OPTIC NERVES. 
Some difficulty no doubt arises when the 
physiologist attempts to reconcile with this 
anatomical arrangement certain facts already 
detailed : it may be remarked, however, that 
arguments derived from the pathology of the 
optic nerve can be but of little value, since 
they have been relied on in turn by the framers 
of each hypothesis as affording proof of their 
_ Own peculiar views ; and in the Museum of the 
Richmond Surgical Hospital, Dublin, the writer 
has seen specimens of atrophied optic nerves in 
man which furnish the most contradictory evi- 
dence upon the subject under discussion. The 
preparations alluded to were cases in which one 
eye had been destroyed either by local disease 
or accident, many years previous to death, and 
_ where in consequence the corresponding optic 
nerve became wasted from disuse, while the 
_ other optic nerve continued healthy. In the 
_ majority of these specimens the wasting has 
been propagated backwards to the opposite 
tractus opticus and has implicated that structure, 
while the corresponding tractus has been spared 
_ (See fig.420, A); in some examples both tractus 
optici have suffered a diminution of size and 
in general to an unequal amount ; and in one 
Fig. 420. 
A 
ltrophy of one optic nerve, the consequence of long- 
_ continued disuse. (From preparations in the Museum 
_ of the Richmond Hospital, Dublin.) 
_A. a, right optic nerve in a state of atrophy; }, 
eit optic nerve healthy ; c, chiasma; d, right trac- 
is opticus healthy ; e, left tractus opticus wasted. 
B. a, right optic nerve atrophied ; 6, left optic 
erve healthy; c, chiasma; d,right tractus opticus 
asted; e, left tractus opticus healthy. 
771 
very remarkable instance the tractus opticus of 
the same side with the shrunken nerve has 
dwindled into a peiete Mc while the other 
retains fully its normal dimensions (fig. 420, B). . 
It may be fair to add, that the case quoted 
from Vesalius is considered by many, and 
amongst others by Gall and Spurzheim, as of 
doubtful authenticity, and the results of expe- 
riments on living animals should be received 
with caution, for to argue from experiments on 
birds to the human subject is plainly fallacious, 
since the structure of the chiasma is not iden- 
tical in the two cases, and the great obstacles 
encountered in the performance of such expe- 
riments on the mammalia renders them of trifling 
value. 
Use of the chiasma.—In the direct junction 
between two corresponding nerves of opposite 
sides displayed in the chiasma, the second pair 
form an exception to a general law; for in no 
other known instance does a similar union 
occur ; it therefore becomes a subject of great 
interest to determine how far this anomaly 
admits of explanation by any unusual properties 
in the nerves so circumstanced. — 
The optic nerves possess the remarkable 
power of conveying to the individual the sen- 
sation of a single impression only, while a 
separate impression affects each nerve simul- 
taneously, so that although a perfect picture of 
the object be depicted on each retina severally, 
nevertheless to the spectator it appears to be 
single (as is really the case); or to speak still 
more intelligibly, a spectator sees an object 
single, although he looks at it with two eyes. 
This property would seem to belong in an 
especial manner to the optic nerves, and: inas- 
much as the second pair differ from all the 
others in possessing a chiasma, there is so far a 
presumption, that the unity of sensation mani- 
fested by the optic nerves depends on the 
chiasma. 
The idea that single vision may be explained 
-by a partial decussation in the chiasma originated 
(upon theoretical grounds) with Newton ; it has 
since been adopted by Wollaston, Solly, and 
others, and many facts have been from time 
to time brought forward in its favour. The 
hypothesis may be thus enunciated. “ Each 
tractus opticus sends some filaments across 
the chiasma ‘to form the inner part of the 
opposite optic nerve, while its outer filaments . 
continue on to form the outer part of the optic 
nerve of its own side; the same arrangement 
of the filaments prevails to the retina, so that 
the right side of each retina comes from the 
right tractus opticus, and the left side of each 
retina from the left tractus opticus; if then 
in vision the pictures of an object be depicted 
simultaneously either on the right sides of 
the two retine, or on the left sides of the two 
retin, the impressions in either case will be 
communicated to one and the same tractus opti- 
cus; such impressions will of course be referred 
to one and the same side of the brain, and 
they will therefore produce the sensation of a 
single impression on/y, although in reality two 
several impressions affect the retine; the unity 
of sensation depending on the fact that the two — 
3 D2 
