THE EPITHELIAL TISSUES. 



the terms ' ' pavement " or " tessellated ' ' are sometimes applied to this type 

 of epithelium. Such arrangement of the squamous type, however, is unus- 

 ual in the human body the alveoli of the lungs, the posterior surface of the 

 anterior capsule of the crystalline lens and the membranous labyrinth being 

 the chief localities in which simple squamous epithelium is found. 



The far more usual disposition of the squamous type of epithelium is as 

 a number of superimposed layers, this constituting the important group of 

 stratified squamous epithelium. Although the free surface of such struct- 



FIG. 14. Simple squamous epi- 

 thelium from the anterior capsule 

 of the crystalline lens. X 360. 



FIG. 15. Stratified squamous epithelium from anterior surface 

 of the cornea. X 465. 



ures presents the mosaic formed by the superficial plates, the entire tissue is 

 by no means composed of flattened cells. When seen in section (Fig. 15), 

 the deepest cells are not scaly, but irregularly columnar, resting on the base^ 

 ipent membrane by slightly expanded bases. The surface of the underlying 

 connective tissue is beset with minute elevations or papillae, which serve as 

 advantageous positions for the terminations of blood-vessels or of nerves. 

 Owing to the more favored nutrition of the deepest stratum, the cells next 

 the connective tissue possess the greatest vitality and are the source of the 



FIG. 16. Isolated surface cells from epithelium 

 lining the mouth. X 320. 



FiO. 17. Epithelial cells from epidermis, show- 

 ing intercellular bridges. X 675. 



new elements necessary to replace the old and effete cells which are continu- 

 ally being removed at the free surface. This loss is due not only to mechan- 

 ical abrasion, but also to the displacement of the superficial elements by the 

 r ..w cells formed within the deeper layers. 



Passing from the basement membrane towards the free surface, the 

 form of the cells undergoes a radical change. The columnar type belongs 

 exclusively to the deepest layer; the superimposed cells assume irregularly 

 polyhedral forms and gradually expand parallel to the free surface, to 

 become, finally, the large thin scales (Fig. 16) so characteristic of the super- 



