5o8 



THE CELL AND MAMMALIAN HISTOLOGY 



THE EPIDERMIS 



The skin is much more than a mere epitheUum, but its outer 

 part, the epidermis, is a good example of a stratified epithelium 

 (Figs. 262, 395). The inner layers, called collectively the stratum 

 of Malpighi, have cells which range from roughly columnar in 

 the deepest part to a flattened polyhedral form above. The 

 nuclei are conspicuous and actively mitotic, and as the cells 

 divide they push the older cells outward ; this accounts for the 

 gradual flattening as the surface is approached. There are no 



Stratum 

 i corneum. 



^ j Stratum 

 XgranuLosum 



i-S,LuCLC/um. 



Stratum 

 ofMaipighi 



Melanocyte. 



Dermis. 



8looc/\ Sweat\ Hair\ Muscle. Sebaceous 

 lessen gland) folLicle, gland. 



Fig. 395. — A diagrammatic transverse section of mammalian skin. 



blood vessels, but there are intercellular spaces which may allow 

 the diffusion of foodstuffs, and the cells are connected by proto- 

 plasmic threads. Outside the stratum of Malpighi is the stratum 

 granulosum, of two or three layers, and then the stratum lucidum. 

 These appear in stained sections, as their names imply, as respec- 

 tively full of granules and clear. They are stages in the death of the 

 cells ; mitosis (p. 688) has ceased, the nuclei become progressively 

 less distinct, and the formation of keratin has begun. The stratum 

 corneum which is on the surface consists almost entirely of kenitin; 

 cells cannot be recognised, and there is no living protoplasm. 

 It may be thicker than all the other layers together. Its outer 



