<html><head><meta http-equiv="Content-Type" content="text/html charset=windows-1252"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;">Hi Barak,<div><br></div><div>Yes, was trying to be funny I suppose. Didn’t mean to imply at all that the mammalian arrangement didn’t make sense or vice versa. Aquatic animals also live in a space with lower photon flux.</div><div><br></div><div>The fact that the neuronal structure of the retina itself is remarkably similar, but the entire structure is flipped, I think makes the point even more strongly that evolution found the same neuronal solution in two sets of animals independently.</div><div><br></div><div>But thanks - isn’t biology remarkable.</div><div><br></div><div>Jim</div><div><br></div><div><br></div><div><br></div><div><div><div><div>On Jan 30, 2014, at 3:52 AM, Barak A. Pearlmutter <<a href="mailto:barak@cs.nuim.ie">barak@cs.nuim.ie</a>> wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite">Jim,<br><br>As a pure aside...<br><br><blockquote type="cite">... accept (sic) that the Cepholapod retina is pointed in the “right”<br>direction - i.e. towards the light source ...<br></blockquote><br>I think this is a bit of a "myth of science". It is true that the<br>mammalian retina has the circuitry and blood vessels between the lens<br>and the photoreceptors, while the cepholapod retina has them the other<br>way around. But the conclusion that this is a design flaw in the<br>mammalian eye is not quite so clear. There are four advantages that I<br>know of to putting the photoreceptors behind the circuitry.<br><br>(1) If the back of the eyeball is reflective (as in a cat) light gets<br>two chances to be caught by a photoreceptor. Resolution when doing this<br>is degraded if the photoreceptors are moved away from the reflective<br>surface.<br><br>(2) It is easier to keep the sheet of photoreceptors smooth.<br><br>(3) If blood vessels are between the eyeball and the photoreceptors, the<br>heartbeat can make the photoreceptors wiggle.<br><br>(4) The photoreceptors are very metabolically demanding, and can be<br>easier to maintain (nourish, clear toxic wastes) from the eyeball.<br><br><span class="Apple-tab-span" style="white-space:pre"> </span><span class="Apple-tab-span" style="white-space:pre"> </span><span class="Apple-tab-span" style="white-space:pre"> </span><span class="Apple-tab-span" style="white-space:pre"> </span><span class="Apple-tab-span" style="white-space:pre"> </span>--Barak.<br>--<br>Barak A. Pearlmutter<br> Hamilton Institute & Dept Comp Sci, NUI Maynooth, Co. Kildare, Ireland<br> <a href="http://www.bcl.hamilton.ie/~barak/">http://www.bcl.hamilton.ie/~barak/</a><br></blockquote></div><br><div apple-content-edited="true">
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