OPTIC NERVES, 
“The size of the optic thalami is not in 
general in direct proportion either to.that of the 
optic nerves or the acuteness of vision in 
animals. 
__ “In most fishes the optic nerves are of great 
_ size, and the perfection of vision is extreme, yet 
in this class the optic thalami are absent. _ 
“In birds, some of which enjoy exquisite 
» powers of sight, the optic thalami are small. 
__ “ In mammalia the optic nerves bear no fixed 
proportion to the optic thalami; for instance, 
the horse, the ox, and the stag have larger optic 
nerves than the human subject, and yet the 
optic thalami in these animals are infinitely 
smaller than in man.” 
The foregoing arguments are not conclusive, 
for if the want of direct proportion between the 
Optic thalami and optic nerves were a proof that 
the optic nerves draw none of their roots from 
_ the optic thalami, the very same principle would 
deprive the tubercula quadrigemina likewise of 
claim to be considered a source of the 
ed 
question and the tubercula quadrigemina ac- 
ually Occur in inverse proportion to each other. 
In considering this question it should be 
ecollected that in the mammalia large optic 
lalami are always found associated with small 
la quadrigemina, and vice versi; and 
he same remark applies to the optic thalami 
irds, the optic thalami are small, but the optic 
bes are of large dimensions: in reptiles the 
me ee prostione are apparent: in fish the 
ptic thalami disappear, but the optic lobes are 
amense, and two inferior lobes (an additional 
urce of the optic nerves) are superadded. 
hese facts favour the presumption that the 
tie nerves derive roots from the optic thalami; 
if (as is most probable) the optic thalami 
and the tubercula quadrigemina both afford 
Origin to the optic nerves, they may be mutually 
supplemental to each other; and in that case 
l€ reciprocal proportions of these eminences 
ill be a matter of no consequence, provided 
nly that their sum be proportional to the 
farther support of the opinion here adyo- 
Fig. 415. 
fetal brain. (From nature.) Lateral view, 
About fourth month. 
a, a, optic nerves; b, chiasma; ¢, right tractus 
“cus; e, right optic thalamus; f, mass of the 
ercula quadrigemina; g, g, cerebellum; d, 
terior extremity of right cerebral hemisphere, 
placed to exhibit the origin of the optic nerve. 
second pair; since in the Mole and some other \s 
Mammalia, already specified, the nerves in WS 
767 
cated, it should be borne in mind, that the 
tractus opticus is clearly traceable to the surface 
of the optic thalamus inthe human adult sub- 
ject, and the writer’s experienee~ has convinced 
him that the same anatomical disposition is 
very apparent in early foetal life (fig. 415). It 
may be well to add that in all the orders of the 
mammalia which he has had an opportunity of 
examining, the tractus opticus derives filaments 
from the optic thalamus: in the horse, although 
a large proportion of the tractus can be traced 
to the nates, its anterior fibres spring most 
distinctly from the optic thalamus (fig. 416); 
Tubercula quadrigemina, together with portions of the 
optic thalami and tractus optici of a horse. (From 
nature. ) 
a,a, nates; b,b, testes; c,c, optic thalami; 
d, d, tractus optici, springing partly from the nates, 
but deriving a great portion of their roots from the 
optic thalami, c¢, c. ' 
in the sheep precisely the same arrangement 
exists: in the hare many filaments of the 
tractus opticus originate in the optic thalamus: 
and in carnivora and quadrumana a similar 
disposition prevails. 
ecent microscopic discoveries in ovology 
(if it be fair to argue from the developement of 
the chigk to the evolution of the human feetus) 
tend to confirm the views here put forward. 
Baer states that on the fourth day of incubation ‘ 
the encephalon of the chick consists of several 
cells, one of which corresponds to the third 
ventricle, and another to the optic lobes, and 
that these two cells are distinct from each other. 
The first rudiment of the eye observable in the 
chick occurs in the form of a vesicle which 
shoots out from the parietes of the cell of the. 
third ventricle, and which becomes gradually 
elongated and drawn out into a canal. On the 
fourth day the eye represents a spherical cavity 
communicating with the third ventricle by a 
canal ; this canal is the rudimental optic nerve, 
which becomes gradually solid, its cavity disap- 
pearing after the sixth day. During the earlier 
periods of growth there is no connection what- 
ever between the optic nerves and the cell of 
the optic lobes, but the nerves just specified 
are from the very commencement in free com- 
munication with the cell of the third ventricle, 
and in the walls of that cell the optic thalami 
are developed. 
