Histology 



307 



produce it and underlie it remain alive. A thin cuticula is present on 

 the epidermis of fishes and larval amphibians and on some internal 

 epithelia. 



Keratin is a nitrogenous organic substance which is formed within 

 some epithelial cells. It is the basis of the horny structures of the 

 vertebrate skin. 



Stratum 

 germinativum. 



Corium 

 (Tunica 

 propria.) 



Fig. 247. Epidermis from the sole of the foot of an 

 adult man. Section perpendicular to surface of skin. 

 External to the stratum germinativum, the strata show 

 successive stages in the production of the stratum 

 corneum. (X 360.) (Courtesy, Bremer: "Text-Book of 

 Histology," Philadelphia, The Blakiston Company.) 



The "horny layer" (stratum corneum: Fig. 247) developed on 

 the skin of vertebrates other than fishes consists of one or more of the 

 outer strata of the epidermis, the cells more or less filled with keratin 

 and strongly adherent to one another so that the whole layer acquires a 

 high degree of mechanical resistance. Completely keratinized cells are 

 dead. No stratum corneum occurs on the skin of fishes, and in most 

 amphibians it is only one cell thick. Hair, feathers, reptilian scales, 

 claws, nails, and hoofs are local specializations of the stratum corneum. 



In amphibians and reptiles the horny layer is usually shed and 

 replaced periodically (ecdysis). In birds and mammals minute flecks 



