in the areolar tissue in their 

 axes, and serve to shorten the villi by their contraction, so that the 

 villi are moved about in the intestinal contents during the process 

 of absorption. In the centre of each villus is a capillary lymphatic 

 vessel arising in a blind extremity near the apex of the villus. 

 These lymphatics open into a lymphatic plexus situated between the 

 muscularis mucosse and the ends of the crypts of Lieberkuhn, and 



