778 
nerves are particularly small ; but the olfactory 
nerves preponderate, and there can be little 
doubt that the superiority of the sense of smell 
in them serves in great measure to supersede 
the necessity for highly-wrought organs of vision, 
and in probably the majority of the Mammalia 
the olfactory nerves preponderate in size over 
the optic, but the corresponding faculty by its 
acutetiess makes amends for any inferiority in 
vision. Thus the keen scent of many Carni- 
vora renders the eye of secondary importance 
in the pursuit of their prey, and the vegetable 
feeders are much indebted to the perfection of 
their sense of smell for the discrimination’ they 
evince in their choice of nutriment. 
FUNCTIONS OF THE OPTIC NERVE. 
The optic nerves when present are essential 
to vision. 
In all animals which possess optic nerves 
they must be considered essential to vision, 
for diseases which destroy the organization of 
the “second pair” invariably deprive the organs 
of sight of their sensibility to.luminous impres- 
sions ; and were other proof wanting, the ex- 
eee instituted of late years by Magendie, 
ayo, and others, would afford sufficient 
evidence of the special function of the nerves 
in question. Magendie found that division of 
either optic nerve in front of the chiasma was 
instantaneously followed by complete loss of 
"vision in the correspondingeye of the animal sub- 
mitted to the experiment; and when both optic 
nerves were thus divided évtal blindness en- 
sued, and no means which could subsequently 
be devised for concentrating light upon the 
eye appeared to excite in the retina the slightest 
sensibility to its accustomed stimulus. 
Although the foregoing facts would warrant 
the conclusion that the only nerves capable of 
endowing the eyes with their special sensibility 
are the optic, nevertheless many considerations 
favour the presumption that the fifth pair exert 
direct influence on the sense of sight, so much 
so that some have considered these nerves 
essential to vision, whilst others have even sup- 
posed that the faculty in question may be main- 
tained through the agency of the fifth nerves 
alone. 
In those animals which possess special optic 
nerves, the fifth pair are totally inadequate 
to support vision. 
There are no facts on record to prove the 
possibility of such animals continuing to see 
after destruction of the second pair. The experi- 
ments already cited may be looked on as con- 
clusive, and those performed by Magendie to 
show that divison of the fifth nerves within the 
cranium in living animals produces blindness, 
can never justify physiologists in the belief that 
the “trifacial” may endow the eyes with their 
special sensibility. 
Certain facts furnished -by the comparative 
anatomy of the second and fifth pairs have been 
A, ape fe time adduced to shew that the 
in the human subject possesses this power. 
Thus it is stated on good authority, that pHs 
mon Mole, the Proteus anguinus, the Mus Ca- 
pensis, the Chrysochlore, the Mus typhlus, and 
J 
OPTIC NERVES. ~ 
the Sorex araneus, in which organs of vision occur, _ 
are not provided with special optic nerves, and — 
that in them the fifth pair furnishes the only 
nerve which the rudimental eye receives. It is” 
argued from these data (and Serres would seem — 
to be one of the ablest advocates for this view) — 
that a branch of the fifth nerve assumes in 
such cases the functions of the optic, becomin: 
endowed with special sensibility to light, and 
that therefore lien analogy the ophthalmi 
division of the fifth in man may be presumed to 
possess similar properties. 33 
Conclusions pit ae at by such reasoning 
should be received with caution in the absence 
of more direct proofs; the weight to 
they are entitled has been already fully ¢ 
cussed under the article Firra Parr 01 
Nerves, and reference is made to that artic 
for further particulars touching this interesti 
topic. The writer fully concurs in the vie 
therein advocated, and feels disposed to attri 
bute little value to arguments founded, as th 
appear to be, on imperfect analogies. Th 
cases of the human subject and the anim 
specified are essentially dissimilar; the pi 
sence of a special optic in the one case, and 
absence in the others, destroys their parallel: 
and may create important differences in 
functions of accessory nerves; and, morec 
the little knowledge we possess of the natut 
and amount of vision enjoyed by animals 
which special optic nerves are wanting, shot 
e hesital argue from them to 
* 
make us hesitate to 
human subject. 
That the fifth pair exercise some influet 
over vision can scarcely be denied, bu' 
nature and amount of this influence are no 
easily determined, and have probably bi 
much exaggerated. 
In the present state of their knowledge phy 
logists have not sufficient proof that in— 
higher animals the influence of the fifth 
is absolutely essential to sight. ial 
Magendie discovered that the section of 
fifth nerve on both sides within the cranium 
a living animal greatly impairs, af it de 
actually annihilate vision, and such a m 
seems, no doubt, to argue that the facul 
sight has a necessary dependence on 
integrity of the “trifacial nerves,” but tl 
periment when critically examined wi 
pear not to warrant such an inferem 
demonstrates that so rude an injury inflie 
the nervous centres deprives an animal ¢ 
of ‘its faculties, and this might 
anticipated on general principles; but: 
not prove that the loss of the di 
on the injury to the fifth nerves alone. 
a destructive proceeding there can be 
surance that the fifth pair have been t 
pores of importance mutilated ; the con 
y much the more probable presumptie 
therefore, the conclusion “ that the fil 
are essentially necessary to vision ” is 1 
deducible from the experiment. But 1 
the facts (detailed even as they 
Magendie) suffice to show, that the eye 
continue sensible to luminous impressio 
complete division of the fifth pair, for ac 
