164 INTESTINAL MUCOUS MEMBRANE. 



they gradually and regularly diminish both in num- 

 ber and in size, until, in the lower part of the ileum, 

 they are recognisable only by a faint thickened line. 

 The villi of the small intestine are minute pro- 

 cesses, analogous to the papillae of the tongue, but 

 more delicate in their proportions. The* best idea of 

 their shape, number, and mode of arrangement, is to 

 be got by placing a piece of the mucous membrane 

 under water, and examining it with a magnifying 

 glass, or a microscope with a low power. They are 



> seen to occupy the whole extent of the surface of the 



membrane, and to be more numerous in the duo- 

 denum and jejunum, than in the ileum / they are seen 

 also to assume two principal forms the flat, or val- 

 vular, and the conical. The flat villi are principally 

 found in the upper portion of the small intestine; 

 they are simple and solitary, or, by running into each 

 other, become compound, in which case they resemble 

 minute valvulce conniventes (PL XXVI. fig. II. 2, 3). 

 The conical villi are found everywhere throughout 

 the small intestine, but they exist in larger proportion 

 in the ileum (fig. III. 1). In some instances, instead 

 of terminating in a point, their apices are slightly 

 bulbous (PL XXVI. fig. XII.) ; their average height 

 is from one-fourth to one-half a line, and their dia- 

 meter from one-sixteenth to one-fourth of a line. 



structure of the Whatever may be the form of a villus its structure 



villi. . J 



is always the same, and, in examining it from its surface 

 inwards we find, first : a single lamina of epithelium 

 which, on a perfectly fresh specimen which has not 

 been roughly handled, presents the appearance of a 

 mosaic, upon the surface of which an unbroken layer 



