Retinal structure, artwork. Light falling on the retina passes from bottom to top. At top is the choroid layer (red), which lines the inside of the eye. At the back of the eye the choroid is covered by the retina. The inner layer of the retina, the retinal pigment epithelium (purple) prevents light passing out of the eye. Light sensitive photoreceptor cells form the next layer and are of 2 types; rods (black) aid vision in dim light and cones (green) allow colour vision. Light triggers electrical impulses in the interneurones (nerve cells, black purple and dark red). The impulses are passed onto the ganglion nerve cells (red, bottom) and then the brain. At left is a Muller cell (blue), a support cell for the neurones.

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