Basic Structure of Vertebrates 



Fig. 245 (Left). 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. (Courtesy, Bremer: 

 "Text-Book of Histology," Philadelphia, The Blakiston Company.) 



Fig. 246 (Right). Skin of lungfish, Prolopteras; section perpendicular to surface; 

 much enlarged, (c) Dermis (corium); (e) epidermis; (g) multicellular gland; (u) 

 unicellular gland. (Courtesy, Kingsley: "Comparative Anatomy of Vertebrates," 

 Philadelphia, The Blakiston Company.) 



on their respiration via the external skin and the pharyngeal lining. 

 Their epidermis and the pharyngeal epithelium may contain blood- 

 capillaries. 



Many epithelia, although "simple" in the sense of being only one 

 cell thick, are not the ideally simple tissue of the definition (p. 301), 

 constituted of cells all "alike in their internal differentiation." Among 

 the special functions of an epithelium are the following: (1) produc- 

 tion of a superficial covering of nonliving, mechanically protective sub- 

 stance ranging, in vertebrates, from the delicate cuticular film over 

 the body of a small fish to the massive horny scales on a big alligator 

 or the horny hoofs of a horse: (2) production of special secretions such 

 as mucus; (3) reception of external stimuli; (4) provision for motile 

 activity. Two or more of these functions may t>e carried on by one 

 "simple" epithelium or by a stratified epithelium. Within the epi- 

 thelium, then, cells will exhibit differentiation of as many types as 

 there are functions. 



1. Most epithelia produce a protective covering at the free surface. 



A cuticula is a dense, tough or hard material, partly nitrogenous, 

 deposited on the exposed surface of an epithelium. The cells which 



