CHAPTER XVIII 



THE SKIN; APPENDAGES OF THE SKIN. PRODUCTION OF HEAT; 

 REGULATION OF HEAT. VARIATIONS IN TEMPERATURE 



THE SKIN 



Functions. — The skin is not, like the kidneys, set apart to per- 

 form one special function. It serves: (1) as a protective cover- 

 ing for the deeper tissues lying beneath it, (2) as a sense organ, 

 (3) as an excretory organ, (4) as an absorbing organ, (5) as an 

 important organ in heat regulation, and (6) as a respiratory organ. 



Structure. — It consists of two distinct layers : — 



(1) Epidermis, scarf skin, or cuticle. 



(2) Derma, cutis vera, or corium. 



Epidermis. — The epidermis is a stratified epithehum, com- 

 posed of a number of layers of cells. The thickness varies in 

 different parts of the body, measuring in some places not more 

 than oi^ inch (0.104 mm.), and in others as much as o^ inch 

 (1.04 mm.). It is thickest in the palms of the hands and on the 

 soles of the feet, where the skin is most exposed to friction, and 

 thinnest on the ventral surface of the trunk, and the inner surfaces 

 of the limbs. It forms a protective covering over every part of 

 the true skin, upon which it is closely moulded. 



It is roughly divisible into two layers : — 



(1) Upper, or Horny. 



(2) Germinative, or Malpighian. 



(1) The horny layer consists of three strata of cells, which are 

 practically dead, and are constantly being shed and renewed from 

 the cells of the germinative layer. 



(2) The germinative layer consists of soft protoplasmic cells. 

 The growth of the epidermis takes place by the multiplication 



of these cells. As they multiply they push upward toward the 

 surface those previously formed. In their upward progress 

 they undergo a chemical transformation, and the soft proto- 

 plasmic cells become converted into the flat, horny scales which 



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