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The blind spot is the region of the retina where the optic nerve and blood vessels pass through to 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 and with information from the other eye, 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:
            X                                                                                                                                                                           O
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 (this should happen about 30 cm away from the paper).
Because the image is inverted by the lens of the eye, this shows that the blind spot is located to the left of (i.e. closer to the nose than) the macula (the most sensitive area, focusing on the X) of your right eye.
The blind spot is also called a scotoma, the general term for any obscuration of the visual field.
All vertebrates have a blind spot, but no cephalopods do. Cephalopod eyes, although superficially similar, have a different structure than vertebrate eyes. See retina for a discussion of this difference.
de:Blinder Fleck
nl:Blinde vlek
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