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Blind spot
The blind spot is the region on the human retina where the optic nerves connect to the back of the eye. Since there are no light receptors there, a part of the field of vision is not perceived. The brain fills in with surrounding detail, so the blind spot is not normally perceived.
You can demonstrate its effect by drawing a small X on a piece of paper, then drawing a small circle about 12-15 centimetres to the right of the X.
With your left eye covered, concentrate on the X. Move the paper closer and further until you find the point at which the circle disappears.
The circle is then positioned directly in line with the blind spot of your left eye.
The blind spot is also called a scotoma, the general term for any obscuration of the visual field.
The mammalian retina is inverted: the rods and cones are on the side away from the lens. The octopus eye is otherwise similar to the mammalian eye, but its retina is rightside-out, thus the octopus has no blind spot. The reason for this apparently inferior design (the light in our eyes has to pass through the nerve before reaching the rods and cones) is that we live where there are bright lights. The heat of a bright light focused on the inverted retina can be carried off by the blood in the adjacent choroid[?].
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