THE SKIN. 339 



Structurally the skin or integument consists of 

 two chief strata that may be subdivided into layers 

 in the following manner: 



I. Epidermis epithelial layers derived from the 

 ectoderm. 



1. Corneum or horny layer, superficial 



epithelial plates. 



2. Stratum lucidum, absent where the 



skin is thin. 



3. Stratum granulosum, absent where the 



skin is thin. 



4. Malpighian or germinal layer, nucle- 



ated growing cells. 



II. Dermis or corium, connective-tissue elements 

 from the mesoderm. 



1. Papillary layer. 



2. Reticular layer. 



Epidermis. The horny layer of the epidermis 

 forms the outer covering of the skin and consists of 

 several layers of scaly epithelial cells in which the 

 nuclei have disappeared. The cells are dead and 

 constantly exfoliating superficially, while new strata 

 are regularly added from below. Bacteria are 

 usually present in its external parts, and in surgi- 

 cal operations, therefore, the skin is thoroughly 

 scrubbed, a process that removes most of this layer 

 and renders the field of operation practically sterile. 

 At birth the horny layer is less compact and of a red 

 color. It is then called the vernix caseosa, which 

 exfoliates in a few days, when the complexion 



