THE STRUCTURE OF THE ELEMENTARY TISSUES. 27 



surface being marked with one or more depressions, into which the heads 

 of the next layer of club-shaped cells fit. Between the lower and nar- 

 rower parts of the second row of cells, are fixed the irregular cells which 

 constitute the third row, and in like manner sometimes a fourth row 

 (Fig. 24). It can be easily understood, therefore, that if a scraping of 

 the mucous membrane of the bladder be teased, and examined under the 

 microscope, cells of a great variety of forms may be made out (Fig. 25). 

 Each cell contains a large nucleus, and the larger and superficial cells 

 often possess two. 



0. Stratified Epithelium. This term is employed when the cells 

 forming the epithelium are arranged in a considerable number of super- 

 imposed layers. The shape and size of the cells of the different layers, 

 as well as the number of the layers, vary in different situations. Thus 

 the superficial cells are as a rule of the squamous, or scaly variety, and 

 the deepest of the columnar form. 



The cells of the intermediate layers are of different shapes, but those 

 of the middle layers are more or less rounded. The superficial cells over- 

 lap by their edges (Fig. 26); they are broad (Fig. 13). Their chemical 

 composition is different from that of the underlying cells, as they con- 

 tain keratin, and are therefore horny in character. 



The nucleus is often not apparent. The really cellular nature of 

 even the dry and shrivelled scales cast off from the surface of the epi- 

 dermis, can be proved by the application of caustic potash, which causes 

 them rapidly to swell and assume their original form. 



The squamous cells exist in the greatest number of layers in the epi- 

 dermis or superficial part of the skin; and the most superficial of these 



FIG. 26. -Vertical section of the stratified epithelium of the Rabbit's cornea, a. Anterior epi- 

 thelium, showing the different shapes of the cells at various depths from the free surface. 6. Por- 

 tion of the substance of cornea. CKtem-) 



are being continually removed by friction, and new cells from below sup- 

 ply the place of those cast off. 



The intermediate cells approach more to the flat variety the nearer 

 they are to the surface, and to the columnar as they approach the lowest 

 layer. There may be considerable intercellular intervals; and in many 

 of the deeper layers of epithelium in the mouth and skin, the outline of 



