SUN'S RAYS 



87 



compensated by increased loss from the animal by such means as a 

 strong wind, the animal suffers no discomfort. Insolation of the 

 skull alone is without effect if the body temperature is kept within 

 normal limits. Our own experiments showed that rabbits died 

 in about an hour if exposed to the sun with their head shaven, 

 and lived if protected from these rays by means of red glass. Aron 

 and Gibbs have also shown that if the human skin is exposed to the 

 sun's rays the temperature of the area so exposed rises as a rule more 

 rapidly and reaches a higher maximum in a dark skin than in a 

 light, until the nerve endingis of the latter are irritated by the pro- 

 longed exposure. The black skin is protective because it guards 

 the nerve endings from irritation, and because of the more rapid 

 radiation by means of which heat is quickly lost, especially from the 

 area in the shade, which is usually greater in extent than the part 

 in the sunlight. Further, it is probable that the relatively greater 

 number of sweat glands in the dark skin is also protective. The air 

 in the human hair, especially in black hair, under the influence of 

 the tropical sun acquires exceedingly high temperatures. There is 

 not the slightest doubt that the pigmentation of native races is 

 protective, and that the older theories of Waltz that carbon was 

 deposited in the tissues owing to imperfect oxidation due to heat, 

 and that of Darwin that it was due to a survival of those best fitted 

 to withstand tropical disease — 'for he believed that pigmentation 

 prevented the native from being attacked by the fatal miasmata of 

 the country — 'Cannot now be seriously considered. 



As to the origin of the pigment, this question must be considered 

 as far from settled. There are two possible sources for the melanin, 

 viz. : — 



(a) The haemoglobin of the blood. 



(b) The cells of the epidermis. 



(a) The Hcemoglohin. — -This theory suggests that haemoglobin is 

 altered into melanin either in the blood-stream, in the connective- 

 tissue cells, or in special melanoblasts, which by amoeboid movement 

 take the pigment to the epidermis. 



(&) The Cells of the Epidermis.— -This theory states that melanin 

 is manufactured in situ by the epithelial cells, and is not derived 

 from haemoglobin, and in view of the histology given above this 

 appears to be probably the correct solution of the problem. 



Melanosis, which varies from mere freckles through the diffuse 

 yellow and brown pigmentation to the jet-black of the African 

 negro's skin, is caused by melanin granules lying in and between 

 the cells of the epidermis. 



With regard to the pigmentation of different races, it must be 

 remembered that it is only absent in albinos, and that it occurs in 

 the epidermis of the areolae and mammillae of the breast, the scrotum, 

 labia majora, and around the anus in white races, being contained 

 chiefly in the large basal cells of the Malpighian layer, and to much 

 less extent in the more superficial layers, and the connective-tissue 



