OPTIC NERVES: 
after making their way to the eye-ball, and 
aap the sclerotic, terminate in the retina. 
hese filaments are arranged at their origin in 
a double linear series, each series streaming 
forth from one of the opposite margins of the 
ganglion. After becoming free of the ganglion 
the filaments of each series approach those of 
the other series, and a most perfect and regular 
interlacement of the two sets ensues ; so com- 
pletely do they decussate that either half of the 
_ Yetina is formed by filaments which spring 
_ from the opposite edge of the ganglion ; those 
filaments which shoot out from the anterior 
margin of that body terminating in, and actu- 
ally forming, the posterior moiety of the retina, 
and vice versa. 
The reciprocal interlacement of these nervous 
filaments reminds one of the manner in which 
__ the two series of threads in a weaver’s loom 
_ (technically called the woof and the warp), cross 
each other; this illustration is exact, for the 
_interlacement of the threads in the one case is 
not more perfect or regular than that of the 
nervous filaments in the other. 
The writer is indebted to his colleague, Dr. 
Power, for a knowledge of this curious arrange- 
ment, which was discovered by that gentleman 
_ while inspecting a preparation of the nervous 
system in the loligo made by the writer, and 
which still exists in the Museum of the Rich- 
mond Hospital School, Dublin. 
__ Subsequent dissections have proved satisfac- 
torily that the decussation in question occurs 
‘variably in the loligo, and that in the octopus, 
which exhibits close affinity to the loligo, the 
same arrangement prevails, In the sepia 
officinalis the optic ganglion is reniform, and 
_ its filaments come off in a double series, as in 
- the loligo ; the majority of the filaments decus- 
" Sate after the same manner as that above de- 
| scribed in the other two species, but some of the 
| extreme filaments of each series pass in a direct 
| course to the retina, without exhibiting any de- 
_cussation. 
__ The peculiar disposition of the optic filaments 
in these cephalopods was unnoticed until the 
| publication of Dr. Power’s paper on the sub- 
| ject in the “ Dublin Journal of Medical Science,” 
—an omission the more surprising, as accurate 
descriptions of the eye in this class of animals 
jn appeared from some of the ablest ana- 
_ Swammerdam long ago described and de- 
ineated the optic ganglion of the sepia offici- 
| nalis, but without making any mention of the 
| mtercrossing of the filaments which emanate 
from that body. Cuvier, in his memoir on the 
Mollusca, describes the eye of the poulpe 
Aectopus) with great accuracy, but makes no 
allusion to the decussation in question ; and in 
the most recent descriptions of the cephalopods 
the same omission occurs. , 
_ The above discovery seemed at first likely 
to afford an explanation of the problem so 
\difficult of solution, namely, the, well-known 
, that objects appear erect to a spectator 
jalthough their images on the retina are inverted ; 
and for some highly ingenious observations 
tending to elucidate this obscure question, the 
ck i 
reader is referred to the paper by Dr. Power, 
already quoted; it may be stated, however, that 
many difficulties remainto_be removed before 
such an explanation can be received, for 
1. The filaments of the optic nerve of the 
loligo have not as yet been traced back to the 
central masses of the nervous system, and there 
can therefore be. no certainty that they preserve 
in the first part of their course the same relative 
positions which they are known to maintain in 
the interval between the optic ganglion and 
point of decussation. 
2. In vision the object is known to be wholly 
reversed in the image on the retina; for ex- 
ample, that which is above in the object is 
below in the image, and vice versa; and that 
which is to the right in the object is to the left 
in the image, and vice vers’; now, although 
the decussation in question might possibly 
explain the correct appreciation of an object 
whose image is reversed in one particular 
direction (say from above downwards), it never 
can fully explain the correct impression made 
by an object of which the image is reversed in 
all directions simultaneously. 
3. In order that this explanation may apply 
to human vision, the same interlacement of 
filaments must first be demonstrated in the optic 
nerve of man, a task which has not as yet been 
accomplished. 
Optic nerves of the compound eyes of Insects. 
These nerves are excessively large and appear 
proportional to the size of the organs of vision 
in the Insect. 
Each nerve on arriving at the eye swells out 
into a bulb, which is convex and varies con- 
siderably in dimensions in different species ; 
this bulb in general represents a segment of a 
sphere, and from its surface nervous filaments 
in immense numbers arise and diverge like the 
radii of a circle. Several thousand of these 
filaments have been counted in a single nerve ; 
each of them is connected by its distal extremity 
to the apex of a small conical transparent body 
interposed between the filament and the cor- 
responding facet of the cornea, but the most 
striking péculiarity of the nerve is found in the 
perfect isolation of each of these filaments by 
the interposition of pigment. 
The pigment is variable in colour, being 
sometimes light, at others dark; it may be 
nearly black, dark violet, dark blue, purple, 
brown, brownish yellow, light yellow, or green, 
and sometimes several layers of different colours 
lie one over the other; the pigment extends 
from the bulb of the nerve to the cornea ; it sur- 
rounds each filament and separates it com- 
pletely from adjacent filaments. 
The effect of this disposition is to isolate the 
rays of light incident on each filament, and 
to prevent the transmission of all rays except 
such as fall in the direction of the axis of the 
filaments ; since all oblique rays must of neces- 
sity impinge upon the colouring matter and be 
in consequence absorbed. 
The reader is referred to the article Insecta 
for further details concerning the optic nerve in 
these animals. 
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