HISTOLOGY 



135 



A cuticula is a dense, tough or hard material formed at the exposed 

 surface of an epithelial cell. The cuticular products of adjacent cells are 

 perfectly continuous thus giving rise to an uninterrupted layer over the 

 epithelium. Such a layer occurs, as stated above, on the stratified epi- 

 dermis of fishes (Fig. 94), and is found also, in varying degrees of develop- 

 ment, on many internal epithelia. 



Keratin is a nitrogenous organic substance which may be formed by 

 epithelial cells. It is the basis of homy structures. Its most character- 

 istic development is seen in the epidermis of vertebrates. Produced within 

 the cell, the keratin is deposited in the peripheral region of the cell and at 

 the expense of the cytoplasm. As the process reaches its limit, the nucleus 

 and remnant of cytoplasm die and dry up. What was a living cell is then 

 merely a minute horny scale — in contrast to the fact that cells which 

 produce a cuticula remain alive. As the keratin is deposited, adjacent 

 cells somehow become strongly adherent so that the entire keratinized or 

 "horny" layer (stratum comeimi) acquires a high degree of mechanical 

 resistance. The process may involve only the outermost tier of cells of 

 the epidermis, as in some amphibians, or, as in reptiles, several or many 

 of the upper layers of cells become horny. On the human body the stratum 

 corneum varies from a thin and flexible layer, as on the back of the hand, 

 to a thick hard and tough layer, as in the callosities of the palm and sole 

 (Fig. 95). 



The stratum corneum is one of the most important epithelial products 

 of a vertebrate. Fishes have merely a cuticular outer layer on the epi- 

 dermis. Apparently amphibians introduced the stratum corneum. The 

 characteristic superficial scales of reptiles, and feathers, hair, claws, hoofs, 

 nails and the hollow horns of ruminant ungulates are all differentiations 

 of the stratum corneum — they are epithelial products. 



In amphibians and reptiles the horny layer is shed periodically and 

 either entire or in large fragments. In birds and mammals minute par- 

 ticles of the layer are constantly sloughing off. The material thus lost is 

 replaced by growth in the deeper part of the epidermis. In animals 

 which shed periodically, a new horny layer is well established beneath the 

 old before the old is shed. The animal therefore passes through no such 

 critical period as the "soft-shelled" stage of a crab. It is this ease of 

 repair and replacement of the outermost layer of the body which makes 

 the stratum corneum incomparably superior to a cuticular layer for the 

 uses of large heavy land animals. 



Calcified structures may be formed by an epithelium. The shells of 

 mollusks are epidermal products. In vertebrates the enamel which caps 

 the spine of a placoid scale of a shark and covers the crown of a tooth is an 

 epithelial product. But, in contrast to cuticular and horny layers which 

 are produced at the free surface of an epithelium, enamel is deposited at 



