224 



DIGESTION. 



[CHAP. xxv. 



Fig. 157 



Of the Mucous Membrane of the Intestinal Canal. The intestinal 

 mucous membrane so far resembles that of the stomach, that it is 

 of the compound variety (see ante, p. 162); and its thickness is 

 caused by the involution of multitudes of tubes, which terminate 

 in blind closed extremities, and rest vertically upon the submucous 

 tissue. Most of the tubes remain undivided from their open to 

 their closed extremities, some are bifurcated towards the blind 

 extremity. Each tube has a separate orifice on the free surface of 

 the mucous membrane, except in the upper part of the duodenum 

 and in the ccecum, where they open by sets of three or more on the 

 floors of shallow cells, as in the stomach. These tubes are com- 

 monly called Lieberkiihn's follicles ; but they were first described 

 by Brunn, or Brunner, who has given a good delineation of them, 

 and their real nature was not known to Lieberkuhn, who regarded 

 them as muscles. 



In examining thin vertical sections of the mucous membrane 



from any part of the in- 

 testine, we see these tubes 

 closely set parallel to each 

 other; they are straight, 

 and, excepting that here and 

 there one is bifurcated, they 

 exhibit no irregularity or 

 bulging of their walls, and 

 are pretty uniform in diame- 

 ter throughout their length 

 (fig. 157). All the elements 

 of the mucous membrane 

 contribute to their forma- 

 tion; the basement mem- 

 brane, the epithelium, and 

 the submucous tissue, which 

 is sparingly interposed be- 



Section of the mucous membrane of the small intestine tW6Cn them. 

 in the dog, showing Lieberkiihn's follicles and villi. ,, .IT . , 



a Villi. b Lieberkiihn's follicles, c Other coats of the The epithelium IS COmm- 

 intestine. A 



nar or subcolumnar; the 



cells are disposed in a single layer, with one end adherent to the 

 basement membrane. The cast-off particles often fill each tube as 

 mucus, which escapes at the orifice on the free surface of the 

 mucous membrane. These tubes are shorter in the large than in 

 the small intestine ; and as the thickness of the mucous membrane 

 is dependent on their length, we find it less thick in the former 



