Some weeks ago I was at my embryology course and we were talking about the eye. Such a complex, performant and awesome organ! It always fascinated me, and to know all of the evolution behind it, it’s really interesting. In our last ten minutes the professor talked about our retina and how it’s organized.
This picture represent everything I need to show you. Focus on the left side. Here we can see our retina: as you can see on the top there is the liquid inside our eyeball, just under it there is an intricate net of neurons with photoreceptors that catch light and create a signal, transported up to the brain which decodify it. Under the neurons there is some blood vessels to carry nutrients to the neurons (which are the most energy consuming part of the whole system) and just under them there is the Sclera, the eye wall.
Ok, everything seem so make sense, but wait a second! Where is the light passing through?? I mean, obviously on the left and on the right of the picture there are other photoreceptors and other vessels. On the bottom of the picture there is a wall, so the light must pass form the top of the picture! Of course, there is present the vitreous liquid, so the light pass through the Iris and pupil, pass through the vitreous liquid and hit the receptors. But isn’t there something between the liquid and the receptors??
That day, I asked my prof: “What is the sense in putting the cables ahead the receptors? It would make more sense if the cables were BEHIND the receptors like an old TV monitor.”
Infact, it turns out that pretty much everything in the eye is shaped to minimize the diffracting of the light through various comparts, so why should you put the neurons net ahead the receptors, this can’t reduce the defraction of the light!
This was kind a Muting Question, because my professor never thought about that! He started to think about it and wasted the remaining ten minutes of the lesson. He promised me to find an answer for the next day. And he actually did it!
Our retina, it turned out, is called Inverted Retina, for a reason! It seems that it’s not really efficient, not as it could be with a Direct Retina. Infact many Cephalopods like Octopuses, which have a normal retina, are famous to have a brilliant eyesight (which is correlated to their high intelligence), of course it is not like ours but this is for other reasons.
Instead, it seems to have other advantages. On the other hand, if the Inverted Retina didn’t bring any advantage to compensate the diffraction, it would not be an option in any eye. Scientists have proposed that this retina has development advantages. Our species like many others, has a particularly big head during the development of the zigote. But much of this space is occupied by the brain, so eyes have to be as much small but functioning as possible. This arrangement seems to be an optimal compromise between size and efficiency. Our sight is pretty good and eyes aren’t too big. Also, the position of the blood vessels are optimal for this kind of arrengement: if we had the receptors in front of the system, we must have the vessels running through the neurons net, which is pretty complicated. A reminder: the fact that we have only a perfect focusing point in our eye, while in other spots the vision is blurred and even lacking (like in the blank spot, which all have), seems to be related to the diffraction caused by the net neurons above the receptors.
Our eyes are pretty cool huh? And still aren’t perfect. Let me tell you a story..