b) II - Optic Nerve


The optic nerve is not a true peripheral nerve but rather an extension of the central nervous system into the back of the eyeball.
The nerve arises at the posteromedial part of the eyeball, and passes posteriorly and medially through the orbit to the apex. In the apex, it enters the optic canal, continuing its course to the optic chiasm. Nerve fibres from the nasal visual field of each eyeball cross over at the chiasm; those from the temporal field do not. Beyond the chiasm, the neurons pass through the optic tracts to the lateral geniculate nucleus of the thalamus.


Within the orbit

Within the orbit, the optic nerve is surrounded by a layer of dura (the optic sheath). The rectus muscles of the eyeball are named for their relation to the nerve. The ophthalmic artery and nasociliary nerve usually pass over the optic nerve to reach the medial wall of the orbit. The ciliary ganglion is lateral to the optic nerve.

Within the optic canal

The ophthalmic artery accompanies the optic nerve through the optic canal. The artery is inferior and lateral to the nerve within the canal.

Relations of the optic chiasm

The chiasm lies superior to the pituitary gland, separated by the cisterna interpeduncularis and a layer of dura mater (the diaphragma sella). The infundibulum and hypothalamus lies posterior the chiasm. The terminal gyrus of the frontal lobe and third ventricle lie immediately superiorly, as does the anterior cerebral artery.