CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 445 



260. The Villi. The villi vary in size and form in different 

 animals, and in different parts of the intestine in the same 

 animal ; each villus moreover varies in form at different times ; 

 they may be generally described as having the shape of a flattened 

 finger but are frequently broader at the free end than at the 

 base ; they have, in man, a length of about 1 mm. and a breadth 

 of from '2 mm. to '5 mm. 



Each villus consists of a body of reticular tissue, the outer 

 surface forming, as explained above, a basement membrane, which 

 is covered by a single layer of epithelium cells. Two kinds of 

 cells, that is cells presenting two sets of characters, make up this 

 single layer of epithelium. 



One kind is a columnar or conical cell, with its broader end 

 forming part of the fred surface of the villas, and its narrower end 

 resting on or filling up a gap in the basement membrane. The 

 greater part of the cell-body is formed of the kind of 'granular' 

 cell substance spoken of as protoplasmic, but differs in appearance 

 and condition according to circumstances; these variations we 

 shall study separately. An oval nucleus is placed vertically at 

 about the lower third of the cell. At the free border of each cell 

 the granular cell-substance changes to a narrow band of clear 

 hyaline refractive material marked, in many prepared specimens 

 and often even in the fresh state, with fine vertical lines so as to 

 appear striated vertically or rather radially; in a section of a 

 villus, optical or actual, the whole villus seems to be surrounded 

 by a band of this clear refractive material. 



A ciliated epithelium bears, as we have seen ( 93), a similar 

 hyaline refractive border from which the cilia project and with which 

 they are connected, but which does not share in the movements of 

 the cilia belonging to it, remaining unchanged in form while 

 these are moving ; its exact nature is at present uncertain. The 

 refractive border of a columnar cell of a villus differs from the 

 similar border of a ciliated cell in that on the one hand it never, in 

 vertebrates, bears cilia, and on the other hand does under certain 

 circumstances change its form. The striation spoken of above 

 appears to be due to the fact that the border is composed of a 

 number of rods imbedded side by side in a substance which is 

 sometimes of the same refractive power as the rods, in which case 

 the whole border appears homogeneous, but which is sometimes of 

 different refractive power, in which case the striation is distinct. 

 The rods, which are thought by some to be hyaline processes of 

 the underlying cell-substance projecting into the above-mentioned 

 cement-substance, are sometimes long and thin, sometimes short 

 and thick, the whole border being in the former case narrow, in the 

 latter broad. Under the influence of reagents or of circumstances 

 the one condition may change into the other, and the change 

 seems to be an active not a passive process, since it will only take 

 place so long as the cells are alive. This refractive border of the 



