<P> The optic nerve is ensheathed in all three meningeal layers (dura, arachnoid, and pia mater) rather than the epineurium, perineurium, and endoneurium found in peripheral nerves . Fiber tracts of the mammalian central nervous system (as opposed to the peripheral nervous system) are incapable of regeneration, and, hence, optic nerve damage produces irreversible blindness . The fibres from the retina run along the optic nerve to nine primary visual nuclei in the brain, from which a major relay inputs into the primary visual cortex . </P> <P> The optic nerve is composed of retinal ganglion cell axons and glial cells . Each human optic nerve contains between 770,000 and 1.7 million nerve fibers, which are axons of the retinal ganglion cells of one retina . In the fovea, which has high acuity, these ganglion cells connect to as few as 5 photoreceptor cells; in other areas of retina, they connect to many thousand photoreceptors . </P> <P> The optic nerve leaves the orbit (eye socket) via the optic canal, running postero - medially towards the optic chiasm, where there is a partial decussation (crossing) of fibres from the temporal visual fields (the nasal hemi - retina) of both eyes . The proportion of decussating fibers varies between species, and is correlated with the degree of binocular vision enjoyed by a species . Most of the axons of the optic nerve terminate in the lateral geniculate nucleus from where information is relayed to the visual cortex, while other axons terminate in the pretectal nucleus and are involved in reflexive eye movements . Other axons terminate in the suprachiasmatic nucleus and are involved in regulating the sleep - wake cycle . Its diameter increases from about 1.6 mm within the eye to 3.5 mm in the orbit to 4.5 mm within the cranial space . The optic nerve component lengths are 1 mm in the globe, 24 mm in the orbit, 9 mm in the optic canal, and 16 mm in the cranial space before joining the optic chiasm . There, partial decussation occurs, and about 53% of the fibers cross to form the optic tracts . Most of these fibres terminate in the lateral geniculate body . </P> <P> Based on this anatomy, the optic nerve may be divided in the four parts as indicated in the image at the top of this section (this view is from above as if you were looking into the orbit after the top of the skull had been removed): 1 . the optic head (which is where it begins in the eyeball (globe) with fibers from the retina; 2 . orbital part (which is the part within the orbit). 3 . intracanicular part (which is the part within a bony canal known as the optic canal); and, 4 . cranial part (the part within the cranial cavity, which ends at the optic chiasm). </P>

Where does the optic nerve enter the eye