26 EPITHELIAL TISSUES 



sense, and will be more fully described as a part of these several 

 organs.* 



STRATIFIED EPITHELIUM. This variety of epithelium oc- 

 curs as a membrane of varying thickness but always comprising 

 several cell layers. A straight line perpendicular to its free sur- 

 face would penetrate from five to thirty or more epithelial cells. 

 But while there is a wide diversity in the thickness of the epithe- 

 lial layers, the character of the cells at any given level is very nearly 

 constant. Thus the deeper cells, those nearest the basement mem- 

 brane, are nucleated, of soft consistence, and may contain mitotic 

 figures, indicating that it is at this level that cell reproduction is 

 most active. Toward the surface of the membrane the cells be- 

 come progressively of firmer consistence, so that the most super- 

 ficial ones present a horny appearance as a result of the gradual 

 keratization of the cytoplasm during the progress of the cell to- 

 ward the surface. The keratization is apparently dependent upon 

 surrounding mechanical conditions, for it is much more marked in 

 the skin, which from constant and rapid evaporation is compara- 

 tively 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 cyto- 

 plasm 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 proto- 

 plasm. In the most superficial cells it is usually impossible to 

 demonstrate the nuclei, except by acting upon their protoplasm 

 with strong reagents such as the caustic alkalis, soda or potassa. 



But the most characteristic change in the cells of stratified 

 epithelium is the progressive transition in shape during their pas- 

 sage from the deeper layers to the free surface. Kew cells, result- 

 ing from indirect 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 perpendic- 

 ular to the surface, becomes progressively shorter the nearer they 



*See chapters on the Eye, the Ear, the Olfactory Organ, the Tongue, and on 

 Nerve End-organs. 



