770 
part of their course. The conclusions arrived 
at by such modes of investigation were un- 
satisfactory, and remarkable for much dis- 
crepancy. 
1. Some maintained that the nerves are 
merely placed in exceedingly close juxta- 
position in the chiasma, without any inter- 
crossing of their respective filaments, and that 
each tractus opticus in reality passes on to form 
the optic nerve of its own side. 
These views were supported by Vesalius,* 
who detailed thé particulars of a case in which 
after death the two optic nerves were. found 
perfectly distinct from each other throughout 
their whole course, and consequently no chiasma 
existed, although, during life, vision had been 
unimpaired ; and the same hypothesis was 
Strengthened by Santorini and others, who, in 
certain instances where one eye had been 
destroyed many years before death, observed on 
the post mortem examination the correspond- 
ing optic nerve atrophied as far back as the 
chiasma, and the tractus opticus of the same 
side wasted, while the nerve and tractus of the 
opposite side were perfectly healthy. 
2. Others were persuaded that a perfect de- 
cussation exists in the chiasma, and that all the 
filaments of the tractus opticus of one side pass - 
fairly across to form the optic nerve of the other, 
and vice versa. In favour of this opinion it 
was urged that in the majority of cases of long- 
continued blindness of a single eye the opposite 
tractus opticus, and not the tractus on the sume 
side with the affected eye, becomes atrophied. 
Scemmerring observed several such cases in the 
human subject, and traced the same appearances 
in the horse, dog, squirrel, rabbit, hog, cat, and 
chamois: and Cuvier preserved in spirits the 
brain of a horse in which the wasting of one 
optic nerve continued backwards into the oppo- 
site tractus. The evident manner in which the 
optic nerves in osseous fish cross each other (see 
fe. 407) was also considered favourable to this 
view, and the results of experiments on living 
animals were confidently appealed to in farther 
confirmation of the same. 
of Rolando, Pourfour Petit, Saucerotte, Hertwig, 
Flourens, and others led to the conclusion, that 
if in the Mammalia one hemisphere of the ce- - 
rebrum be injured deeply or removed, vision 
becomes impaired or destroyed in the opposite 
eye; if the ¢wo cerebral hemispheres be suc- 
cessively subjected to the same treatment, vi- 
sion becomes successively impaired or destroyed 
in both eyes; if one of the nates be removed, the 
‘animal sees no longer on the opposite side; and 
if both be removed, blindness affects both eyes 
in succession and occurs constantly in the eye 
opposite to the injured tubercle; if in birds one 
of the cerebral hemispheres be removed, vision 
becomes extinguished in the opposite eye ; if the 
two hemispheres be removed in succession, a 
cross paralysis affects the éwo retine : if one of 
the optic tubercles be removed, the sight of the 
opposite eye fails, and if both be removed, per- 
fect blindness ensues. 
3. Another class of physiologists believed 
* Vesalius de Corp, Hum. Fab., |. iv. c. iv. 
hus the experiments - 
OPTIC NERVES. 
that a partial decussation occurs in the human 
chiasma, and that some of the filaments of each 
tractus opticus continue on into the nerve of their 
own side, while others cross obliquely into the 
optic nerve of the opposite side. facts in 
pathology seemed difficult of explanation on — 
any other supposition ; thus cases of long-con- ~ 
tinued blindness of a single eye, the result of 
accident, have been met with, in which after 
death the corresponding optic nerve was four 
atrophied se fir back ap the chiaiiall (the optic 
nerve belonging to the healthy eye being of 
fully its ordinary dimensions, or 
than natural) while both tractus optici 
wasted. : an 
That a partial decussation does 
human chiasma can now no longer 
section, and it can be rendered a to 
naked eye by the precaution of teardening the 
nervous substance before the dissection is com: 
menced. According to the writer’s experience 
immersion in mie hardens the prereaaam 
the most satisfactory manner. performing 
the dissection the neurilemma must be first care- 
fully removed from the chiasma, and also \ 
the adjoining portions of the optic nerves and 
tractus optici ; after this pein optic 
nerve should be divided transversely a little in 
front of the chiasma ; a transverse incision car- 
ried horizontally into the cut surface of thi 
cerebral extremity of each nerve will ther 
enable the operator to split it in a directiot 
backwards towards the chiasma, and by pro. 
ceeding cautiously a horizontal division of th 
chiasma, and of a part of the optic nerves am 
. ¢ Fig. 419. 
E 
: 
the tractus optici, will be effec 
The annexed 
Fig. 419. gure represents — 
reparation made 
the manner de 
bed, and it is esse 
tially similar to 
diagram of the h 
man chiasma pt 
lished some ye 
Sy pines 
Human Chiasma. ( Froma dis- geet: ar 
section made by the writer. ) of ‘course fate 
aa, opticnerves ; 6b, trac- but the directic 
tus optici. the larger fast 
admits of not the slightest question; the 
fasciculi of each tractus opticus cont 
onwards without interruption to form the 
of the optic nerve of the same side 
middle fasciculi cross the chiasma oblis 
and after decussating the corresponding fé 
of the other tractus, contribute to the for 
of the optic nerve of the opposite side; 2 
internal fasciculi cross the posterior part 
chiasma transversely and uniting direct 
the corresponding fasciculi of the other’ 
seem to be strictly commissural; acro 
front of the chiasma some fasciculi ta 
arched course, and being prolonged ~ 
along the inner edges of the optic nen 
are likewise apparently commissural. — 
7-2 
a 
