OPTIC NERVES. 
rience proves that in the majority of such cases 
one or other retina is wholly paralysed, and not 
unfrequently vision continues perfect in one 
eye although extinguished in the other. More- 
over, although the theory in question affords an 
ingenious explanation of the defect in vision 
noticed by Wollaston, such explanation can 
_ scarcely be the true one, for Mayo has known 
_ “ this visus dimidiatus to alternate in the same 
individual with temporary insensibility of the 
centre and circumference of the retina,” and, 
as he observes, “the three phenomena. being 
_ alternative no doubt proceed from the same 
_ organic source, but as the hypothesis will not 
_ explain ¢wo of them, it is probably not the 
right explanation of the third.” 
3. The structure of the human chiasma does 
“Rot afford satisfactory explanation of the so 
_ called “ identical” and “ non-identical ” parts 
of the two retine as laid down by Miiller, and 
this has been so clearly shown by himself that 
his own words are quoted :— 
* With reference to their identity of sensa- 
tion, the two retinze must be considered as in- 
cluded one within the other, so that all points 
_ of the two retin which lie within the same 
degrees of latitude and longitude (the eyes being 
regarded as globes) are identical in their sensa- 
opposed to each other or different, just as any 
two yee in the retina of the same eye. 
_ . “If the image fall on identical points in 
- both eyes it will be seen single, and if the 
image does not fall on such identical points it 
will appear double. 
| The two globes of the eyes are most mi- 
nutely divided into degrees, minutes, and 
_ seconds of latitude and longitude; at all cor- 
| responding points they are identical, at all dif- 
ferent points non-identical. The outer lateral 
_ portion of one eye is identical with the inner 
portion of the other eye ; the upper part of one 
' retina is identical with the upper part of the 
other, and the lower parts of the two eyes are 
identical with each other. 
/_ “The left half of the retina A, from 1 to 5, 
however, (fig. 421,) is not, as a whole, identical 
_ with the left half of the retina B, from 1 to 5, 
_ but certain points only of the left halves of both 
retine are identical, viz. those which in the two 
Fig. 421. 
Diagram to represent the supposed identical parts 0 
the fas Satins, CAfter Muller.) f 
1, 2, 3, &c., the identical parts of the two retinz ; 
} ¢, ¢, the optic axes. 
| retin occupy the same degrees of latitude and 
longitude: 1 is identical with 1, 2 with 2, 
| and so on ; but 1 in the one eye is not identical 
with 5 in the other eye. 
tions; all other points in the two retine are 
773 
“To explain the single vision, therefore, it 
is necessary that not ‘merely each root of the 
optic nerve, but each primitive fibre of each 
root should in the chiasma divide into two 
branches for the two optic nerves, so that the 
identical fibres of the two nerves might com- 
municate with the brain at one point only, viz. 
by one radical fibre, as in the annexed wood- 
cut (fig. 422). But such a division of the fibres 
in the chiasma does not exist: Treviranus and 
Volkmann were unable to detect any division 
of fibres in the chiasma,and I also was unsuc- 
cessful in my search for such dividing fibres. 
( Fig. 422.) 
Fig. 422. 
Li 
Diagram to represent an ideal division of the fibres in 
the chiasma suitable to this theory. ( After Miller.) 
a, a, optic nerves ; b, b, tractus optici; c, c, sup- 
posed division of each radical fibre in the chiasma 
into two branches, one for each optic nerve. 
. 4. If single vision in man be explained on 
the assumption that certain parts of the two 
retine are reciprocally identical, and that such 
identity depends upon a partial decussation in 
the chiasma, single vision in animals should of 
course admit of explanation upon the same 
principles; and if this be granted, the relative 
directions of the optic axes in the vertebrate 
classes ought to afford a good criterion of the 
extent to which the retine are reciprocally 
identical ; for when the optic axes have a 
strictly lateral direction (as in many osseous 
fish), the same object can never be depicted on 
both retine simultaneously, and consequently 
it may be inferred that, in such cases, no parts 
of the two retine are reciprocally identical. 
Again, when the optic axes are very divergent, 
as in many quadrupeds, the respective fields of 
vision must comprise in great measure different 
objects, and under such circumstances it may 
be presumed that the two retine have but little 
identity. And when the eyes are so set that the 
optic axes are parallel, or capable of becoming 
parallel, or convergent (as in man), the same 
objects, or nearly the same, will almost con- 
stantly occupy the two fields of vision; and in 
such case the greatest amount of reciprocal 
identity may be assumed to occur in the two 
retine. 
Now, if the relative directions of the optie 
axes in animals bear relation to the amount of 
reciprocal identity in their retine, and if this 
reciprocal identity depend upon the decussa- 
tion in the chiasma, as has been assumed, the 
structure of the chiasma in animals generally 
should vary as the relative directions of their 
optic axes. 
Such variation in the structure of the chiasma 
has not, however, been proved to occur gene- 
