Nerve · Head

Optic nerve

Nervus opticus

The optic nerve is not really a peripheral nerve at all — it is a tract of the brain that happens to lie outside the skull. Its fibres are the axons of retinal ganglion cells, insulated by the same cells that insulate the brain, and it is wrapped in all three meningeal layers with a sleeve of cerebrospinal fluid that runs the whole way from the skull to the back of the eyeball. About four centimetres long between eye and chiasm, it leaves the orbit through the optic canal in the lesser wing of the sphenoid, carrying the ophthalmic artery alongside it. At the optic chiasm, directly above the pituitary gland, the fibres from the inner half of each retina cross to the other side — the point at which the two eyes' information is regrouped by which half of the world it came from rather than which eye saw it.