34 



EPITHELIUM. 



[CH. III. 



consists of smaller cells fitted into the inequalities of the cells 

 above them, as in the trachea (fig. 42). 



The epithelium which is met with lining the urinary bladder 

 and ureters is, however, the transitional par excellence. In this 

 variety there are two or three layers of cells, the upper being 

 more or less flattened according to the full or collapsed condition 

 of the organ, their under 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 narrower parts of the second 

 row of cells are fixed the irregular cells which constitute the 

 third row ; sometimes a fourth row is present (fig. 44). It can 

 be easily understood, therefore, that if a scraping of the mucous 



Fig. .44. Epithelium of the bladder, a, one 

 of the cells of the first row ; b, a cell of 

 the second row; c, cells in situ, of first, 

 second, and deepest layers. (Obersteiner.) 



Fig. 45. Transitional epithelial cells 

 from a scraping of the mucous 

 membrane of the bladder of the 

 rabbit. (V. D. Harris.) 



membrane of the bladder is teased, and examined under the micro- 

 scope, cells of a great variety of forms may be made out (fig. 45). 

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

 cells often possess two. 



Stratified Epithelium. 



The term stratified epithelium is employed when the cells 

 forming the epithelium are arranged in a considerable number of 

 superimposed layers. The shape and size of the cells of the 

 different layers, as well as the number of the layers, vary in 

 different situations ; but 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 super- 

 ficial cells are broad and overlap by their edges (fig. 46). Their 

 chemical composition is different from that of the underlying cells, 

 as they contain keratin, and are therefore horny in character. 



