The color of the iris is known to be dependent on two pigments, one within the two epithelial layers of the posterior part of the iris (pars iridica retinae) and the other within the cells of the stroma. When only the first pigment is present, the iris appears to be blue. In gray, brown and black eyes the second pigment also is present. As to black eyes, it should be kept in mind that at the age of puberty the iris becomes slightly lighter, in contrast to the darkening of the skin and of the hair. Dark brown eyes are rare in adults of the white race. The origin of the pigment of the iris is not clearly understood. It is likely that in man, as in Amphibia, certain substances are produced in the body at a certain time of embryonic life and are carried by the blood stream into
HESS L. HETEROCHROMIA OF IRIS. Arch Ophthalmol. 1943;30(1):93–104. doi:10.1001/archopht.1943.00880190111011
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