THE INTEGUMENTARY SYSTEM 



177 



light. A mole or nevus is an elevation of the skin due to local prolifera- 

 tion of epidermis and corium, and is usually excessively pigmented. When 

 a mole is congenital and involves blood capillaries, it forms a "birthmark." 

 Since pigments like those of vertebrates are found also in invertebrates, 

 there is no reason to question their common origin. Many animals 

 below the mammals have their pigments in special cells, the chromato- 

 phores, which expand or contract under the influence of hormones and 

 thus alter the color of the skin. The colors of lizards, which are often 



STRATUM CORNEUM---; 



mn 



CORIUM PAPILLA 

 / 



mmiiiinMjiniK- 



,PILLA--^**S1*'' 



STRATUM GERMINATIVUM 



EVOLUTION OF 

 FEATHER 



EVOUUTION OF 

 HAIR 



PAPILLA--^ 



A. EVOULTTION OF PLACOID 

 SCALE 



Fig. 135. — A series of hypothetical stages in the phylogenesis of cutaneous append- 

 ages of vertebrates. A common origin of placoid scale, horny scale, feather, and hair 

 from a corium papilla is assumed. (Redrawn after MatviefT.) 



brilUant, are not in their scales, but in chromatophores of the underlying 

 corium. 



Widely among vertebrates, pigments of scales, skin, hair, or feathers 

 often show striking and elaborate patterns that serve for protection, 

 warning, recognition, or sexual allure; but in man chiefly the region of the 

 nipples and the external genitals are slightly darker than the rest of the 

 body. In man and some other hairless mammals, such as the elephant, 

 the function of the skin pigment is to check ultraviolet light before it 

 penetrates to Uving cells. Everyone has observed the effect of the 

 sun's rays upon unwonted skin, and the promptness with which the skin 

 responds by tanning. Lacking skin pigment, men could not live in 

 some parts of the earth. 



