Gallus gallus retina
This image depicts the freshly dissected retina of a chicken (Gallus gallus) viewed en face from the photoreceptor side. Birds have an elaborate color visual system consisting of five cone photoreceptor types which are evenly distributed across the retina: four single cones maximally sensitive to ultraviolet, blue, green and red light that mediate tetrachromatic color vision; and an unusual cell type called a double cone which detects changes in luminance. Each of the five cone types contains a differently colored, carotenoid-containing oil droplet in its inner segment (shown in the image) which filters the incoming light before it reaches the light-sensitive outer segment, thereby improving color discrimination. Such brightly colored oil droplets are found in turtles and birds, animals that frequently use carotenoids to pigment their integument. Thus, carotenoids serve a complementary role in both seeing and in coloring what is seen.
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