490 A TEXTBOOK OF PHYSIOLOGY 



When clothing is worn, the increased C0 2 and water output occurs 

 at a lower temperature : 



C0 2 per Hour. 



0-33 gramme 



These observations were carried out in still air in a respiratory 

 chamber. 



In frogs and kindred animals, the exchange is great enough to 

 enable the animal to live without lungs. There is a special pulmo- 

 cutaneous circulation for this purpose. 



The Function of Pigment. The value of pigment in the Malpighian 

 layers of the skin of man is to protect against the lethal effect of 

 intense sunlight. Rays are absorbed by the pigment of the skin, 

 and converted into heat-rays, which in their turn increase the 

 transpiration of water from the cutaneous capillaries, and stimulate 

 the cutaneous nerve-endings, and provoke sweating. Thus, the 

 energy of the sun is sweated off the body of the black man. On the 

 other hand, rays penetrate to the blood of the white man, and are 

 absorbed by the haemoglobin, and there converted into heat-rays. 

 Moreover, the rays produce sunburn in the skin of the white man, 

 resulting in pigmentation. The white man, therefore, to keep white . 

 has to wear clothes and to shelter himself from the sun, while the 

 black man is happy naked. The white man wears white clothes in 

 the tropics to reflect and scatter the sun's rays ; also these, by 

 entangling air, lessen the loss of heat by convection and evapora- 

 tion. Thus, the clothed white man cannot do field labour and be 

 comfortable in the tropics, and fans are of the utmost necessity for 

 securing his comfort and efficiency indoors. The naked black man 

 is physiologically efficient for life in the tropics. 



The absorption of rays by the skin of the negro is probably 

 the reason why the photograph of the naked negro is less distinct 

 than that of the white man, from whose skin more rays are reflected. 

 The pigment is not derived from blood-pigment, but belongs to 

 a group of bodies known as melanins. The pigment of the hair and 

 skin of the negro has been found to contain about 15-5 per cent, of 

 nitrogen, 3-6 per cent, of sulphur, and a trace of iron, the pigment of 

 the hair containing less nitrogen than that of the skin. The pigment 

 is probably derived from tyrosin by the action of an oxidase. 



In addition to the function of protection against the sun's rays, 

 pigment is used among animals for various other purposes. Animals, 

 such as the Arctic fox and hare, may undergo seasonal change of 

 colour for protective purposes, changing a brown, soil-colour summer 

 coat to a white, snow-colour winter coat. The change seems to be 





