780 . 
justify the inference that the fifth nerve endows 
the eye with its general sensibility, and also 
exerts some influence over the nutrition of that 
organ. 
is distinguished physiologist divided the 
fifth nerve within the granium of an animal 
and found the tactile sensibility of the surface 
of the eye completely abolished by the expe- 
riment; opacity, ulceration, and sloughing of 
the cornea, followed by expulsion of the hu- 
mours, and total destruction of the eye subse- 
quently ensued; and nearly the same results 
have been observed to oceur in the human 
subject from disease affecting the fifth nerve 
within the cranium. It must be perfectly ob- 
vious, however, that these facts have no bearing 
upon the question more immediately under 
discussion. 
An impartial review of this highly interesting 
question leads to the conviction that notwith- 
standing the great plausibility of the arguments 
by which the contrary view has been sustained, 
there is as yet no evidence that in man any 
other nerve than the optic enjoys special sensi- 
bility to light. 
Ordinary tactile sensibility—Although the 
optic nerves are endowed with such acute 
sensibility to the influence of light, they would 
seem to possess little ordinary tactile sensibi- 
lity,—a circumstance the more surprising, as 
it is difficult to imagine any impressions more 
delicate than those of light; and where nerves 
evince such exquisite susceptibility of excite- 
ment from the stimulus of that imponderable 
agent, an equal obedience to those rougher 
stimulants which produce such marked effects 
on the common sentient nerves might at least 
be expected. 
Magendie infers from his experiments on 
living rabbits that the retina in them is not 
susceptible of pain from mechanical irritation ; 
so much so, that puncturing or tearing that 
hervous expansion appeared to him to cause 
these animals no sort of suffering.* Precisely 
the same results followed from injuries inflicted 
by him on the retina in fish and reptiles, 
although in birds, cats, and dogs similar expe- 
riments seemed to create some uneasiness. 
This physiologist asserts that the human retina 
also is devoid of ordinary tactile sensibility, for 
in operating for .cataract he has proved that 
the membrane in question exhibits little if any 
susceptibility of pain. 
His experiments were likewise extended to 
the optic nerves; and in the course of his in- 
vestigations frequent opportunities were afforded . 
for testing the comparative sensibility of the 
second and fifth pairs. 
whether the injury was inflicted in front of or 
behind the chiasma, the second pair seemed 
quite insensible to mechanical irritation; and 
whenever the slightest disturbance affected the 
fifth nerve, the animal, by its cries and struggles, 
immediately manifested the most acute suffer- 
ings. 
The phenomena noticed in extirpation of the 
human eye are favourable to the same views, 
* Journal de Physiologie, t. iv. 
In all the mammalia, 
OPTIC NERVES. 
for division of the optic nerve in this operation 
is not attended with the agonizing torture which 
an equal amount of injury to a nerve of the — 
same dimensions endowed with common sensi- — 
bility would unavoidably produce. This fact — 
should have some weight with physiologists in 
their attempts to form a just estimate of the 
properties of the nerve under consideration, 
although the results of such observations are 
inconclusive; for in many cases of extirpation 
of the organ the optic nerve is itself diseased, 
and under such circumstances it would be 
unfair to argue from the known effects of 
injuries on a diseased structure, to the probable 
effects of injuries on the same structure when 
healthy. ») 
The optic nerve is not singular in its insensi- 
bility to pain from mechanical irritation, for 
experiments on other nerves of special sense 
countenance the belief that some of them labour 
under the same disability. Magendie laid bare 
the olfactory of a dog, and the animal did 
not manifest the slightest pain when the nerve 
was compressed, pinched, or even torn; an 
when the auditory of a rabbit was subjected to 
similar rough treatment at his hands, the animal 
afforded no indication of suffering.” ; 
The specific stimulants of the organs of sense 
act however at times so intensely as to produc 
painfully disagreeable impressions on their 
spective organs, and it therefore becomes diffi- 
cult to reconcile, with the foregoing statemen 
facts such-as the following, which apparent! 
favour the opinion that the optic as well as 
other nerves of special sense commor 
sensibility. “ An intense light dazzles the ey 
so as to become actually insupportable. A harsl 
or discordant sound produces a most distress 
ing impression on the organ of hearing; 
certain odours are disgusting and intolerable 
the pituitary membrane.” ~ = 
In estimating the weight to which these latte 
facts are entitled, it should be recollected 
the above sensations still preserve their speci 
characters, no matter how intensely disagreeah 
they become : thus light, although sufficien 
brilliant to dazzle the retina, still continue 
be a luminous impression, and in like mai 
sonorous vibrations and odours, though actual 
offensive to their respective organs, are § 
nothing more than sounds and scents. So 
on the whole, however questionable may be t 
propriety of such experimental zeal as wot 
induce a French physiologist to test the 
bility of the human retina in operations fore 
ract, the general proposition that the optic 
in man and the higher animals enjoys litt! 
any, tactile sensibility, seems pretty 
tablished. "i 
Effects of stimulants—Although the o 
nerve betrays little indication of pain in | 
sequence of injuries, nevertheless mec 
and other stimulants produce upon it pect 
effects: mechanical injuries and irritants are 
its special sensibility instead of exciting pai 
sensations—their ordinary effects on ce 
sentient nerves. 4 
. 
* Journal de Physiologie, t. iv. 
, 
