248 DIGESTION. 



vessel rarely looped or branched (Fig. 81); besides granular 

 matter, fat-globules, &c. 



The epithelium is of the columnar kind, and continuous with 

 that lining the other parts of the mucous membrane. The 

 cells are arranged with their long axis radiating from the sur- 

 face of the villus (Fig. 81), and their smaller ends resting on 

 the basement-membrane. Some doubt exists concerning the 

 minute structure of these cells, and their relation to the deeper 

 parts of the villus. 



Beneath the basement or limiting membrane there is a rich 

 supply of bloodvessels. Two or more minute arteries are dis- 

 tributed within each villus ; and from their capillaries, which 

 form a dense network, proceed one or two small veins, which 

 pass out at the base of the villus. 



The layer of the muscularis mucosce in the villus forms a kind 

 of thin hollow cone immediately around the central lacteal, 

 and, is therefore, situate beneath the bloodvessels. The ad- 

 dition of acetic acid to the villus brings out the characteristic 

 nuclei of the muscular fibres, and shows the size and position 

 of the layer most distinctly. Its use is still unknown, although 

 it is impossible to resist the belief, that it is instrumental in the 

 propulsion of chyle along the lacteal. 



The lacteal vessel enters the base of each villus, and passing 

 up in the middle of it, extends nearly to the tip, where it ends 

 commonly by a closed and somewhat dilated extremity. In 

 the larger villi there may be two small lacteal vessels which 

 end by a loop (Fig. 81.), or the lacteals may form a kind of 

 network in the villus. The last method of ending, however, 

 is rarely or never seen in the human subject, although com- 

 mon in some of the lower animals (A, Fig. 81). 



The office of the villi is the absorption of chyle from the 

 completely digested food in the intestine. The mode in which 

 they effect this will be considered in the chapter on Absorp- 

 tion. 



Structure of the Large Intestine. 



The large intestine, which in an adult is from about 4 to 

 6 feet long, is subdivided for descriptive purposes into three 

 portions, viz. : the ccecum, a short wide pouch, communicating 

 with the lower end of the small intestine through an opening, 

 guarded by the ileo-ccecal valve ; the colon, continuous with the 

 caecum, which forms the principal part of the large intestine, 

 and is divided into an ascending, transverse, and descending 

 portion ; and the rectum, which, after dilating at its lower part, 

 again contracts, and immediately afterwards opens externally 



