288 



THE ALIMENTARY TRACT, ETC. 



these folds and between them are smaller, 

 secondary folds, partly irregularly arranged, 

 partly longitvidinal [d d). 



Towards the middle of the small intestine 

 this valvular arrangement is lost, to be replaced 

 by an irregular net-like folding ; beyond this 

 longitudinal folds arise, which proceed in a 

 sinuous course towards the large intestine. 



The whole mucous membrane, both on the 

 folds and otherwise, is covered with a simple 

 layer of columnar epithelium, which is con- 

 tinued into numerous simple follicles (glands 

 of Lieberkiihn) found throughout the mucous 

 membrane of the small intestine. The cells 

 are placed on a basement membrane, which 

 rests on a thin layer of loose connective-tissue, 

 intervening betAveen the epithelial coat and 

 the muse /dan's mucosae. The ejiithelial cells 

 are intermixed with a large number of goblet- 

 cells, and have between them fine processes 

 from the connective-tissue corpuscles of the 

 subjacent layer; many of these processes 

 extend to or even l)eyond the free margin of 

 the epithelial cells. 



The individual cells are columnar, possessed 

 of a well-marked cell-wall, and have distinct, large, oval nuclei, 

 containing one or more nucleoli. The protoplasmic contents are 

 granular, and with proper treatment show a very distinct intra- 

 cellular network. The free margins of the cells are sharply marked 

 off from the cell-contents, and are more firmly attached to the cor- 

 responding portions of adjacent cells than the rest of the cell-wall. 

 This margin has a longitudinal striation, which owdng to the im- 

 portant function performed by this part of the intestine, namely, 

 absorption of the fat, has been the subject of many important 

 investigations. 



[In the following brief summary of the earlier researches on the minute structure 

 of the intestinal epithelium, in which the intestine of the frog was chiefly used, the 

 memoirs in which these investigations are recorded are referred to in the order of 

 time. 



1837. Henle first described the border as a thickened, highly refractive portion 

 of the cell-wall. 



18.^5. Kolliker and Funke, after independent research, described the longitudinal 



Mucous membrane of the 

 pyloric end of tlie stomach 

 and the duodenum. 



% Mucous membrane of 

 the stomach. 



7I Commencement of 

 duodenum. 



h Duodenal mucous 



membrane arranged 

 in irregular net- 

 work. 



(■ 1 Semi-lunar folds of the 



I'l / mucous membrane. 



(/ Longitudinal folds of 

 the mucous mem- 

 brane. 



