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The colour of the iris and the iris-layers

        THE COLOUR OF THE IRIS AND THE IRIS-LAYERS
The colour of the iris determines the appearance of the eyes. We distinguish in general three natural basic colours: blue, grey, and brown.
Each of these colours has a physiological basis, and is conditional upon the degree of pigmentation of the iris.
The iris appears blue when its surface layers are colourless, and the deepest dark layer of the iris (retinal epithelial pigment) shows through. If the middle vascular layer of the iris—the
stroma—is coarse and compact, then the iris appears grey. However, the more dark coloured material is deposited in this stroma, the more the iris is darkened in its colouring, and the appearance tends towards brown. There are occasionally seen in a less pigmented iris, local accumulations of brown-to-black coloured substance which strikingly appear as dark-reddish flecks in the otherwise grey or blue iris. These are referred to scientifically as naevi irides
(iris-birthmark). We call them 'toxin-flecks'.
In the case of albinos, the iris layers are completely transparent. There is a lack of all pigment. These eyes appear reddish, because of the visibility of the blood vessels in the deep layer of the iris—the retina.
In the new-born, the iris is at first dark-violet to blue-grey. Only in the course of development does there appear a lightening or darkening through alteration in the pigment content. With advancing age the stroma becomes more compact and coarser and thereby acquires a grey appearance.
The change of blue to brown iris is sometimes limited to an individual iris or even to a part, so that in the same person, one iris can be blue with the other brown, and also a smaller or larger brown sector may be seen in the blue iris. This is referred to as Heterochromia. Discolourations of the iris following organic diseases are of especial
significance in Iriscopy.
The structure of the iris is best viewed when the pupil is contracted, using a strong beam of light, either with natural vision, or better still with a loupe of 3 or 4 magnifications.
The iris is rich in changes, and is especially characterised by elevations and depressions of the anterior surface. This is referred to as the iris-relief.

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