762 
OPTIC NERVES. Under this heading it 
is proposed to describe the special nerves of 
vision in man, for although other names are 
also employed by anatomists to denote the 
nerves in question, the above is preferred as 
being expressive of their functions. It should 
be borne in mind that the optic nerves are like- 
wise fiequently called “the second pair;” a 
term derived from their numerical position on 
the base of the brain, as they are the second 
from before backwards on the under surface of 
the encephalon. , 
The anatomy and physiology of the optic 
nerves in man constitute the more immediate 
subjects of the present article, but as these 
would be imperfectly treated without the aid 
of comparative anatomy, the reader will find 
in the following pages frequent references to 
the condition of the nerves of vision in other 
animals also. 
DescripTIVE ANATOMY. 
Apparent origin.—The optic nerves com- 
mence by two broad medullary tracts (the 
tractus optici), each of which becomes first 
apparent at the under surface of the corres- 
ponding optic thalamus. 
Tractus opticus.—This appears to derive its 
principal origin from the corpus geniculatum 
-externum: from that tubercle a narrow band 
arises which is soon reinforced by another (not 
in general equally large or distinct,) from the 
corpus geniculatum internum, and by the 
junction of the two the tractus opticus is 
formed: thus constituted, the tractus opticus 
takes a course forward and inward around the 
outer and inferior surface of the crus cerebri: 
it is at first deeply concealed from view in the 
great cerebral fissure, being overlapped from 
without by the middle lobe of the cerebrum, so 
as to be invisible until a portion of the brain, 
together with the arachnoid membrane and pia 
mater, have been displaced. Emerging from 
under cover of the middle lobe, the tractus next 
gains the front of the crus, runs along the 
margin of the tuber cinereum, and at length 
unites with the other tractus opticus to form 
the chiasma. 
At the crus cerebri the tractus opticus in- 
creases in breadth, and of its two edges the 
“anterior or external is here the thicker, while in 
the vicinity of the chiasma the tractus loses its 
flattened appearance, and becomes nearly cylin- 
drical. 
The tractus opticus is soft in texture through- 
out, being devoid of the tough neurilemma from 
‘which the proper optic nerve derives its uncom- 
mon firmness. 
_ The tractus opticus receives a very extensive 
investment from the pia mater, which covers 
and adheres to all its surface: anteriorly, 
where the tractus is approaching the chiasma, 
nearly two-thirds of its .circumference are 
clothed by pia mater; and further back, that 
membrane even insinuates itself a short distance 
between the posterior or inner margin of the 
‘tractus, and the adjacent surface of the crus 
cerebri. The arachnoid has a far less extensive 
relation to the tractus opticus: in the early 
of its course the tractus has no serous covering, 
OPTIC NERVES. 
but in the interval between the middle lobe of 
the brain and the chiasma, the eymersi . 
beneath the tractus opticus, and so ita 
partial investment. " 
The anterior or external margin of the tractus 
opticus is so closely connected to the crus — 
cerebri that in attempts to separate them the 
medullary substance is torn, and consequently 
some anatomists are of opinion that the crus 
furnishes filaments of origin to the tractus; but 
the posterior or inner edge of the tractus is no’ 
identified with the crus, for there the two 
structures can be separated without any vic 
to either. ‘ 
The third and fourth nerves, before react 
the cavernous sinus, cross ee ractus 
opticus, but not immediately, for Dt 
ad vascular membranes of the brain, and in 
general the edge of the middle cerebral le 
are interposed; the posterior communic 
artery also across the tractus inferiorly 
and the artery of the choroid plexus, in it 
course to the great cerebral fissure, runs 
neath it, the pia mater alone intervening be 
these bloodvessels and the tractus opticus. 
The chiasma is somewhat ilat 
and receives by each posterior angle the 
responding tractus opticus, while its anteri 
angles are prolonged respectively into eith 
optic nerve; when “ in situ,” it is supported 
a transverse groove of the sphenoid bone 
front of the sella Turcica. P iorly, 
identified with the tuber cinereum, 
upper surface the peculiar greyish memt 
which closes up the third se is adher 
The chiasma has complex relations to h 
vessels ; behind and below this body the 
rior portion of the coronary sinus is situ 
external to the chiasma the termination of 
internal carotid artery is placed, and in fre 
it are the anterior communicating, and a» 
of the anterior arteries of the cerebrum. 
Optic nerve proper.—This proceeds 
the chiasma, and after passing throug! 
foramen opticum into the orbit, and arrivit 
the eye-ball, it perforates the sclerotic 
choroid coats, and terminates in the retina 
First stage.—In the short interval b 
the chiasma and the optic foramen, the 
nerve is directed forward and outward ; its 
is perceptibly greater than that of the t 
opticus : it is not perfectly aaa ns 
being slightly flattened above and below; 
covered immediately by a dense tough 
lemma, and provided besides with a 
sheath of arachnoid membrane, whie 
accompanying the nerve fairly into 
the sphenoid bone, becomes reflected | 
— of dura mater lining that a 
hortly after its commencement the opti 
is separated from the olfactory by the a 
artery of the cerebrum. The ophthalmi 
leaves the cranium by the foramen optiet 
and lies beneath the optic nerve’ and to? 
side, being there enveloped in a spec 
of foore preg ‘oie 
cond stage.—Having entered 
the optic Bt inclines more directly fort 
in consequence of this change of direc 
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