THE STRUCTURE OF THE ELEMENTARY TISSUES. 



37 



(b) Transitional Epithelium. 



This term has been applied to cells, which are neither arranged in a 

 single layer, as is the case with simple epithelium, nor yet in many 

 superimposed strata as in laminated; in other words, it is employed 

 when epithelial cells are found in two, three, or four superimposed 

 layers. 



The upper layer may be either single columnar, columnar ciliated, 

 or squamous. When the upper layer is columnar or ciliated the second 

 layer consists of smaller cells fitted into the inequalities of the cells 

 above them, as in the trachea (fig. 30). 



The epithelium which is met with lining the urinary bladder and 

 ureters is, however, the transitional par excellence. In this variety. there 



Fig. 31. 



Fig. 32. 



Fig. 31. Epithelium of the bladder, a. One of the cells of the first row; 6, a cell of the second 

 row; c, cells in situ, of first, second, and deepest layers. (Obersteiner.) 



Fig. 32. transitional epithelial cells from the mucous membrane of the bladder of a rabbit. 

 Highly magnified, a, Large flattened cell of superficial layer; a', similar cell in profile; 6, pear- 

 shaped cell of second layer. (Klein.) 



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, and in like manner sometimes a fourth 

 row (fig. 31). 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. 

 32). Each cell contains a large nucleus and the larger and superficial 

 cells often possess two. 



(c) 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. Thus the 



