The optic nerve (n. opticus), or cranial nerve II, is considered as a component of the special somatic afferent system but it is not a peripheral nerve based on its structure. It is equivalent to a central nervous system tract because the ganglion cell layer of the retina is a development of the optic vesicle from the neural tube. The axons from the neuronal cell bodies in the ganglion cell layer of the retina invade the hollow stalk of the original neuroectodermal outpouching that forms the optic vesicle. Centripetal growth of the ganglion cell axons begins at approximately day 32 of gestation and is not completed until after birth.
Because the eye forms as an outgrowth of the brainstem (see section on development), the myelin of the optic nerve is of the central nervous system type, being formed by oligodendrocytes. Ganglion cell axons are unmyelinated in the nerve fiber layer of the retina. They become myelinated as they turn from the nerve fiber layer into the optic nerve. The intraocular myelinated portion is the white or gray optic disc seen grossly. Myelin may extend a variable distance from the optic nerve head onto the retina, giving the disc a large and ragged appearance. The size and shape of the optic disc varies widely among dogs (see section on the retina).
The optic nerve is surrounded by outer and inner sheaths (vagina externa et vagina interna n. optici), which are continuations of the dura mater, and the arachnoid and pia mater of the brain, respectively. The space within the vagina interna (between the arachnoid and pia) is continuous with the intracranial subarachnoid space and contains cerebrospinal fluid, and a linear relationship between pressure in the cerebrospinal fluid and optic nerve subarachnoid space has been demonstrated. This space serves as a conduit for spread of infection between the eyeball and the brain. Optic neuritis is a form of encephalitis. The ciliary vessels and nerves are closely applied to the outer surface of the external sheath. Radicles of the internal ophthalmic and ciliary vessels supply the intraorbital portion of the optic nerve.
Brooks and colleagues (1995) reported a mean (N = 4) of 24,610 (±8791) myelinated axons (range 16035-37246) representing 16.6% of the total axons in the optic nerve. The mean total area of the individual axons per optic nerve was 43,466 (±10,807) µm2 (range 38,081-59,267). This was estimated to represent a mean axonal diameter of 1.49 µm (range 0.4 to 23). Axons >2 µm in diameter represented approximately 12.7% of axonal optic nerve fibers. The mean crosssectional area of the entire optic nerve was 2.93 (±0.81) mm2 (range 2.03-3.99). The total number of optic nerve axons for the normal dog was estimated to be 148,303 (±58,865) (range 95,755-232,690). Overall, the axon density was 51,725 axons/mm2.
Using magnetic resonance imaging, Boroffka et al. (2008) reported the following mean measurement of the optic nerve and related structures (N = 5): optic disk width 5.2 (±0.17) mm (range 5-5.5), intraorbital diameter of the optic nerve sheath complex 3.7 (±0.27) mm (range 3.6-3.8), optic nerve 1.7 (±0.06) mm (range 1.6-1.8), intracanalicular and intracranial diameter of the optic nerve 2.2 (±0.15) mm (range 2-2.5), optic chiasm height 2.1 (±0.08) mm (range 2.1-2.4), mm and optic chiasm width 4.8 (±0.16) mm (range 4.4-5).
The optic nerve follows an undulating course from its origin lateral and inferior to the posterior pole of the eyeball to the optic canal of the presphenoid bone. The nerve traverses the optic canal to the rostroventral floor of the middle cranial fossa, where fibers are exchanged with the opposing optic nerve at the optic chiasm. In the dog, approximately 75% of the optic nerve axons cross to join the contralateral optic tract caudal to the chiasm. The crossing axons are derived from the central and medial retinal areas; axons from the lateral retina join the ipsilateral optic tract. Abnormal eye movements (pendular nystagmus) are observed in dogs with congenital mutations involving absent or malformed optic chiasms.
Each optic nerve is composed of approximately 150,000 axons, all of which are myelinated. There is a correlation between the size of the eyeball and the number of axons, but larger eyes have fewer axons relative to retinal surface area.
In humans and cats, centrifugal axons, as well as the centripetal ganglion cell axons, have been identified in the optic nerve. It is probable that centrifugal axons also occur in the dog. Their function has not yet been elucidated.
The optic nerve is longer than the straight-line distance from the posterior pole of the eyeball to the optic canal of the presphenoid bone to allow for the rotation of the eyeball. The degree of undulation in the nerve depends on the position of the eyeball and the degree of retraction or protrusion. In dolichocephalic and mesaticephalic dogs, the eyeball can be almost completely retracted into the orbit. In the dog, the optic nerve is sufficiently long that proptosis can occur without rupture of the nerve.