MUCOUS MEMBRANES. 



239 



Fig. 27. 



Villi of the Human Intestine, 

 with their capillary plexus in- 

 jected. 



The mucous surface of the small intestine, from the pyloric orifice to the 

 csecum, is thickly beset with vitti, which are prolongations of the basement 

 membrane, having somewhat the form of the finger 

 of a glove, copiously furnished with bloodvessels 

 from the subjacent surface (Fig. 27), and also con- 

 taining lacteal tubules in their interior. In form 

 they are sometimes nearly cylindrical, sometimes 

 rather conical, and not unfrequently become flat- 

 tened and extended at the base, so that two or more 

 coalesce. Their length varies from l-4th to l-3d of 

 a line, or even more ; and the broad flattened kinds 

 are about l-6th or l-8th of a line in breadth. In 

 the upper part of the small intestine, where they are 

 most numerous, it has been calculated by Krause 

 that there are not less than from 50 to 90 in a square 

 line ; and in the lower part, from 40 to 70 in the 

 same space. An approach to the villous structure 

 is presented by the portion of the mucous membrane 

 of the stomach, in the neighborhood of the pylorus; 

 but the prominences which are here found between 

 the orifices of the gastric follicles, are much smaller 

 than the true villi of the intestine, and contain no 

 lacteal vessels. There can be no doubt that the pro- 

 per intestinal villi are the chief instruments of absorption, by means both of 

 their bloodvessels, which take up soluble matters by simple imbibition, and of 

 their lacteals, which absorb certain special products of the digestive operation. 

 In the selection of these, it will be hereafter shown that the epithelial cells of 

 the villi are the instruments chiefly concerned; these filling themselves with 

 the materials of chyle from the contents of the alimentary canal (Fig. 134), and 

 then delivering them up to be absorbed by the lacteals beneath (CHAP. vin. 



SECT. 1). 



235. The inversion of the mucous surface into follicles, gives to it a character 

 precisely the reverse of the preceding, both as regards structure and function. 

 These follicles, in their most elementary form, may be regarded as originating 

 in a recession of the basement membrane (as if 

 the finger of a glove were pushed back into the 

 interior of the palm) ; they are nearly of a cylin- 

 drical shape, their orifices opening upon the free 

 surfaces of the mucous membrane in the inter- 

 spaces of the vascular network (Fig. 28), 

 while their csecal extremities, which are some- 

 times simply rounded, sometimes loculated, abut 

 against the submucous areolar tissue. Such fol- 

 licles present themselves along the whole extent 

 of the gastro-intestinal mucous membrane; but 

 although very similar in their appearance in dif- 

 ferent parts of its length, their secretion is pro- 

 bably very different. In the stomach they are known as the " gastric follicles," 

 and the digestive solvent is prepared and poured forth by them. Along the 

 course of the intestine, on the other hand, they are known as the " follicles of 

 Lieberkuhn ;" and it is doubtful whether they form any other secretion than 

 that of protective mucus. Such follicles are not known to exist in any other 

 than the open state ; and they seem to have a permanent character, continually 

 discharging new broods of epithelial cells. The secreting action of these folli- 

 cles may be best observed in those of the stomach ; which during the intervals 



Fig. 28. 



Distribution of Capillaries around folli- 

 cles of Mucous Membrane. 



