44 EPITHELIAL TISSUES 



of the cell toward the surface. The keratization is apparently dependent 

 upon surrounding physical conditions, for it is much more marked in 

 the skin, which from constant and rapid evaporation is comparatively 

 dry, than in the mouth, esophagus, or conjunctiva, where the epithelium is 

 constantly moistened by glandular secretions; the margins of the lips, 

 eyelids, etc., present an intermediate state of keratization. 



With these chemical changes in the composition of the cytoplasm 

 there are corresponding changes in its nucleus. In the deeper cells, the 

 nucleus is oval or spherical and highly chromatic. Toward the surface, 

 the nucleus becomes more and more flattened and more and more obscured 

 by the cornification of the cell protoplasm. In the most superficial cells 

 it is usually impossible to demonstrate the nuclei, except by acting upon 

 their protoplasm with strong reagents such as caustic alkalies, soda or 

 potassa. 



But the most characteristic change in the cells of stratified epithelium 

 is the progressive transition in shape undergone during their passage 

 from the deeper layers to the free surface. New cells, resulting from indi- 

 rect division of the cells in the deeper layers, are by continued reproduction 

 gradually pushed toward the surface, whence they are constantly being 

 desquamated in small scaly masses. The pressure exerted in this process 

 tends to gradually flatten these cells, so that their vertical diameter, that 

 perpendicular to the surface, becomes progressively shorter the nearer 

 they approach the free surface ; on the other hand, their transverse diam- 

 eter, that parallel to the surface of the epithelial membrane, is correspond- 

 ingly increased. The deepest cells of the stratified epithelium those 

 which rest upon the basement membrane are elongated in their vertical 

 diameter and possess an irregularly columnar shape. Their nuclei are 

 likewise elongated, oval or elliptical in shape. In the skin of brunettes 

 and the dark-skinned races, and in the epithelium of the skin of the 

 scrotum, perianal region, and areolae of the breasts, these cells contain 

 small granules of the pigment to which the color of the cuticle is largely 

 due. This columnar cell layer is then described as the layer of pigment 

 epithelium. Superficial to these, but still in the deeper layers, are poly- 

 hedral cells with spherical nuclei, which are known as priclde cells be- 

 cause of their prominent intercellular bridges. Superficial to the prickle 

 cells, the epithelial cells become progressively more flattened, until at the 

 surface, they are mere scales. This gradual transition from columnar 

 and polyhedral cells below, to thin flat scales on the surface is character- 

 istic of all stratified epithelium. 



The thin superficial scales resemble very closely in shape and appear- 



