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The section of retina shows a layered arrangement of the various retinal cells. Starting from the inside (where the light strikes first) is the nerve fibre layer, ganglionic layer, amacrine cell layer, bipolar cell layer, horizontal cell layer and finally rods and cones (and that too in inverted position, i.e. the actual receptor portions are farther inwards).

Why is this stratification inverted? If the receptors would have been the innermost cells, they could have received the maximum quantity of unaltered light. Does this "inverted" arrangement, i.e. arrangement with the receptor cell behind a screen of several other layers not hinder the process of photoreception by blocking a lot of incident light and hence the formed image? What is, if any, advantage of this reverse arrangement?

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I don't have an other explanation than pointing to evolution. It was obviously not a problem to evolve the way it did. One article I found (see "The Inverted Retina: Maladaptation or Pre-adaptation?") called it a sign of pre-adaptation, where forms exist before they get their final function. Another pretty comprehensive review can be found here: "Evolution of Phototransduction, Vertebrate Photoreceptors and Retina."

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  • $\begingroup$ Thank you. And great references, as usual. $\endgroup$ Dec 25, 2013 at 10:04
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There is most likely no advantage to this arrangement and it probably is the result of the evolutionary history of the eye. Evolution always work by means of small adjustments from something that works into something that works better. During the course of evolution of the vertebrate eye the orientation perhaps by chance, perhaps for some reason unrelated to vision itself, became fixed in the inverted way it is today and as it appears to work very well, even though it appears as an inefficient design, it is still there. The point is substantiated by looking at the cephalopod eye. Squids, octopuses and cuttlefish have eyes that are anatomically remarkably similar to vertebrates, yet their retina is oriented the 'right' way, with sensory cells pointing towards the light and not away from it. Both designs seems to work.

The inverse structure of the eye is a good example to illustrate that evolution does not 'seek' to develop optimal solutions. Natural selection will improve whatever already works, which sometimes leads to solutions that from a purely engineering point of view are sub-optimal. Other very good examples are our extremely fragile knee joints and the extremely dangerous arrangement where all food and drink has to pass over the wind pipe to get to the esophagus, with danger of suffocation every time we put something into our mouth.

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