48 



ZOOLOGY. 



Fig. 10. SQUAMOUS 

 EPITHELIUM. 



Surface view. 



perpendicular to the surface of the epithelium (figs. 

 8, 12). 



We may also distinguish simple epithelia (consisting of 

 one layer of cells only), and compound 

 epithelia (consisting of many layers). 



This does not by any means exhaust 

 the varieties of epithelium, but is merely 

 a rough provisional classification, sufficient 

 for our present purpose. 



One kind of squamous epithelium, in 

 which the thin ends of one cell overlap 

 those of others, can easily be obtained from 

 the inside of the human cheek by gentle 

 scraping with the handle of a scalpel. If the material so 

 obtained is mounted in a drop of water and examined 

 under the microscope, the cells can easily be seen (fig. 10), 

 The addition of a little acetic acid makes the nucleus very 

 plain, and carmine or magenta will stain it deeply. 



3. Endothelium. In another kind of squamous 

 epithelium there is only one layer of cells and they are 

 exceedingly flat and thin, the nucleus making a little bulge ; 

 t he edges of the cells do not over- 

 lap, but meet like the tiles in a 

 mosaic pavement (figs. 9s, 11). 

 Hence this kind of epithelium is 

 often called pavement-epithelium : 

 it is called endothelium when it 

 lines closed cavities, as very fre- 

 quently it does. 



The ccelom and all the blood- 

 vessels are lined by endothelium Fig. 11. i 



the Walls of Capillaries Consist of Surface view. From peritoneum. 



nothing but endothelium, those of ^gyf**** are Bbown 

 veins and arteries have, in addition, 



coats of connective- ana muscular tissue around this. The 

 alveoli of the lungs are also lined by pavement-epithelium, 

 but as they are not closed cavities, it is not called endothelium. 



4. Ciliated Epithelium. Columnar epithelium is very 



