STRUCTURE OF THE SMALL INTESTINE. 



387 



commencement of the duodenum, and are large and well marked in the 

 duodenum, and remain so as far as "the upper half of the jejunum, where 

 they begin to become smaller, and finally disappear about the lower part 

 of the ileum.] The villi 

 are characteristic of the 

 small intestine, and are 

 confined to it ; they occur 

 everywhere as closely-set 

 projections over and be- 

 tween the valvulre conni- 

 ventes (Fig. 149). When 

 the inner surface of the 

 mucous membrane is 

 examined in water, it 

 has a velvety appearance 

 owing to their presence. 

 [They vary in length 

 from r \ to -aV of an inch, 

 are most numerous and 

 largest in the upper part 

 of the intestine, duo- 

 denum, and jejunum, 

 where absorption is most 

 active, but they are less 

 abundant in the ileum. 

 Their total number has 

 been calculated at four 

 millions by Krause.] 

 Each villus is a projection 

 of the entire mucous 

 membrane, so that it 



Fig. 149. 



Mucous membrane of the small intestine of the dog; 

 the lacteals are black and the blood-vessels lighter 

 a, artery; b, lymphatic; c, plexus of capillaries in 

 the villi ; d, lacteal ; e, Lieberkiihn's glands. 



contains within itself 

 representatives of all the 

 tissue elements of the 

 mucosa. The orifices of 

 the glands of Lieberkiihn open between the bases of villi (Fig, 151). 



Each villus, be it cylindrical or conical in shape, is covered by a single layer 

 of columnar epithelium, whose protoplasm is reticulated, and contains a well- 

 defined nucleus with an intranuclear plexus of fibrils. The ends of the epithelial 

 cells directed towards the gut are polygonal, and present the appearance of a 

 mosaic (Fig. 150, D). When looked at from the side, their free surface is seen to be 

 covered with a clear, highly refractive disc or " cuticula," which is marked with 

 vertical strife. These stria? were supposed by Kolliker to represent pores for the 

 absorption of fatty particles, but this has not been confirmed, while Brettauer and 

 Steinach regarded them as produced by prisms placed side by side. 



According to some observers (v. Thanhoffer), however, this clear disc is the 

 optical expression of a thinning of the cell membrane, comparable to the thickened 

 flange around the bottom of a vessel, such as is used for collecting gases. On this 

 supposition, the upper end of each cell is open, and from it there projects pseudo- 

 podia-like bundles of protoplasmic processes (Fig. 150, B). These processes are 

 supposed to be extended beyond the margin of the cell and again rapidly retracted, 

 and in so acting they are said to carry the fatty particles into the interior of the 

 cells, much as the pseudopodia of an amoeba entangles its food. [This view has 

 not been confirmed by a sufficient number of observers.] Between the epithelial 

 cells are the so-called goblet-cdls (Fig. 150, C). [Each goblet-cell is more~or less 



