THE SMALL INTESTINE. 



917 



begin to appear about one or two inches beyond the pylorus. In the lower part 

 of the descending portion, below the point where the bile and pancreatic ducts 

 enter the intestine, they are very large and closely approximated. In the trans- 

 verse portion of the duodenum and upper half of the jejunum they are large and 

 numerous ; and from this point, down to the middle of the ileum, they diminish 

 considerably in size. In the lower part of the ileum they almost entirely dis- 

 appear ; hence the comparative thinness of this portion of the intestine as com- 

 pared with the duodenum and jejunum. The valvulse conniventes retard the 

 passage of the food along the intestines, and afford a more extensive surface for 

 absorption. 



The villi are minute, highly vascular processes, projecting from the mucous 

 membrane of the small intestine throughout its whole extent, and giving to its 

 surface a velvety appearance. In shape, according to Rauber, they are short and 

 leaf-shaped in the duodenum, tongue-shaped in the jejunum, and filiform in the 

 ileum. They are largest and most numerous in the duodenum and jejunum, and 

 become fewer and smaller in the ileum. Krause estimates their number in the 

 upper part of the small intestine at from fifty to ninety in a square line ; and in 

 the lower part from forty to seventy, the total number for the whole length of 

 the intestine being about four millions. 



Structure of the Villi (Fig. 497). The structure of the villi has been studied 

 by many eminent anatomists. We shall here follow the description of Watney, 1 

 whose researches have a most important bearing on the physiology of that which 

 is the peculiar function of this part of the intestine, the absorption of fat. 



Capillaries. 

 Lymph trunk.' 



t f Lymph 1nn,k. 



iff Capillaries. 



Small artery.' Lymphatic plexus. 



FIG. 498. Villi of small intestine. (Cadiat.) 



The essential parts of a villus are the lacteal vessel, the blood-vessels, the 

 epithelium, the basement membrane and muscular tissue of the mucosa, these 

 structures being supported and held together by retiform lymphoid tissue. 



These structures are arranged in the following manner : situated in the centre 

 of the villus is the lacteal, terminating near the summit in a blind extremity ; 

 running along this vessel are unstriped muscular fibres : surrounding it is a plexus 

 of capillary vessels, the whole being enclosed by a basement-membrane, and covered 

 by columnar epithelium. Those structures which are contained within the base- 

 ment-membrane namely, the lacteal, the muscular tissue, and the blood-vessels 



1 Phil. Trans., vol. clxv., pt. ii. 



