OPTIC NERVES. 
to his own account, “the eye of the rabbit on 
sudden exposure to the sun’s rays, after the fifth 
pair had been divided, was ‘still sensible to 
strong solar light; and the effect was more 
marked when a lens was used to test its 
sensibility.” 
Mayo’s experiments on pigeons afford still 
more convincing proof of the ability of the 
optic nerve, unaided by the fifth, to maintain 
_ the special sensibility of the eye; this physio- 
_ logist succeeded in dividing the fifth nerve 
within the cranium of a living pigeon (leaving 
the optic uninjured,) without rendering the 
retina insensible to light. 
The results of pathological observations on 
man furnish also abundant evidence that vision 
continue after disease has destroyed the 
_ fifth nerve. Opportunities do not ofteri occur 
_of bringing this to the test of dissection, for in 
most of these cases changes of structure involve 
other parts of the nervous centres simultane- 
ously with the fifth nerve, and so deprive them 
of their greatest value; and the destructive in- 
flammation of the eye-ball, which so constantly 
“accompanies morbid alterations of the fifth pair, 
is another fruitful source of embarrassment in 
attempts to investigate their history, but even 
a few well-attested observations are amply 
ufficient to establish a negative proof. Miiller 
cites a case of disease involving the whole trunk 
of the fifth nerve of the left side, in which 
insensibility of the entire left side of the head 
d the corresponding side of the tongue and 
eye, occurred, while vision remained perfect ; 
| and in the article Frrra Parr or Nerves, other 
imilar examples are related. 
The conjecture that the fifth pair is essential 
| to vision receives probably its strongest support 
‘from the occasional results of injuries to cer- 
tain branches of that nerve, for numerous 
cases are on record in which wounds or con- 
usions of its frontal twigs have been fol- 
‘lowed by blindness, and the same unfortu- 
‘Mate event has resulted (though rarely) from 
itations affecting some of its other branches. 
Thus, Mr. Travers has known amaurosis to 
ginate from irritation of the dental nerves. 
He says, “I have seen.an incipient amaurosis 
arrested by the extraction of a diseased tooth, 
when the delay of a similar operation had 
teasioned gutta serena on the opposite side 
t os before.” And Professor Galenzowski 
of Wilna “ observed severe neuralgia and 
blindness produced by a splinter of wood. be- 
coming entangled in a diseased tooth, and 
these symptoms were cured by the-extraction of 
he tooth together with the offending material.” 
The value of such facts as these in assisting 
ysiologists to determine the influence exerted 
the fifth nerve over vision, appears to have 
been much overrated ; for in a large proportion 
‘these cases it may be inferred with great pro- 
ability that the same injury which affected the 
jupra-orbital nerves exerted also pernicious in- 
juence on the deeperseated contents of the orbit, 
d that the optic nerve, or retina, or even the 
in itself participated in the effects of the vio- 
ence, although from the more superficial posi- 
and greater exposure to danger of the frontal 
oe 
) 
779 
branches of the fifth,\they alone were believed 
to have suffered. This explanation will undoubt- 
edly not apply to cases in which blindness has 
been produced by very trivial injuries, such as 
simple incised wounds or punctures of the 
nerves in question ; but nevertheless the weight 
of evidence which these latter cases would seem 
to afford is much diminished by the consideration 
that loss of sight has likewise ensued from inju- 
ries and affections of other nerves, to which, 
while healthy, no participation in the support of 
vision can be conceded. For example, Dr. Jacob 
recites the case of an officer in whom amaurosis 
occurred in consequence of injury inflicted by 
a ball on some branches of the portio dura ;* 
and irritations in the digestive organs (dyspeptic 
disturbance of the stomach more especially) 
are well known to produce at times amaurotic 
symptoms. Now, although these facts un- 
questionably establish the existence of curious 
pathological affinities between the nerves of the 
part thus irritated and those which are subser- 
vient to vision, no physiologist would be hardy 
enough to infer from such premises that the facial 
nerve, the par vagum, or those which supply the 
intestinal tract, exercise in the normal state 
any control over the faculty of sight. 
If, in addition to these considerations, it be 
recollected that blindness occurs only as an 
occasional consequence of injuries to the frontal 
nerves, and that loss of vision is found to ensue 
very rarely from the irritations to which other 
branches :of the fifth are so peculiarly liable, - 
the importance of such cases in determining 
the question must be still further lessened. 
The observations of Dr. Jacob on this sub- 
ject appear to the writer so apposite that 
he is induced to insert them. ‘This gentle- 
man writes, “* Blindness does not seem to have 
followed any of the operations formerly so 
much practised of dividing the branches of this 
nerve, and in some of the worst cases of that 
form of neuralgia called tic douloureux, vision is 
not impaired. Moreover, thousands of children 
suffer from dentition and thousands of adults 
from tooth-ache, yet none of these become 
blind in consequence.” + 
The coincidence of loss of sight with injuries - 
or irritations affe¢ting branches of the fifth’ 
nerve, admits of explanation on other principles 
without assuming the fifth to be essential to 
vision ; the hypothesis that in such cases reflex 
irritation becomes propagated from the parts pri- 
marily affected through the nervous centres to 
the optic nerve, seems in the present state of 
physiological science sufficiently plausible; 
for while it applies to cases of amaurosis 
resulting from abnormal conditions of other 
peripheral branches of the nerve as well as its 
ophthalmic division, it also affords a solution 
of the still more obscure dependence of the 
same disease on irritations in remote organs. 
The experiments of Magendie, confirmed 
as they have been by pathological observations, 
* Cyclopedia of Practical Medicine, art. AMAU- 
ROSIS. 
t+ On Paralytic Neuralgia and other Nervous 
Diseases of the Eye, by Arthur Jacob, M.D. 
