THE SKIN 323 



of an inch, total area 16 to 20 square feet in man, and 

 12 to 16 square feet in woman. 



The skin secretes a clear, colorless fluid, the sweat; 

 it acts as a protection to the underlying structures, and 

 aids in the excretion of waste-products of metabolism, 

 possessing an accessory function in conjunction with 

 the lungs and kidneys of elimination of these products 

 from the body. It also assists in regulating the 

 temperature of the body by promoting heat dissi- 

 pation. 



The skin must be studied under the microscope in 

 order to properly grasp its structure. There are two 

 principal layers, superficial and deep; the former is 

 termed the epidermis, the latter the derma or corium. 



The Epidermis. This consists of epithelial cells 

 derived from the ectoderm. There are two layers 

 within the epidermis: superficial or horny layer, and 

 deep or Malpighian layer. The horny layer consists 

 of non-nucleated scaly cells composed of keratin. 

 The surface cells of this layer are being continually 

 rubbed off, and are replaced by cells from the Mal- 

 pighian layer underneath, which undergo a change 

 and are converted into keratin as they approach the 

 surface. The Malpighian or deep layer is divided into 

 four layers, named from without inward stratum 

 lucidum, stratum granulosum, stratum mucosum, and 

 .the stratum germinativum. The stratum mucosum 

 contains the prickle cells which contain the pigment 

 granules that give to, the skin its different individual 

 and racial characteristics in regard to color. 



The Derma, Corium, or True Skin. This is com- 

 posed of the two layers: superficial or papillary layer, 

 and the reticular. 



The superficial or papillary layer lies beneath the 

 epidermis. It contains numerous projections called 

 papillae, and i composed of a net-work of fine bundles 

 of fibrous tissue. The papillae are composed of fibrous 



