THE EPIDERMIS 



263 



of the body, being thickest upon those surfaces which are exposed to the 

 greatest mechanical violence, e.g., the palms of the hands and soles of 

 the feet ; and thinnest in the least exposed portions, e.g., inner sides 

 of the arms and the back. 



The layer of stratified squamous epithelium composing the epidermis 

 differs from that of the mucous mem- 

 branes in that its superficial cells con- 

 tain an abundance of keratin, a peculiar 

 horny material. The production of 

 keratin in the cells of stratified epi- 

 thelium appears to be more or less de- 

 pendent upon the desiccation which oc- 

 curs in those cells which form the com- 

 paratively dry cutaneous surface. The 

 cornification can scarcely be demon- 

 strated in the stratified squamous epi- 

 thelium of the moistened mucous mem- 

 brane of the mouth, esophagus, etc. ; it 

 is present though not pronounced in 

 the partially moistened margins of the 

 eyelids, lips, labia minora, glans penis, 

 etc. In the epidermis, however, corni- 

 fication is pronounced and characteris- 

 tic in all portions of the body. 



The thickness of the cornified lay- 

 ers appears to be in proportion to, 

 if not entirely dependent upon, the 

 amount of mechanical violence to which 

 the cutaneous surface is subjected. Ac- 

 cordingly the increased thickness of 

 the epidermis covering the palms and 

 soles is found to be due almost entirely 

 to an increase in the superficial horny 

 portion of the epidermis, the germinal 

 layers being no more pronounced than in other portions of the body. 



The epidermal tissue is divisible into a superficial horny portion 

 consisting of flattened, desiccated, cornified cells the stratum corneum 

 or horny layer and a deeper protoplasmic, so-called 'mucous' portion, 

 which consists of polyhedral and cylindrical cells stratum germinati- 

 vum, or nmcosum, rete mucosum, rete Malpighii. 



Vg 



il 



' 



FIG. 260. EPIDERMIS OF THE FOOT. 



a, flattened cells; b, stratum luci- 

 dum; c, granular layer; d, germinal 

 layer; e, cylindrical cell layer; /, 

 derma. Picrocarmin. Moderately 

 magnified. (After Ranvier.) 



