As it turns out, the supposed problems Dawkins finds with the inverted retina become actual advantages in light of recent research published by Kristian Franze et. al., in the May 2007 issue of PNAS . As it turns out, "Muller cells are living optical fibers in the vertebrate retina." Consider the observations and conclusions of the authors in the following abstract of their paper:
Although biological cells are mostly transparent, they are phase objects that differ in shape and refractive index. Any image that is projected through layers of randomly oriented cells will normally be distorted by refraction, reflection, and scattering. Counterintuitively, the retina of the vertebrate eye is inverted with respect to its optical function and light must pass through several tissue layers before reaching the light-detecting photoreceptor cells. Here we report on the specific optical properties of glial cells present in the retina, which might contribute to optimize this apparently unfavorable situation. We investigated intact retinal tissue and individual Muller cells, which are radial glial cells spanning the entire retinal thickness. Muller cells have an extended funnel shape, a higher refractive index than their surrounding tissue, and are oriented along the direction of light propagation. Transmission and reflection confocal microscopy of retinal tissue in vitro and in vivo showed that these cells provide a low-scattering passage for light from the retinal surface to the photoreceptor cells. Using a modified dual-beam laser trap we could also demonstrate that individual Muller cells act as optical fibers. Furthermore, their parallel array in the retina is reminiscent of fiberoptic plates used for low-distortion image transfer. Thus, Muller cells seem to mediate the image transfer through the vertebrate retina with minimal distortion and low loss. This finding elucidates a fundamental feature of the inverted retina as an optical system and ascribes a new function to glial cells
"Any engineer would naturally assume that the photocells would point towards the light, with their wires leading backwards towards the brain. He would laugh at any suggestion that the photocells might point away, from the light, with their wires departing on the side nearest the light. Yet this is exactly what happens in all vertebrate retinas. Each photocell is, in effect, wired in backwards, with its wire sticking out on the side nearest the light. The wire has to travel over the surface of the retina to a point where it dives through a hole in the retina (the so-called 'blind spot') to join the optic nerve. This means that the light, instead of being granted an unrestricted passage to the photocells, has to pass through a forest of connecting wires, presumably suffering at least some attenuation and distortion (actually, probably not much but, still, it is the principle of the thing that would offend any tidy-minded engineer). I don't know the exact explanation for this strange state of affairs. The relevant period of evolution is so long ago."
inverted retinas seem to have some at least marginal if not significant advantages based on the needs of their owners. We also have the evidence that the best eyes in the world for image detection and interpretation are all inverted as far as their retinal organization. As far as the disadvantages are concerned, they are generally not of practical significance in comparison to overall relative function. Even Dawkins seems to admit that his uneasiness is mostly one of aesthetics. Consider the following admission from Dawkins:
With one exception, all the eyes I have so far illustrated have had their photocells in front of the nerves connecting them to the brain. This is the obvious way to do it, but it is not universal. The flatworm keeps its photocells apparently on the wrong side of their connecting nerves. So does our own vertebrate eye. The photocells point backwards, away from the light. This is not as silly as it sounds. Since they are very tiny and transparent, it doesn't much matter which way they point: most photons will go straight through and then run the gauntlet of pigment-laden baffles waiting to catch them.
Is Our ‘Inverted’ Retina Really ‘Bad Design’?
Light at various wavelengths is capable of very damaging effects on biological machinery. The retina, besides being an extremely sophisticated transducer and image processor, is clearly designed to withstand the toxic and heating effects of light. The eye is well equipped to protect the retina against radiation we normally encounter in everyday life. Besides the almost complete exclusion of ultraviolet radiation by the cornea and the lens together, the retina itself is endowed with a number of additional mechanisms to protect against such damage:
The retinal pigment epithelium produces substances which combat the damaging chemical by-products of light radiation.
The retinal pigment epithelium plays an essential part sustaining the photoreceptors. This includes recycling and metabolising their products, thereby renewing them in the face of continual wear from light bombardment.
The central retina is permeated with xanthophyll pigment which filters and absorbs short-wavelength visible light.
The photoreceptors thus need to be in intimate contact with the retinal pigment epithelium, which is opaque. The retinal pigment epithelium, in turn, needs to be in intimate contact with the choroid (also opaque) both to satisfy its nutritional requirements and to prevent (by means of the heat sink effect of its massive blood flow) overheating of the retina from focused light.
If the human retina were ‘wired’ the other way around (the verted configuration), as evolutionists such as Dawkins propose,2 these two opaque layers would have to be interposed in the path of light to the photoreceptors which would leave them in darkness!
Thus I suggest that the need for protection against light-induced damage, which a verted retina in our natural environment could not provide to the same degree, is a major, if not the major reason for the existence of the inverted configuration of the retina.
Last edited by Admin on Thu Feb 28, 2019 8:23 am; edited 7 times in total