764 
views were still more powerfully advocated by 
Gall and Spurzheim, although they admitted 
that the nerves derive a reinforcement from the 
corpora geniculata externa and the tuber 
cinereum. 
Tiedemann (although fully aware that some 
filaments of the optic nerves are traceable to 
the surface of the optic thalamus both in the 
foetus and adult) yet believed the nates and 
geniculata externa to be the true origins 
of the nerves under consideration, and in this 
opinion he was strengthened by the Report on 
the Memoir of Gall and Spurzheim, made to 
the Institute by Cuvier, Portal, Sabatier, and 
Pinel. 
_ According to Serres the tubercula quadri- 
gemina are the proper sources of the optic 
nerves, and by Leuret the second pair are traced 
to a triple cerebral attachment, viz. the nates, 
testes, and optic thalamus. 
It is proposed to examine in this place some 
of the grounds on which the foregoing opinions 
have been founded, and to this inquiry the aid 
of comparative anatomy is indispensably re- 
quisite. 
Fisu.—In these animals the optic nerves 
are distinctly traceable to two of the ganglia 
which compose the diminutive brain. The 
gunglia in pens are called “ optic lobes,” 
from being the principal sources of the nerves 
of vision ; they are hollow, and their position 
in the brain is between the cerebral hemispheres 
and the cerebellum (fig.407.) The optic lobes 
Fig. 407. 
Brain of a Hake. ( From nature. ) Side view seen from 
below. 
aa, optic nerves; 6b, oblique crossing of ditto ; 
ce, ovtic lobe of left side, being the chief source of 
the right optic nerve; dd, two inferior lobes from 
which the nerves of vision in fishes generally 
derive roots, 
in fish very generally bear proportion to the size 
of the optic nerves (a proof of their physiological 
relations); and this ahaa becomes par- 
ticularly apparent in fish which possess either 
unusually small organs of vision, as the Eel ; or 
eyes of different dimensions, as the Pleuronectes. 
Fig. 408. 
Brain an Eel. (A 
Solly. ae from ibs sa 
a, optic nerve ; bb, o 
tic lobes, which are small, 
being proportional to the 
size of the optic nerves. 
In many kinds of fish the optic nerves de- 
rive some of their-filainents fiom a pair of 
OPTIC NERVES. 
Brain of a Halibut. ( From nature. ) 
A, seen from above. _B, seen from below. — 
__¢, large optic nerve in both ; d, small optic 
in both ; e, large optic lobe in both; f, small 
lobe in both ; gg, inferior lobes ing the 
proportion to each other in size that the oy 
obes exhibit. 
N. B. The optic nerves derive their rot 
from the large lobes, and the small optic nerves 
their origin in the small lobes. = 
tubercles placed on the under surface of 1 
encephalon beneath the optic lobes (fig. 411 
Fig. 410. ) 
The writer does not: 
sume to decide wh 
these tubercles are re 
identical with the m 
millary eminences of 
human brain as m 
tained by Desm 
and others; or 1 
the tuber cineream 
Carus, Spurzheim, 
have’ contended : 
that they have a § 
in the origin of the: 
nerves is certain, 
in those fish whic 
two optic nerves ¢ 
equal size, the tub 
to which allusion is 
Brain of a Ray. (From present espo 
nature. ) Seen from rE differences 7 
aa, optic nerves; by gj Jf 
pre te ec, inferior > tha (fi. Aa 
lobes from which the op- whtbton 
class the optic 
tic nerves derive some of 
their roots ; dd,opticlobes are derived fro 
the principal sources of lobes v 
the optic nerves. those in ; 1 
two in number and interposed bets 
cerebral hemispheres and the cerebellu 
size is proportional to the developeme 
optic nerves, and they are best seen att 
or dorsal surface of the brain (fig. 41 
Birps.—In birds the optic nerves 6 
chiefly in two lobes situated at the 
lateral aspect of the brain, and ca 
class also “ optic lobes.” The 
lobes is in proportion to that of the optie 
and organs of vision, and they are 
ae 
