THE KERATINIZED TISSUES 



55 



Certain of the polysaccharide fibres from vertebrate skin are said to yield 

 an X-ray diffraction pattern like that of cellulose (p. 24). 



The configuration of the boundary between the two layers is much 

 influenced by the presence of specialized appendages — hairs, scales, etc., 

 formed by the epidermis. In the absence of these, it may be smooth and 

 run parallel to the external surface; when present they dip deeply into 

 the dermis. In mammals there is a marked development of small regularly- 

 placed dermal papillae and ridges which, being more prominent the 



Fig. 25. A generalized, stratified, keratinized, squamous epithelium 



resting on a basal membrane backed by a collagen-containing dermis. 



In all keratinized tissues (except the thinnest skins) the several layers 



shown on the right-hand side may be distinguished. 



thicker the epidermis, are related probably to the nutrition of the superficial 

 cells. They have the effect of greatly increasing the area of contact 

 between the two formations and thus should facilitate the transfer of 

 nutrient from the vasculated dermis to the epidermis. 



The Epidermis 



Among the vertebrates the epidermis is always a stratified epithelium 

 consisting of a few to many layers of cells produced by the proliferation 

 of a single basal layer (Fig. 25). Epithelia, more-or-less regular and 

 compact layers of cells, are common tissues and usually cover the external 

 surfaces of an organism or its internal cavities. Their superficial location 

 appears to impose certain common histological features. The cells may 



