Photoreceptor cells (rods and cones) are communicated with the nerve cells called bipolar cells. These bipolar cells synapse with the ganglion cells, which transmit the impulses to the brain via axons in the optic nerve.
Optic nerve is formed by the axons of retinal ganglionic cells, which leaves the eye through optic disk. Optic nerves transmit the sensations to the brain. The medial fibres (of optic nerves) of both sides cross at optic chiasma, which is present near the center of the base of cerebral cortex.
Thus, the blank can be filled with retinal ganglionic cells.