774 
rally throughout the animal series ; on the con- 
trary, the chiasma usually conforms to the type 
evalent in the class to which the animal 
longs, without evincing in its conformation 
much regard to the relative directions of the 
optic axes ; and examples are not unfrequent in 
which the anatomy of the optic nerves is at 
variance with what the relative directions of the 
optic axes would require theoretically. 
Thus in the pleuronectes fish, (fig. 409,) the 
optic axes are so directed that the two retinee may 
be inferred to have a certain amount of identity, 
and nevertheless, in such of them as have been 
examined by the writer, the optic nerves are 
severally derived from opposite sides of the 
brain, and cross each other without forming a 
chiasma; or in other words, retine evincing 
mutual identity are supplied by optic nerves 
which have no identity of origin; the type pre- 
valent in osseous fish being preserved, without 
respect to the directions of the optic axes. In 
many of the cetacea, the direction of the 
optic axes is such that the retin can have no 
identity, and nevertheless a perfect chiasma, 
such as occurs in other mammalia, exists in 
these animals. And in the owl the eyes look 
more directly forwards than those of most other 
birds, from which it may be presumed that the 
amount of mutual identity in the two retine is 
much greater in them than in those birds whose 
eyes have a lateral aspect; but nevertheless 
the structure of the chiasma in the owl appears 
in nothing different from that which prevails in 
birds whose optic axes have a strictly lateral 
direction. 
But further, when the optic axes are very 
divergent, as in some quadrupeds, any object 
which can be depicted upon both retine simul- 
taneously, will throw its images on the outer 
parts of the two retin, and in order to explain 
Fig. 423. 
@ 
am to shew how in eyes with di 
axes, 
the images of every object placed so as to be seen by 
both eyes si fh on the outer parts of the 
two retine. (After Miller. ) 
x, 2’, y,y, axes of the eyes diverging, a,b,c, 
objects seen by both eyes; a’, a”, parts ot the two 
retine on which the object a is depicted; J’, b”, 
parts of the two retine on which the object 5 is 
depicted ; c’, c’, parts of the two retin on which 
the object c is depicted. 
OPTIC NERVES. 
single vision under such circumstances, the 
outer of the two retine should therefore 
be reciprocally identical; but these are 
formed by the outer filaments of i 
nerves, which come respectively from the cor- 
responding sides of the brain, so that here, 
reciprocally identical parts of the two retine, 
instead of having a common origin at one and 
the same side of the brain (as’ they in 
order to suit theoretical views), are derived seve- 
rally from opposite sides of the organ, and con- 
sequently bave no identity of origin (, 
These considerations are sufficient to 
explanation of single vision put forward ; 
and the writer is of opinion that hitherto the 
exact use of the chiasma has not been disco- — 
vered. (See Visron.) i 
Some remarkable varieties of optic nerves. — 
Optic nerves in certain c —The — 
loligo or calamary exhibits in its optic nerves 
a singularly beautiful arrangement, which the — 
physiologist cannot but contemplate with the 
greatest interest, as it presents the most perfect — 
decussation of nervous filaments hitherto dis-— 
covered. The loligo, in common with some of 
the allied families of the cephalopoda, posse 
an extremely perfect organ of vision; so elabo- 
rate is the mechanism of the. eye that it h 
attracted a considerable share of attention from 
comparative anatomists ; and the developement 
of the optic nerve bears proportion to the per- 
fection of the other parts of the visual apparatu 
The nervous system of the loligo conform: 
to the cyclogangliate type, and from each late. 
ral surface of the supracesophageal ganglion 
(fig. 424,) one of the optic nerves comes 0 
After pursuing a short course outwards th 
nerve swells into a large ganglion (the optic) 
this body is oval in shape, and of enormou 
dimensions ; it exceeds considerably the volu 
of the supracesophageal ganglion’ One su 
of the optic ganglion is directed towards 
eye, and emits an immense number of filamen 
which spring chiefly from its edges, and 
Organ of vision, together with the optic nere 
supra-@. lion in the loligo, 
, nified, Frees Sinncton iy the writer.) 
a, supra-cesophageal ganglion; 6, optic | 
c, optic ganglion; d, d, one series of fi 
springing from the edge of the ; 
second orcs ¢ Slemente foe ( | 
posite edge of the ganglion, and in view 
Sear thelr tersaisation only ; f, f, points at’ 
ies decussate 5 — 
the filaments of the two series 
of the eye. 
