OPTIC NERVES. 
For example: firm pressure applied to the 
globe of the eye when the lids are closed and 
light excluded, gives rise to the sensation of 
luminous spectra which present different colours. 
Concussion of the eye-ball is often followed by 
the same results: division of the optic nerve in 
extirpation of the organ of vision generally 
causes the patient to perceive a great light ; 
and an electric current transmitted through the 
optic nerve, or its immediate vicinity, seems to 
roduce a flash of light. The second pair, in 
Sccer their special sensibility excited by such 
varied stimulants, merely conform to the laws 
by which other nerves of special sense are go- 
verned ; for electricity applied to these nerves 
severally may be made in each case to elicit 
the peculiar sensibility of the nerve which hap- 
pens to be the subject of the experiment: in the 
optic nerve it produces the sensation of a flash 
fi of light ; in the auditory it excites a loud sound ; 
in the gustatory it gives rise to a peculiar taste; 
_ in the olfactory it developes a particular smell ; 
and in the common sentient nerves it causes 
“painful sensations. In like manner a blow 
may occasion the optic nerve to flash fire, the 
_ auditory nerve to hear sounds, the common 
sensitive nerves to feel pain; and more examples 
| might be added to this catalogue. 
+ The nerves of special sense seem in general 
to be endowed with but one determinate sort 
of sensibility; and though this is commonly 
excited by a specific stimulus only, it may be 
elicited occasionally by other means. 
_ Excito-motory properties—The optic nerve 
is one of those paths through which incident 
impressions are propagated so as to excite 
| reflex motions. The impression of light on 
' the retina is instantaneously followed by con- 
traction of the pupil, a phenomenon indicative 
of reflex motion developed in the iris; and 
_ the sudden closure of the eye-lids under the 
| influence of a strong light or a threatened blow 
| is also a familiar example of reflex motion 
| produced by impressions upon the terminal 
_ expansion of the optic nerve. 
is Te tet, Fontana, and Caldani, have de- 
monstrated that the optic nerve is the channel 
gh which the incident impression travels 
im order to excite reflex motion in the iris. 
n their experiments, rays of light transmitted 
through a hole in a sheet of paper, and by this 
contrivance conveyed through the pupil di- 
rectly to the retina, produced immediate mo- 
tion of the iris; but when the light was al- 
9a to impinge upon the iris a/one without 
‘Teaching the retina, no contraction of the pupil 
_ Mayo’s experiments on pigeons taken in 
‘connection with the foregoing facts appear par- 
ticularly instructive, proving as they do that in 
the bird, irritation propagated along the optic 
nerve in a centripetal direction may excite reflex 
otion in the iris. When the optic nerves were 
divided within the cranial cavity of a living 
pigeon by Mayo, the pupils became fully dilated 
and were no longer obedient to luminous im- 
| pressions even when dazzling light was admitted 
mto the eyes. When a pigeon was decapitated 
the same experimentalist, and its optic nerves 
781 
subsequently divided within the cranial cavity, 
irritation of that portion of the divided nerves 
which continued in connection with the eye pro- 
duced no effect on the iris; but contraction of 
the pupil immediately ensued when the other 
extremity of the nerves, viz. that which re- 
tained its connection with the brain, was irri- 
tated. 
In general the reflex motion is developed in 
the iris of the same eye on which the impres- 
sion is incident, or in other words light falling 
on the right retina produces in general con- 
traction of the right pupil and not of the left, 
and vice versa; but it sometimes happens 
that an impression propagated along one optic 
nerve (for instance the right) may cause the 
reflex phenomena to appear in the iris of the 
other eye (viz. the left). 
Certain forms of amaurosis in which the dis- 
ease affects but one eye while the other con- 
tinues healthy will serve for illustration. 
In such cases it occasionally happens that 
little or no difference in the size of the two 
pupils can be detected so long as both eyes 
remain exposed to the light, the iris of the dis- 
eased organ contracting and dilating simulta- 
neously with that of its healthy neighbour; 
but as soon as the lids of the sound eye are 
closed, the pupil of the amaurotic eye becomes 
dilated, and the most intense light admitted into 
this diseased organ takes no effect on the iris, 
now become perfectly motionless. 
The explanation of these phenomena is found 
in the preceding proposition; so long as the 
healthy eye continues exposed to light, the 
impression falling on a sound retina excites 
reflex motion in the pupils of both eyes, as 
well the amaurotic as the sound; but when the 
light is excluded from the healthy retina, the 
influence of that agent upon the diseased retina 
of the other eye has no longer the power to 
excite reflex motions. 
. The exercise of these excito-motory proper- 
ties of the optic nerve is generally accompanied 
with excitement of its special sensibility; thus 
a.person is in general conscious of the lumi- 
nous impression, or, in other words, he sees 
the light which causes contraction of his pupil ; 
but the reflex phenomena may be manifested 
by the iris, althoagh the incident impression 
pass unnoticed by the individual. 
In certain cases of general insensibility (as, 
for example, concussion occasionally) the pupil 
contracts upon the admission of light while 
the patient remains perfectly unconscious, and 
something similar seems to occur at times even 
in health; for the iris varies its dimensions 
‘with each successive change in the volume and 
- intensity of the light, although from inatten- 
tion we do not perceive these trifling changes. 
Some degree of attention appears requisite in 
order that weak or transitory impressions should 
arouse the special sensibility of the optic nerves, 
whereas attention is not a condition essential 
to the production of reflex phenomena. 
Mayo’s experiments here again admit of 
application; he found that in decapitated pi- 
geons in which the optic nerves were subse- 
quently divided, irritation of the cerebral ex- 
