Alberta Classic Eye Sign

Below are some definitions as used on this website, especially the dicussion forum at
http://www.albertaclassic.com/cgi-bin/cutecast/cutecast.pl

Please guide your mouse over the various terms.

It shares many similarities with our own eye, the major difference being the very large pecten, the functions of which include:

• To keep the retina nourished and healthy although it is not covered by blood vessels as is the case with the mammalian eye's retina.
• To keep the fluids in the vitreous body at the right pressure.
• To absorb light to reduce the chance of reflections inside the eye, which can distort vision.
• To help birds to perceive motion.

There are also some other functions possible but hard scientific evidence for these is lacking:

• To provide shade from the sun?
• To sense the earth's magnetic field?

Let us therefore look at an image of a typical pigeon eye:

1. A: represents that part of the iris covered by many blood vessels which hide the radial muscles underneath the surface. Contraction of these radial muscles causes the pupil to dilate.
2. B: represents that area of the iris which contains many circular muscles, an indication of their presence being the "distance lines" in this "circle of correlation". Contraction of these circular muscles causes the pupil to constrict.
3. G: is a fine membrane which we encountered already as the "circle of adaptation".

There are many lights/structures inside the pupil and it is important to ask ourselves what they could possibly be or we may run the risk of assuming structures which do not really exist in this eye. D: is a candidate for such wrong identification as many pigeon fanciers classified it as a "cluster". What bothered me about this identification was that I noticed the same similar looking "object" in a few pigeon eyes but:

• One would not expect to find abnormalities at such high a rate and all looking so much alike, even though it can be seen in only a few images and only in images where the light is directed toward the pupil, never where the center of the light strikes the retina.
• The frequency with which these "structures" are observed is proportional to the brightness of the light source and inversely proportional to the distance of the light source to the eye.

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