1292 



THE ORGANS OF DIGESTION 



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 approxi- 

 mated. In the transverse 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 consider- 

 ably in size. In the lower part of the ileum they almost entirely disappear; hence the com- 

 parative thinness of this portion of the intestine as compared with the duodenum and jejunum. 

 The valvulae conniventes retard the passage of the food along the intestine, and afford a more 

 extensive surface for absorption. 



The villi (villi intestinales) (Figs. 1018 and 1019) are minute, highly vascular processes, 

 never larger than 1 millimeter, projecting from the mucous membrane of the small intestine 

 throughout its whole extent, and giving to its surface a velvety appearance. They spring 

 from the valvulae conniventes and also from the spaces between them. 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. Kraus 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. 



Capillaries. 

 Lymph trunk. 



j$m Lymph trunk. 



Capillaries. 



Small artery.' Lymphatic plexus. 



FIG. 1019. Villi of small intestine. (Cadiat.) 



Structure of the Villi (Figs. 1018 and 1019). 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. 



The essential parts of a villus are the lacteal vessel, the bloodvessels, the epithelium, the 

 basement membrane, and muscle and lymphoid tissues of the mucosa, these structures being 

 supported and held together by retiform tissue. 



These structures are arranged in the following manner: Situated in the centre of the villus 

 is a space, the lacteal, terminating near the summit in a blind extremity; running beside this 

 vessel are unstriped muscle fibres; surrounding it is a meshwork of fibroelastic tissue supporting 

 a plexus of capillary vessels and diffuse lymphoid tissue, the whole being enclosed by a basement 

 membrane, and covered by simple columnar epithelium and goblet cells. Nerve fibres are con- 

 tained within the villi; they form ramifications throughout the reticulum. 



The lacteals are in some cases double, and in some animals multiple. Situated in the axis of a 

 villus, each commences by a dilated cecal extremity near to, but not quite at, the summit of the 

 villus. The walls are composed of a single layer of endothelial cells, the interstitial substance 

 between the cells being continuous with the reticulum of the matrix. The muscle fibres are 

 derived from the muscularis mucosae, and are arranged in bundles around the lacteal vessel. 



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



