BLUE SKIN

SUMMARY.— Blue colours in the skin arise from a particular optical relationship between collagen and melanin which leads to a “subtractive colour mixing” in the light reflected from the dermis. Unimportant in this type of colour production are scattering of light, the red‐brown transmission colours of melanin, and the optics of the epidermis in general. In primates the blood pigments have an adjuvant effect in varying the blue colour. It is suggested that cyanosis may not always imply the presence of reduced or abnormal haemoglobin. The evolutionary significance of the dermal melanocyte system is briefly mentioned in relation to colour vision, and sundry metabolic, protective and socio‐sexual functions.