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Nutrition is expensive. There is a price to pay to nourish any cell, and this is especially true of the eye. Evolution has had to be creative to nourish portions of the eye so as not to interfere with vision. If the retina is relatively simple and is thinner than about 150 μm, diffusion from the choroid is sufficient. But, if the ecology demands better vision, the retina must thicken to permit more amacrine and horizontal cells to increase retinal processing. To assure inner retinal cells have adequate nutrition and proper cellular metabolism, evolution has found creative and seemingly contradictory mechanisms.
Primates have intraretinal vessels, and must actually look through these vessels and the oxygen carrying pigment of the red blood cells that degrades the image, however slightly. Some animals, though, demand the best possible vision and have shunned any obscurations from vascularisation—birds! Yet, even though birds do not have obvious inner retinal nourishment, they must somehow provide nutrients and oxygen to the high concentration of amacrine, horizontal, and ganglion cells present in the inner retina. How can this be done?
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