The Biology of Vision
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The Biology of Vision
The human eye is an organ that has been cited by proponents of intelligent design as one too perfect and complex to have developed naturally on its own. However, the eye and the biological process of vision has counterintuitive steps and so many opportunities for something to go wrong that, given the hardware we have, its amazing how well our brains allow us to see at all.
When a person sees something – the face of a friend, for example – the first step is for light to pass through the transparent protective cornea and enter the pupil. Depending on the amount of light coming into the pupil, the colorful muscle surrounding the pupil called the iris expands or contracts the pupil. If the person sees her friend in the hallway at school, her pupils will be less dilated than if she sees him at a dance with very low lighting.
A curved lens behind the pupil focuses the light onto the retina, the eyes light-sensitive back surface. The image focused onto the retina is imperfect: it is upside-down and blood vessels block some of the light from hitting the retina. The friends face would have streaks of missing light. If there is a problem with the lens, the image is even worse. A nearsighted persons lens focuses distant images in front of the retina, so when the light reaches the retina it is not in sharp focus anymore and the image is blurry. In farsighted people, the lens focuses nearby images behind the retina.
The light hits receptor cells within the retina called rods and cones, which generate neural signals. The rods are sensitive to light intensity, allowing the person to distinguish the friends outline against the slightly-darker backdrop of the hallway. The cones allow us to see color and fine detail: they handle the information that the friend has brown hair, freckles, and chapped red lips. The neural signals pass through a layer of bipolar cells, connecting the rods and cones to the ganglion cells whose axons converge to form the optic nerve. There is a small tunnel structure at the back of the eye through which the optic nerve goes through the retina to the brain. Since its a hole in the retina, it creates a blind spot.
Neural messages from each eye come together at a point in the brain called the optic chasm before passing through the thalamus to the visual cortex, in the back of the brain. Special cells in the visual