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Why is the retina Back-to-Front?


One of the extraordinary facts about the human eye is that the retina is "The wrong way round", it’s like a camera with the film put in back to front. Why is this?

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The 'Design Flaw'

In the human eye the retina is made up of several layers of different kinds of cells. The sensitive rods and cones are at the back of the retina, facing away from the light. To reach them the light first has to travel through blood vessels, nerve fibres and then several layers of retinal nerve cells. The octopus eye, it seems, evolved independently to ours and whilst it is similar in many ways the octopus got the retina in ‘the right way’. Its rods and cones face forwards, and blood supply and nerves come from behind.

How could evolution have made such a blunder with us and with other vertebrates?

Is it a 'Design Flaw'?

My belief is that it is not a blunder at all. The design of our eyes is just that bit more crafty.

Richard Gregory in his book "Eye and Brain" [World University Library, 1976] gives one reason why the design flaw is less serious than it seems. He notes that the blood vessels and nerve fibres visible with an opthalmoscope skirt around the area centralis where visual acuity is crucial, so they interfere very little with precise vision.

Here is a diagram of a cross section through the retina showing the path the light has to travel:
Retina Cross-Section An important feature can be seen on this diagram. The diagram shows a layer behind the rods and cones, the choroid layer, a second system that provides nutrition and oxygen to the cells. According to Bill Garoutte "Surveys of Functional Neuroanatomy" [Jones Medical Publications, 1987]:
"Neither system (anterior or posterior supply) actually penetrates the retinal layer, so that nutritive substances and oxygen must get to the retinal cells by diffusion. Both systems appear to be necessary for normal retinal function."
It seems that we have a greater need for high acuity vision than the octopus does and accordingly our eyes have a richer supply of nutrients than the octopus does. The retinas of our eyes get nutrition both from the blood vessels in front and from the choroid layer directly behind the rods and cones. The blood vessels in front supplement the supply from behind.

Garoutte’s description also states that the substances in the blood reach the retina by diffusion, rather than directly.

That description doesn't make it clear whether or not diffusion take place via the intermediary of the vitreous humour, the clear liquid which fills the eyeball, but my assumption is that it does. If this is the case it makes it clearer why the blood vessels do not need to be in close contact with retinal cells in the foveal region - in spite of that being where there is greatest expenditure of metabolic energy.

The possibility that the vitreous humour plays an important role in transporting nutrients is supported by existence of a structure in the eye in certain birds. Birds, like us, have a back to front retina. The special structure in avian eyes is called ‘a pecten’ or ‘comb’ and it has an excellent blood supply.

The pecten is a complex folded structure projecting into the centre of the eyeball. It is presumed to have a nutritive function. (Milton Hildebrand "Analysis of Vertebrate Structure" [John Wiley & Sons, Inc. 1988]). As the pecten is in contact with the vitreous humour it may be passing nutrients to it.
So, the blood vessels on the front surface of the retina avoid areas where acute vision is essential and may instead supply nutrients via diffusion into the vitreous humour. The blood vessels could also serve a second purpose, as blood vessels have important roles in repair. It is possible that they can grow out across the retina in response to damage, bringing white blood cells to the place of damage and taking away debris, and then retract later when damage has been repaired.
Gregory sees the main 'design flaw' of the vertebrate eye not as the having of blood vessels in front so much as the extra layers of cells that the light must pass through. If you look at the diagram again, you will see that there are three layers of neural derived cells light must pass through before reaching the rods and cones.
It is not unreasonable to speculate that the first layer of these cells (the ganglion summation cells) act in some respects like the objective lens in a telescope. From their position and known function their feedback controlled growth processes will be affected by the rods and cones whose signals they receive. This is likely to be particularly relevant in the macular region where each rod or cone projects to just one ganglion cell. Success for the ganglia means strong signals, which in turn means focusing light on to the cells they receive signals from.
The optical system of the eye cornea and lens is designed to bring point objects to a focus at a point. However the rods are not points, they are much longer than their diameter. The rod will gain in sensitivity if the light reaching it is collimated, that is travelling as a parallel beam rather than converging to a point and then diverging. The full length of the sensitive part of the rod will then be being used. If this is indeed what happens in human eyes, then the sensitivity lost through having additional layers of cells to pass through is more than made up for by the correct focusing of light onto the rods and cones by these additional cells themselves.
James Crook, April 1998.


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Postscript:

Since writing the article above, I've noticed that there is a structural analogy between the view of the eye I am putting forward and multifacetted eyes of insects. If neurally derived cells in vertebrate eyes do in fact collimate light onto the rods and cones then they are behaving in a manner analogous to the lenses of individual facets in an insect's compound eye. This could be relevant to evolution of eyes - it suggests a common origin is possible


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"Inverted Retinas - Did the Octopus do Better?" page last updated 5-July-2003