Is the Human Eye Badly Designed?

 

Some believe that the human eye is badly designed because the sensors are wired from the front rather than from the back (as in, say, an octopus). I would like to try and demonstrate that the eye is adequately designed for its purpose.

 

Many designs have conflicting requirements.

Consider the following case: The fastening mechanism on the Tower of London gate merely has to keep it secure so that the designer could make it as substantial as he could with the available materials. He was therefore able to construct a fastener which might last 1000 years or more and survive the efforts of bad guys with a battering ram trying to defeat it. Conversely the designers of the catch on the cargo door of a 747 airliner had to ensure that there was an equal chance of it not opening but they were far more constrained by weight and size. The Tower of London guy would have laughed at this mechanism if he had seen it in Henry VIII's day but the design is (or should be) adequate for the purpose.

 

The human eye is in the category of the airliner. It has many requirements, resulting in design features whose purpose may not be immediately obvious. For instance it has to perform in lighting conditions from almost compete darkness to the sunlight experienced trekking across the Sahara desert. As well as focussing light, lenses focus heat and without adequate cooling the sensitive retina would be destroyed by the build up of this heat. A camera sensor can dissipate this heat on a copper heat sink of some type. The human eye has a liquid cooling arrangement whereby excessive heat is removed by substantial blood vessels just behind the retina. To get round this problem the nerves taking the visual signals to the rest of the brain are wired from the front. The much simpler octopus eye is wired from the back but it does not require a cooling system as it spends all its time in the murky ocean depths. The human eye also needs to rotate freely. It is mechanically much more straightforward to have the nerves enter the eyeball from a fixed point rather than have a complex web of nerves having to be dragged around as the eyeball moves. The simpler octopus eye is fixed so does not need this.

 

The cost of this arrangement is of course the blind spot. In practice though the brain suppresses our consciousness of this and we would not know about it without taking a specially designed test. When you consider the way the eye functions, the blind spot is irrelevant anyway. The eye does not function like a TV camera with an eye within the brain watching it. Rather the eye is an extension of the brain (Gregory: Eye and Brain). The image we actually see is something like the MP4 picture we might see on our personal video player: the brain builds up an image of what it sees over a short period of time and then just registers the changes.

 

The eye has many amazing mechanisms. The sensors have an ongoing replacement plan so in a normal human lifetime a reasonable performance is maintained. There is a mechanism that suppresses visual noise so that you don't see sparklies on a dark night as you would with a digital camera. The eye has a mechanism for detecting hazards approaching from the side. Sometimes it does go wrong as any complex mechanism does (including the lock on the 747 cargo door) but who can deny that the design is brilliant unless you have a particular agenda to justify of course.

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