CHAP. II.] EPITHELIAL TISSUE. 9 



epithelial tissue in this situation called epidermis, because it 

 lies upon the surface of the true skin. In other situations, 

 epithelial tissue usually receives the general name of epithelium. 



Classification. We may classify the varieties of epithelium 

 according to the shape of the cells which compose them, or 

 according to the arrangement of these cells in layers. Adopt- 

 ing the latter and simpler classification, we distinguish three 

 main varieties : the stratified, consisting of many layers ; the 

 transitional, consisting of two or three layers; the simple,' con- 

 sisting of a single layer of cells. 



1. Stratified epithelium. The cells composing the different 

 layers of stratified epithelium differ in shape. As a rule, the 



FIG. 4. SECTION OF STRATIFIED EPITHELIUM, c, lowermost columnar cells ; P, 

 polygonal cells above these ; fl, flattened cells near the surface. Between the cells 

 are seen intercellular channels, bridged over by processes which pass from cell to cell. 



cells of the deepest layer are columnar in shape ; the next, 

 rounded or many-sided, whilst those nearest the surface are 

 always flattened and scale-like, the protoplasm of the cell being 

 finally converted into a horn-like substance. The deeper soft 

 cells of a stratified epithelium are continually multiplying by 

 cell-division, and as the new cells which are thus produced 

 in the deeper parts increase in size, they compress and push 

 outwards those previously formed. In this way cells which 

 were at first deeply seated are gradually shifted outwards and 

 upwards, growing harder as they approach the surface. The 

 older superficial cells are being continually rubbed off as the 

 new ones continually rise up to supply their places. 



Stratified epithelium covers the anterior surface of the eye, 

 lines the mouth, the chief part of the pharynx, .the gullet, the 

 vagina, and the neck of the uterus, but its most extensive distri- 

 bution is over the surface of the skin, where it forms the epider- 

 mis. Whenever a surface is exposed to friction we find stratified 



