1022 



THE ORGANS OF DIGESTION. 



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shark family. The large folds project about one-third inch into the lumen, and 

 often connect at one end obliquely with a smaller fold. Sometimes a valve ter- 

 minates abruptly, or it bifurcates at one or both ends. They are so close that in 



a relaxed condition they cover the intes- 

 tinal surface like roof-tiles. These valves 

 are most abundant in the duodenum and 

 jejunum ; they decrease and disappear at 

 the lower end of the ileum. Their total 

 number is 800 or 900. They begin with 

 the commencement of the descending duode- 

 num, there being usually none in the first 

 portion. Just beyond the point of entrance 

 of the bile and pancreatic ducts they are 

 very large and regular and closely packed. 

 About two feet from the lower end of the 

 ileum they cease. From this point up to 

 the middle of the jejunum they are indis- 

 tinct and irregular, smaller, and farther 

 apart. They are seen at their best from the lower part of the descending duode- 

 num through the upper half of the jejunum. Their function is to retard the 

 passage of food and to afford an extensive absorptive surface. 



Mouth of crypt of 



Lieberkiihn 



FIG. 640. Small portion of mucous mem- 

 brane of the small intestine. (Rauber.) 



FIG. 641. Diagrammatic section of a villus. (Watney.) ep. Epithelium only partially shaded in. /.Cen- 

 tral chyle-vessel: the cells forming the vessel have been less shaded to distinguish them from the cells of the 

 parenchyma of the villas, m. Muscle-fibres running up by the side of the chyle-vessel. It will be noticed that 



membrana propria. It will be seen that there is hardly any difference between the cells of the parenchyma, 

 the endothelium of the upper part of the chyle-vessel, and the cells of the ineiubrana propria. v. mood-vessels. 

 z. Dark line at the base of the epithelium formed by the reticulum. It will be seen that the reticulum IK 'ni- 

 trates between all the other elements of the villus. The reticulum contains thickenings or "nodal points." 

 The diagram shows that the cells of the upper part of the villus are larger and contain a lanrer zone of proto- 

 plasm than those of the lower part. The cells of the upper part of the chyle-vessel differ somewhat from those 

 of the lower part, in that they more nearly resemble the cells of the parenchyma. 



The villi are minute vascular processes, consisting entirely of tissues of the 

 mucosa, projecting from every part of the inner surface of the small intestine 



