This chapter deals with the occurrence of melanin in sites other than skin and hair, and it will explore the putative role of the pigment in these situations. With the exception of melanin in the eye all the other melaninbearing tissues are internal and are totally shielded from light. Hence arises the problem of what likely use melanin would have in locations where it is deprived of its primary functions of photoprotection, thermoregulation and ecological adaptation (e.g. camouflage). Indeed, unlike beauty, melanin is not skin deep!

Eye

Iris

This is the visible pigmented region of the eye, and in describing an individual's eye colour one is referring to the pigmentation of the iris. The iris consists of several layers but, from the standpoint of colour, the two most important portions are the anterior layer together with its underlying stroma (both containing melanocytes) and the posterior pigmented epithelium.

Eye colour depends partly on the amount of melanin pigment in the anterior layer and stroma and partly on optical phenomena. In brown and dark brown irises there is an abundance of melanocytes and melanosomes in the anterior layer and stroma. Blue eyes are not the effect of a blue pigment, but represent Tyndall scattering (see p. 72). In blue-eyed individuals the anterior layer and stroma contain very little (if any) melanin. As light traverses these relatively melanin-free layers, the minute protein particles of the iris scatter the short blue wavelengths to the surface. The blue colour is heightened because the longer wavelengths (yellow and red) are absorbed by the dark background of the posterior pigmented epithelium, which also obscures the reddish hue of the adjacent blood vessels.