198 



COLUMNAR EPITHELIUM. 



columnar and ciliated transitional epithelia differ, however, so very slightly from the 

 corresponding simple epithelia viz., merely in the presence between the fixed ends 

 of the columnar and ciliated cells of smaller and probably younger epithelium cells 

 irregularly disposed that they do not seem to merit any special description. But 

 the scaly transitional epithelium which is met with lining the urinary bladder and 

 ureters presents several peculiarities. It consists of three or four layers of cells, of 

 which the inner or most superficial are large flattened scales when examined from 

 the distended bladder (fig. 227, a) ; almost cubical in shape when taken from the 

 collapsed organ ; smooth over their free surface, but pitted on the opposite side, 

 being moulded over the rounded ends of the cells which form the next layer. 

 These are pyriform, and the smaller end of the pear is set upon the subjacent 

 connective tissue, whilst the larger end has the position just mentioned (fig. 227). 

 Filling up the intervals between these tapering cells are the smaller irregular cells 

 of the third layer (fig. 226, c). All these cells have distinct nuclei, and in the flattened 

 superficial cells two nuclei may often be seen in the cell. If this is an indication 

 that the cell is about to divide, the mode of growth of this kind of epithelium must 

 be different from that of the stratified scaly variety, in which the multiplication of 

 the cells appears to take place exclusively in the deeper layers. 



Fig. 228. PAVEMENT EPITHELIUM, SCRAPED FROM A SEROUS MEMBRANE. 

 , cell-body; b, nucleus ; c, nucleoli. (Henle.) 



Fig. 229. PAVEMENT EPITHELIUM (ENDOTHELIUM) FROM THE OMENTUM OF THE RABBIT. 

 NITRATE OF SILVER STAINING. (E. A. S.) 



Pavement epithelium. In this the cells form polygonal plates or scales, which 

 fit together by their edges like the tiles of a mosaic pavement. The lines of 

 junction of the cells may be straight, or they may be more or less jagged or sinuous. 

 The flattened mesoblastic epithelia (endothelia), such as the epithelium of serous 

 membranes and of the vessels and that lining the anterior chamber of the eye belong 

 to this variety, but it includes also the epithelium lining the alveoli of the lungs 

 which is of hypoblastic origin ; that covering the outer surface of the membrana 

 tympani, and that lining the mammary ducts, both of which are epiblastic. 

 , Columnar epithelium. A second variety of simple epithelium is the columnar, 



