Sight

Sight is dependent not only on a substantial portion of the cerebral cortex, but also upon six cranial nerves (II-VII). Perception is the function of the retina, optic nerve, tract, radiation and cortex. The oculomotor, trochlear and abducens nerves move the eye. Eyeball sensations such as pain, touch and pressure are mediated by the ophthalmic nerve, and the facial nerve innervates orbicularis oculi muscle. This Chapter deals with the optic pathway: eye movements and their control come later.

The optic nerve

The optic pathway transmits visual impulses from the retina to the brain. The optic nerve is the name given to the pathway between the eyeball and the optic chiasma. It is an artificial subdivision of the optic pathway. Like the olfactory nerve (Chapter 18), the optic nerve is not really a nerve. It is an outgrowth of the diencephalon (the thalamic structures). As in the olfactory system, the primary sensory neurons are bipolar and are confined to the sensitive epithelium (retina), the axons of secondary sensory neurons forming the optic nerve, chiasma and tract.

Visual pathways (Fig. 20.1)

Described from the eyeball back to the forebrain attachment.

Retina

Two layers: neural (next to vitreous) and pigment (next to choroid). Rods and cones in deepest parts of neural layer, with terminal processes of rods and cones in contact with pigment layer. Rods and cones synapse with bipolar cells (primary sensory neurons). Bipolar cells, confined to retina, synapse on ganglion cells. Axons of ganglion cells form optic nerve.