OPTIC NERVE- Cranial nerve II

The optic nerve extends from the back of the eye about 3 mm medial and slightly superior to the true posterior pole of the eye. The optic nerve varies in length from 35 to 55 mm. The optic nerve is divided into 4 segments along its course: intraocular, orbital, intracanalicular and intracranial.

1. Intraocular optic nerve is subdivided into retinal choroidal and scleral portions or alternatively into: a. optic disc b. prelaminar c. laminar and d. retrolaminar portions.

The intraocular portion is about 1 mm in length and is thinnest at the oval shaped disc before nerve fibers have myelinated and measures about 1.5 mm ( H) x 1.75 mm (V) diameter. It is conical in shape as myelinated fibers are accumulated. The blood supply includes both retinal and posterior ciliary arteries.

2. The orbital portion of the optic nerve is about 25-30 mm in length and about 3.5 mm in diameter. There is slack in the optic nerve of about 7-12 mm so the eye will not be constrained upon turning. The blood supply includes the intraneural branches of the central retinal artery, which enters the eye about 6-12 mm posterior to the sclera, as well as the pial network of the nerve derived from the adjacent branches of the ophthalmic artery.

3. The intracanalicular portion of the optic nerve measures 11.4 mm and enters the canal with the outer portion of the dura fusing to the bone. The optic canal measures about 3.6 (H) mm x 4.8 mm (V).These attachments constrain the expansion of the optic nerve during infections resulting in a compartment syndrome and may also account for pain of eye movement in optic neuritis. The blood supply is derived from the ophthalmic artery.

4. The intracranial portion of the optic nerve includes that from the end of the optic canal to the optic chiasm and measures about 10 mm in length. The blood supply is derived from the ophthalmic and internal carotid arteries.