OPTIC NERVES. 
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- 
letely 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 
tmouscles 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 
xth nerve, and the ophthalmic vessels in their 
irst 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- 
¥ placed farther forwards than the artery. 
th the optic nerve, the inferior division 
of the third nerve is placed, and those twigs 
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 
le ee 
d external aspects, but nevertheless one or 
wo of the ciliary branches of the nasal nerve, 
as well as one or more from the lenticular gan- 
glon, 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 
tated tothe optic nerve, some. of the latter 
763 
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 other  nerves—The 
optic nerves have no direct communication with 
the other cerebral nerves,tbut certain anatomists 
have traced filaments from the ganglionic 
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 the arteria 
centralis retinze 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 (Mémoires de la Société Médi- 
cale d’Emulation) has asserted, that a minute 
subdivision of the cavernous plexus extends 
along the arteria centralis retin, 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 pate her a special provision for 
the security of the second pair Thirdly. The 
arteria centralis retine 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 
