Optic nerve proper.—This proceeds f the chiasma, and after passing through foramen opticum into the orbit, and arriving the eye-ball, it perforates the sclerotic a clioroid coats, and terminates in the retina.
First stage.—In the short interval betw the chiasma and the optic foramen, the o nerve is directed forward and outward ; its s. is perceptibly greater than that of the tract opticus : it is not perfectly cylindrical in sha being slightly flattened above and below ; it covered immediately by a dense tough ne lemma, and provided besides with a disti sheath of arachnoid membrane, whic accompanying the nerve fairly into the the sphenoid bone, becomes reflected process of dura mater lining that a Shortly after its commencement the opti is separated from the olfactory by the a artery of the cerebrum. The ophthalmic art leaves the cranium by the foramen opticu in al and lies beneath the optic nerve and to its ou side, being there enveloped in a special shea of fibrous membrane.
Second stage.—Having entered the orbit, the optic nerve inclines more directly forwards; in consequence of this change of direction it appears slightly bent at the optic foramen, the • convexity of the curvature being turned outwards, and it traverses the fibrous and vascular coats of the eye at a point not exactly corresponding to the axis of the organ, but a little inferior and internal to that imaginary line.
Whilst in the orbit the optic nerve is com pletely cylindrical, but a circular constriction indents it just before piercing the sclerotic.
In this, its second stage, the optic nerve has still its neurilemmatous investment, and in ad dition, a perfect sheath of fibrous membrane, derived from and clearly traceable to the dura mater; this latter covering of the nerve pos sesses great strength and density; it is white and tough, and admits of ready separation from the proper neurilemma ; moreover, it becornes continuous with the sclerotic, as the nerve is perforating that tunic.
In its course through the orbit the optic nerve is related to many of the important parts in that cavity ; on leaving the foramen opticum it is surrounded by the posterior attachments of the muscles of the eye, and afterwards proceeds forwards to its destination through the centre of the space which has the recti for its limits. The
nerve is here imbedded in a quantity of soft fat, from which it derives protection, and wherein other nerves and bloodvessels are irnmersed.
The nasal branch of the ophthalmic division of the fifth nerve (immediately after entering the orbit), the lenticular ganglion with its roots, and some of the ciliary nerves at their origin, the sixth nerve, and the ophthalmic vessels in their first stage, intervene between the outer surface of the optic nerve and the external rectus muscle.
Between the upper surface of the optic nerve and the superior rectus muscle, the superior division of the third nerve, the nasal branch of the ophthalmic division of the fifth nerve, and the ophthalmic artery and vein (in the second stage,) take their course ; the vein being gene ' rally placed farther forwards than the artery.
Beneath the optic nerl're, the inferior division I of the third nerve is placed, and those twigs of the latter which are destined for the inferior and internal recti muscles separate the optic from the inferior rectus.
To the inside of the optic nerve and upon a higher plane, the ophthalmic vessels in their 1 third stage, and the nasal branch of the ophthalmic division of the fifth nerve are situated; their position is rather above the upper edge of the internal rectus, but the branch of the third nerve which supplies that muscle I separates it in part from the optic nerve.
? The ciliary nerves run from behind forwards.
. Closely approximated to the optic nerve, they I appear mostly above the nerve and on its lower and external aspects, but nevertheless one or two of the ciliary branches of the nasal nerve, las well as one or more from the lenticular gan *lion, before piercing the sclerotic coat, gain, in general, the inner side of the opticInerve.
I The long and short ciliary arteries in their bourse to the globe of the eye are intimately .elated to •the optic nerve, some. of the latter 1 1 vessels appearing actually to twine around it in a spiral manner : and many of the muscular branches of the ophthalmic artery lie immersed in the surrounding adipose_ tissue, at no very great distance from the nerve in question.