by Jerry Bergman* and Joseph Calkins

Abstract

It is often claimed that the human retina is poorly designed because light must travel through the nerves and blood vessels to reach the photoreceptor cells, which are located behind the eye’s wiring. Many specific reasons exist for this so-called backward placement of the photoreceptors.  A major one is that it allows close association  between the rods and cones and the pigment epithelium required to maintain the photoreceptors. It is also essential in both the development and the normal function of the retina. Both the rods and cones must physically interact with retinal pigment epithelial cells, which provide nutrients to the retina, recycle photopigments, and provide an opaque layer to absorb excess light.

Introduction

One of the most common examples of putative poor design in both the popular and scientific literature is the mammalian retina. The retina is the thin, light-sensitive organ located at the back of the eyeball. The claim is made that the vertebrate eye is functionally suboptimal because the retina photoreceptors are oriented away from incoming light (Ayoub, 1996, p. 19). Oxford professor Richard Dawkins considers this an example of poor design because he concludes that an 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!) (Dawkins, 1986, p. 93)….

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