The left illustration shows a human eye, which is round and filled with vitreous humour. The optic nerve and retinal blood vessels exit the back of the eye. At the front of the eye is the lens with a pupil in the middle. The lens is covered by the iris, which in turn is covered by the cornea. The aqueous humour is a gel-like substance between the cornea and iris. The retina is the lining of the inner eye. A second illustration is a blowup which shows that the optic nerve is at the surface of the retina. Beneath the optic nerve is a layer of ganglion cells, and beneath this is a layer of bipolar cells. Both ganglia and bipolar cells are nerve cells with root-like appendages. Beneath the bipolar cell layer are the rods and cones. Rods and cones are similar in structure and column-like.
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