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CHORDATE ANATOMY 



Simple Epithelium. An epithelium only one cell in thickness is 

 termed simple. There is, however, great variation in the thickness of 



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Fig. 79. — Columnar ciliated epithelium from human trachea. Most of the cells 

 are slender, with axes more or less curved, and extend from the basement membrane 

 to the free surface of the epithelium. Occasional short cells, basal cells, lie at or near 

 the basement membrane and do not extend to the free surface. Several swollen mucous 

 cells ("goblet" cells) are shown. (From Bremer, "Text-book of Histology.") 



simple epithelia. The cells, seen in sections perpendicular to the surface, 



may be approximately square in outline. Such an epithelium is called 



cuboidal (Fig. 77), but incorrectly for 

 the cells are usually hexagonal prisms. 

 A simple epithelium consisting of tall 

 prismatic cells (Fig. 78C) is called 

 columnar. At the extreme of thinness 

 are epithelia (flat or squamous) each of 

 whose cells is a broad flat plate, hexag- 

 onal in outline. (Figs. 77, 78S) 



Stratified Epithelium. On Amphi- 

 oxus, a slender marine animal only four 

 or five centimeters long, an epidermis one 

 cell thick afi'ords adequate protection. 

 On an elephant it would not. Surfaces 

 of large heavy animals are exposed to 

 excessive mechanical friction and impact. 

 Loss of material at the surface is best 



compensated for by a stratified epithelium whose lower layers persistently 



grow to replace the loss. 



A stratified epithelium may be two or several or many cells in thickness 



(Fig. 'jSD-F). In all vertebrates the epidermis is stratified (Fig. 80). 



Fig. 80. — Skin of lung-fish, Proto- 

 pterus; section perpendicular to sur- 

 face; much enlarged, c, dermis 

 (corium); e, epidermis; g, multi- 

 cellular gland; u, unicellular gland. 

 (From Kingsley.) 



