So now let's go on and talk about the retina proper. It's a complicated piece of neural machinery and it is neural machinery. This is the interface between the non-neural parts of the eye and the neural parts of the eye. And here is what you would see if you took out a little piece of retina and looked at it in a microscopic section. This is obviously not a microscopic picture, but just a diagram. But here are the kinds of features that you would see in the retina at large, which is here. And in the diagrammatic representation of the fovea, which is here. The fovea is about two millimeters in extent. And what you see here is that the eye is layered, and we'll look at these layers diagrammatically in a minute and describe them. But the layers consist of a choroid layer in the back of the eye which has the pigment epithelium. I'll describe the importance of that epithelium in a minute. And then it has the photoreceptive layer, the layer in which the rods and cones that we'll discuss are located. And then it has zones that have other kinds of nerve cells. And the output layer of ganglion cells, whose axons form the optic nerve that we looked at a minute ago when seeing the optic nerve head. So, what's interesting here and what's very important is that, as you reach the fovea, and again this is just a rough diagram. It doesn't look exactly like this, but pretty much. The layers, the overlying layers of the photoreceptors which are remember back here in this layer are reduced, they're taken away. And you might expect that, because the overlying layers of the retina diminish the light that's getting to the retina. And in the area of greatest sensitivity, the fovea, you want to take away the components that are going to scatter the light, absorb some of the light and diminish the sensitivity, the accuracy of what you would be able to see were those elements not there. That's why they are not present over in the foveal region. So, let's look at this in some detail. Here is again, a little piece of the retina. And we're going to take this piece out diagrammatically and blow it up, and look at the elements that are in the retina. And let's look at that in a little bit more detail than in this picture. So light is coming in here, and right away you see something that is odd and seems an unusual arrangement. And that is that the photoreceptors, the cells that actually transduce the light by capturing photons and turning them into the beginnings of the neural signals that end up going to the brain. Are at the back of the eye, not the front and the light therefore has to go through all of this junk. Well, it's not junk. It's obviously critical processing machinery, but it has to go through all that stuff that degrades the light by scattering it, by absorbing some of it before the light reaches the photoreceptors. That's why these elements are taken away in the phobias I told you a minute ago, but in the vast majority of the retina, the light is passing through these other elements. So why is that? Well, the reason is that the photo pigment layer, which is here, is an absolutely necessary component that has to be adjacent to the photo receptors. The reason is that the photo receptors have a high metabolic rate and the disks that contain the photo pigment molecules are actually being turned over at a high rate and the disks are being degraded. And that degraded detreiutus has to be dealt with and removed so that the rods, and there are two types of photoreceptors that we'll talk more about in a minute. That needs to be removed so that they can continue to function on an ongoing basis properly. That's the reason that the photoreceptors are in the back of the eye. And the next question we should ask is well what are all these other cells doing? First of all, there's a straight through pathway of light that's mediated by bipolar cells that is basically the pathway that's forming the high-resolution that one gets from vision. But there are also these other cells that are integrating information across the retinal service, and they are the Amacrine cells and the Horizontal cells that are processing information in this lateral direction. The information that is processed in this way by these retinal layers, and there are five of them in the retina, ends up in the Ganglion cell layer. And the Ganglion cells are the output layers of the retina that send the axons to the optic nerve and form the optic nerve head that we talked about a minute ago. So, again, the retina as a legitimate part of the brain, is doing a lot of processing. We normally think of that as happening in the cerebral cortex or higher up the visual system, but it's beginning in the retina. It's not completely understood. People have spent an enormous amount of time and energy trying to understand the nature of this processing. But suffice it to say that that work is still in progress and one knows a lot about the retina, but not fully what the retina processing accomplishes. In any event, what it clearly does accomplish is providing an output from photoreceptor cells to Ganglion cells that carries that information to the rest of the brain.
