916 



THE ORGANS OF DIGESTION. 



are granular in appearance, and possess a clear, oval nucleus. At their superficial 

 or unattached end they present a distinct layer of highly refracting material, 

 marked by vertical striae, which were formerly believed to be minute channels 

 by which the chyle was taken up into the interior of the cell, and by them 

 transferred to the lacteal vessels of the mucous membrane. 



The mucous membrane presents for examination the following structures con- 

 tained within it or belonging to it : 



Valvulre conniventes. ( Duodenal glands. 



Villi. Glands < Solitary glands. 



Simple follicles. ( Fever's or agminate glands. 



The valvulae conniventes (valves of Kerkring) are large folds or valvular flaps 

 projecting into the lumen of the bowel. They are composed of reduplications 

 or folds of the mucous membrane, the two layers of the fold being bound together 

 by submucous tissue ; they contain no muscular fibres, and, unlike the folds in 

 the stomach, they are permanent, and are not obliterated when the intestine is 

 distended. The majority extend transversely across the cylinder of the intestine 



FIG. 497. Diagrammatic section of a villus. (Watney.) ep. Epithelium only partially shaded in. /.Central 

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

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

 each muscle-fibre is surrounded by the reticulum, and by this reticulum the muscles are attached to the cells 

 forming the membrana propria, as at e', or to the reticulum of the villus. /c. Lymph-corpuscles, marked by a 

 spherical nucleus and a clear zone of protoplasm. I'. Upper limit of the chyle-vessel, e, e, e?. Cells forming 

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

 the endotheliumof the upper part of the chyle-vessel, and the cells of the membrana propria. V. Blood-vessels. 

 z. Dark line at base of the epithelium formed by the reticulum. It will be seen that the reticulum penetrates 

 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 larger zone of protoplasm 

 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. 



for about one-half or two-thirds of its circumference, but some form complete 

 circles, and others have a spiral direction ; the latter usually extend a little more 

 than once round the bowel, but occasionally two or three times. The spiral 

 arrangement is the characteristic one of the shark family of fishes. The larger 

 folds are about one-third of an inch in depth at their broadest part ; but the 

 greater number are of smaller size. The larger and smaller folds alternate with 

 each other. They are not found at the commencement of the duodenum, but 



