470 THE VILLI. [BOOK n. 



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 epitnelium. 



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

 forming part of the free surface of the villus, and its narrower end 

 resting on or filling up a zap 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 igranular 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 move- 

 ments 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 

 change its form. The striatjon spoken of above 



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

 number of rods imbedded side by side in a substance whi'-h i- 

 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, sometime 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, 

 whatever be the exact way in which it is carried out, is of such a 

 kind that it will only take place so long aw the cell* are alive. 



