772 
impressions are referred to the same side of the 
brain, and not to opposite sides of the brain as 
occurs in other cases.” 
Wollaston adopted this hypothesis in conse- 
quence of the ready explanation it affords of 
certain cases of visus dimidiatus: he on several 
occasions in his own person experienced a tem- 
porary defect of vision, in consequence of which 
one and the same lateral half of every object 
became invisible, whilst he still continued to 
see the other half distinctly; and the same 
amount of blindness subsisted whether he em- 
ployed one or both eyes in looking at the 
object. 
Examples of the same form of amaurosis are 
of no very unfrequent occurrence, dnd according 
to the theoretical notions just propounded this 
partial loss of vision originates in some func- 
tional affection of one tractus opticus whilst the 
other remains healthy ; those parts of the two 
retine which derive their origin from the faulty 
tractus being supposed to labour under a tem- 
porary amaurosis, while all other parts retain 
their ordinary sensibility. 
In further support of this hypothesis, Mul- 
ler’s researches have convinced him that in 
man single vision by the two eyes occurs only 
when certain parts of the two retinz are affected 
simultaneously, and that under other circum- 
stances double vision ensues. This conclusion 
has been arrived at, chiefly from the results of 
experiments upon the eye-ball. Thus, when 
the eye-lids are closed in a dark room, if 
pressure be applied deeply to the eye so as to 
affect the retina, luminous spectra are produced. 
When certain parts of the two eye-balls are 
subjected to pressure at the same time, a single 
Spectrum occurs, and when other parts of the 
two eye-balls are pressed upon simultaneously 
two spectra appear. Those parts of the two 
retina which in the above experiment furnish a 
single spectrum are styled “ identical,” in con- 
sequence of their identity of sensation; and 
those parts which produce two spectra are de- 
nominated “ non-identical,” for obvious reasons. 
Some of the conditions under which single and 
double vision respectively take place, seem to 
be confirmatory of this doctrine of “ identical ” 
and “ non-identical” parts in the two retine ; 
thus double vision is a common consequence 
of any cause having a tendency to disturb the 
relative directions of the optic axes: for exam- 
ple, diplopia frequently occurs in cases of 
strabismus, and double vision may (according 
to Muller) be produced in perfectly healthy 
eyes by a simple experiment: ifa spectator fix 
his eyes upon an object and then press on one 
of them in such manner as to alter the direction 
of its axis, the object which at first seemed 
single will assume a double appearance. These 
phenomena admit of explanation on the suppo- 
sition that in consequence of the distortion of 
the axes of the eyes, the visual impressions take 
effect on “ non-identical” parts of the two 
retine, which therefore propagate two impres- 
_ sions instead of one to the sensorium. 
These views apparently strengthen Newton's 
hypothesis, for it may be presumed that the 
“ identical” parts of the two retine are those 
OPTIC NERVES. 
which derive their origin from the same tractus 
opticus, and the “ non-identical” on the con- 
trary those which come from different tractus 
optici. 
the comparative anatomy of the nerves in 
uestion furnishes some facts favourable to 
ewton’s hypothesis; thus many animals in 
which the eyes are directed /aterally, in such 
manner that each embraces a totally different 
field of vision, have no chiasma, and their optic — 
nerves cross each other, so that the right retina 
is in connection solely a the left side of the — 
brain, and vice versa. is arrangement. pre- — 
vails in the majority of osseous fish, and, for so 
far, affords negative a of the h esis 
under consideration. following retical 
explanation may be offered. “ In theseanimals, 
owing to the position of their eyes, the same 
object can never be depicted on the two re- 
tine simultaneously ; consequently, in them no 
provision to ensure single vision of the same 
object by both eyes is required, and 
no parts of the two retine have a common con- 
nection with one and the same side of the br 
the two optic nerves being derived respecti 
from opposite sides of the organ.” ; 
Again, Mr. Solly has shown that in r 
fish, such as the skate, (in which the eyes ang 
so set that the respective fields of vision ma 
comprise in a great measure the same ob 
a chiasma exists: and the anatomy of th 
chiasma in birds is likewise on the whole 
vourable to the hypothesis; for in these anit 
the optic axes are in general very diverg 
and consequently the respective fields of visic 
can have but little identity ; a fact which ag 
theoretically with the pam Or ; 
observable in their laminated chiasma. 
Although this explanation of single vi 
has been sanctioned by the authority of N 
and Wollaston, and supported by | 
tomical facts and analogies, it will scar 
stand the test of critical examination ; s 
lidity has therefore been much questioned 
apparently with justice, for the following reasoi 
1. “ Identity of sensation” is not exclus 
a special attribute of the second pair; alt 
it exists in them in great perfection, other ni 
must also be admitted to the same 
perty ; thus notwithstanding that both ear 
commonly employed simultaneously for the 
nary purposes of hearing, thesensation of a 
impression of sound is in general propaga 
the sensorium; and although both nai 
used in the appreciation of odours, the sen 
of single impressions of scents is most 1 
produced: now since neither the § 
nor the auditory nerves are provided ¥ 
chiasma, and nevertheless these nerves 
niably evince a unity of sensation, there 
reason for scepticism when the be sal 
perty in the optic nerves is attribu’ 
sence of a chiasma. . 
2. Many facts in pathology are obvi 
variance with Newton’s theory; if it1 
true explanation of single vision, mort 
tions of one side of the brain (wh 
ductive of amaurosis) ought to imptical 
or less of one half of each retina, wheres 
