PERSONAL APPEARANCES IN HEALTH AND DISEASE. 79 



of brown verging on black, and irides of a nut-brown 

 colour. 



The difference in coloration of the skin between inha- 

 bitants of hot climates and those of temperate races, is 

 probably due to an exaggeration of the conditions which 

 are undergone by white-skinned men after prolonged 

 exposure to heat. The cause of the colour of the negro, 

 rests in the great accumulation of pigment in the deep 

 layers of the epidermis ; that pigment is derived from the 

 colouring matter of the blood, and the reason of its 

 excessive production perhaps depends upon some deeper 

 seated changes in blood formation about which we can only 

 form conjectures. It is suggested that the liver, which is 

 the organ most readily influenced by changes in tempera- 

 ture, and which secretes a product largely pigmented, viz., 

 the bile, may have its function so modified that an excess 

 of pigment is, as it were, left in the blood. At any rate 

 there is a reason for the coloration of dark skins, since 

 they serve as a protection from the effects of heat to the 

 delicate structures beneath. 



The " tanning " of the skin of exposed parts, which 

 occurs after a few days' exposure to the air and solar 

 rays, is due doubtless in part to increased vascularity and 

 escape of pigment from the blood, just as any irritant 

 applied to the skin will produce pigmentation from the 

 congestion it causes. Other and deeper influences may 

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