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Why is the retina considered an extension of the brain?
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Photoreception
The mammalian eye has a cornea and a lens and functions as a dioptric system, in which light rays are refracted to focus on the retina.
The retina is considered an extension of the brain because it is formed embryonically from neural tissue. During development, the eyes originate as a pair of shallow grooves on the sides of the forebrain that extend outward as optic vesicles.123
These vesicles remain connected to the brain by an optic stalk, which eventually serves as the pathway for the optic nerve. The optic vesicles then invaginate to form double-walled optic cups, with the thick inner layer developing into the sensory, neural retina. Because the retina is essentially an outgrowth of the forebrain, the optic nerve is considered a central tract connecting two regions of the nervous systemβthe retinal neurons and the visual relay stations within the brain.234