OPTIC NERVES. 



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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 becomes 

 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 

 immersed. 



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 (m 4 the second 

 stage,) take their course ; the vein being gene- 

 rally placed farther forwards than the artery. 



Beneath the optic nerve, the inferior division 

 of the third nerve is placed, and those twigs 

 of the latter which are destined for tie 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 

 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 

 separates it in part from the optic nerve. 



The ciliary nerves run from behind forwards. 

 Closely approximated to the optic nerve, they 

 appear mustly above the nerve and on its lower 

 and external aspects, but nevertheless one or 

 two of the ciliary branches of the nasal nerve, 

 as well as one or more from the lenticular gan- 

 glion, before piercing the sclerotic coat, gain, in 

 general, the inner side of the optic nerve. 



The long and short ciliary arteries in their 

 course to the globe of the eye are intimately 

 related to the optic nerve, some of the latter 



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. 



Communication with oilier nerves. The 

 optic nerves have no direct communication with 

 the other cerebral nerves, but certain anatomists 

 have traced filaments from the gangiionic 

 system to them. Arnold (Icones nervorum 

 Capitis, Tabula Sexta) has described and 

 delineated two slender threads which run from 

 the spheno-palatine or Meckel's ganglion to 

 the optic nerve, and Hirzel observed in several 

 instances the same arrangement. Tiedemann 

 has seen an excessively delicate filament from 

 the lenticular ganglion accompanying thearteria 

 centralis retinae through the optic nerve : he 

 has also discovered branches of the ciliary nerves 

 taking the same course, and has even suc- 

 ceeded in following them as far as the retina ; 

 and M. Ribes (Memoires de la Societe Medi- 

 cale d'Emulation) has asserted, that a minute 

 subdivision of the cavernous plexus extends 

 along the arteria centralis retinae, being derived 

 from that division of the plexus which accom- 

 panies the ophthalmic artery. 



Organization. The organization of the optic 

 nerve is in many respects peculiar. Firstly. 

 From the chiasma to its distal extremity it is 

 enveloped by a strong coating of neurilemma, 

 and from the inner surface of this tunic a 

 number of processes are detached which divide 

 the interior of the envelope into longitudinal 

 canals wherein the medullary substance is 

 lodged ; the optic nerve is not therefore a mere 

 bundle of nervous cords (the structure prevalent 

 in other nerves), but it is "a cylinder of collected 

 tubes." Secondly. From the optic foramen to 

 the sclerotic a sheath of dura mater is super- 

 added to the optic nerve, and since none of the 

 other cerebral nerves possess a similar covering, 

 it must be considered a special provision for 

 the security of the second pair. Thirdly. The 

 arteria centralis retina runs through the centre 

 of the optic nerve (an anatomical arrangement 

 of exceedingly rare occurrence) : and the pri- 

 mitive fibres of the optic nerve evince a marked 

 tendency to appear " varicose," a condition 

 discovered by Ehrenberg, and considered by 

 him and others peculiar to certain parts of the 

 nervous system. 



Real origin. Anatomists have entertained 

 very conflicting views upon this interesting 

 question, so that from time to time different parts 

 of the human encephalon have been considered 

 the true origin of the optic nerves. 



The older writers very generally believed 

 that these nerves originate in the optic thalami, 

 as the names " thalami nervorum opticorum " 

 still applied to the bodies in question suffi- 

 ciently attest, and Eustachius, Varolius, 

 Lieutaud, Haller, &c. supported this opinion. 



Others conceived that the nates (or anterior 

 pair of the tubercula quadrigemina) are the 

 principal source of the optic nerves; this 

 was maintained by Ridley, Winslow, Zinn, 

 Morgagni, Sanctorini, Girardi, Hildebrandt, 

 Boyer, Bichat, and Scemmerring ; and the same 



