The eyes have it!
Structure of the Iris
The iris is made up of four layers:
- The anterior border layer (the front layer facing out)
- The stroma
- Two layers of endothelium (at the back of the iris)
The double layer is responsible for dilating the pupil and absorbing any stray light that reaches the back of the iris. It is only the first two layers that determine iris colour.
The anterior border layer contains melanocytes. Everyone’s body contains about the same number of melanocytes, but the amount of melanin in these cells is genetically determined. Melanin absorbs light and is the principle pigment in hair and skin. Differing levels of melanin account for the differences in skin and hair colour between races and individuals. People with dark skin and hair have a generally higher level of melanin than pale, blond people. As a result, people with darker skin and/or hair are more likely to have brown eyes. In the eye, low levels of melanin absorb less light and have a yellow appearance, while high levels look brown.
The stroma is a connective tissue layer which contains collagen, blood vessels and the iris sphincter. The iris sphincter is the muscle which constricts the pupil. White light entering the stroma is scattered by the collagen. The collagen absorbs most of the colours apart from blue or grey, these are reflected back by the collagen. The blood vessels and sphincter scatter the light in different ways giving different patterns of flecks. The ring that can sometimes be seen in the iris is the minor iridic circle, which is the artery ring supplying the iris with blood. Freckles and darker patches on the iris are caused by round groups of pigment and are called clump cells.
Whether the eye is blue or grey depends on the arrangement of the collagen fibres: fine arrangement causes blue eyes while a coarser arrangement causes grey ones.
Different Eye Colours
In a brown eye there is a lot of melanin in the anterior border layer. This absorbs the light and gives a brown velvety appearance.
In a blue eye there is not much melanin in the anterior border layer. The light passes into the stroma where the collagen fibres scatter the light back as blue.
In a green eye (or a hazel one) there is a variable level of melanin, so that some of the light is absorbed by the melanin and some is scattered by the collagen. The brown layer looks yellow as it is thinner, and so the yellow and blue mix to make green.
Red irides1 are a result of albinism. Albinism is where there is no melanin in the melanocytes at all. Therefore all of the blood vessels (in the iris and retina) are seen and a redder appearance is given. In practice only very few albinos have red eyes, the blue reflections of the collagen show up stronger and so most have blue/grey or even brown. The mixing of red and blue reflections can also give rise to violet eyes.
Why the Pupil Usually Looks Black
The retina of a human eye looks red because it has lots of blood vessels supplying the cells with metabolites. One reason you don’t see the red colour is because the retina absorbs nearly all of the light which enters the pupil. In normal circumstances very little light is reflected and so the pupil looks dark. When a very strong light is shone on the pupil, some of the light is reflected back and the pupil looks red (so you sometimes get red-eye in photographs).