300 DIGESTION. 



licles o f LieberliuJin are simple tubular depressions of 

 the intestinal mucous membrane, thickly distributed over 

 the whole surface both of the large and small 

 intestines. In the small intestine they are visible 

 only with the aid of a lens; and their orifices 

 appear as minute dots scattered between the villi. 

 They are larger in the large intestine, and 

 increase in size the nearer they approach the anal 

 end of the intestinal tube ; and in the rectum their 

 orifices may be visible to the naked eye. In 

 length they vary from -^ to -^ of a line. Each 

 tubule (fig. 74) is constructed of the same essential 

 parts as the intestinal mucous membrane, viz., 

 a fine structureless membrana propria, or base- 

 ment membrane, a layer of cylindrical epithelium 

 lining it and capillary blood-vessels covering its exterior. 

 Their contents appear to vary, even in health; the varieties 

 being dependent, probably, on the period of time in rela- 

 tion to digestion at which they are examined. At the 

 bottom of the follicle, the contents usually consist of a 

 granular material, in which a few cytoblasts or nuclei 

 are imbedded ; these cytoblasts, as they ascend towards 

 the surface, are supposed to be gradually developed into 

 nucleated cells, some of which are discharged into the 

 intestinal cavity. The purpose served by the material 

 secreted by these glands is still doubtful. Their large 

 number and the extent of surface occupied by them, seem, 

 however, to indicate that they are concerned in other and 

 higher offices than the mere production of fluul to moisten 

 the surface of the mucous membrane, although, doubtless, 

 this is one of their functions. 



The glands of Peyer occur exclusively in the small intes- 

 tine. They are found in greatest abundance in the lower 

 part of the ileum near to the ileo-csecal valve. They are 



Fig. 74. A gland of Lieberkiihn. 



