766 
occur in the form of two masses; they persist 
as such during two-thirds of foetal existence; 
they are hollow at first and not covered by 
the cerebral hemispheres, and their size is im- 
mense in proportion to the bulk of the ence- 
phalon. As developement advances, a trans- 
verse groove appears on the surface of the 
future tubercula quadrigemina; this divides 
them into four eminences, which are now for 
the first time really entitled to be called “ qua- 
drigemina :” nervous matter is gradually depo- 
sited from within on their walls, in conse- 
om of which they henceforth become solid ; 
their growth in size is arrested, and the» cere- 
bral hemispheres having grown backwards, 
overlap and conceal them from view. This 
overlapping occurs in all except a few of the 
lowest families of the mammalia, in which the 
tubercula quadrigemina remain permanently 
uncovered. 
From the foregoing exposition it appears 
that during their ae Rca the tubercula 
quadrigemina in man and mammalia assume 
tora time all the characters which the optic 
lobes of birds, reptiles, and fish exhibit in the 
anent condition ; and hence it can scarcely 
questioned that the nates and testes of the 
former class are identical with the optic lobes 
of the latter animals; but since the optic 
nerves in the oviparous Vertebrata are trace- 
able to the optic lobes and manifestly derive 
from them the greater proportion of their roots, 
there is so far prima facie evidence that the 
optic nerves in man have their origin in part 
from the tubercula quadrigemina. In further 
confirmation of the same view it may be re- 
marked that some of the roots of the optic 
nerves in certain orders of the mammalia are 
seen to spring from these bodies ; for example, 
in the horse a large proportion of the nerves 
can be traced distinctly to the nates. In Ro- 
dentia and Carnivora numbers of the fibres of 
the nerves emanate obviously from the same 
ir of tubercles, and in the Ruminants a simi- 
anatomical arrangement prevails. 
As an additional proof Tiedemann asserts 
that although much difficulty is encountered in 
attempts to follow the optic nerves to the tuber- 
cula quadrigemina in the adult human subject, 
he has succeeded in tracing them to the nates 
in foetuses of the third month, and at the fourth 
and fifth months he has frequently repeated 
the same observation. 
Human pathol would seem to furnish 
“some corroborative facts: thus in every case of 
long-continued atrophy of the optic nerve, 
where the wasting had involved the tractus 
opticus, Gall and Spurzheim found the nates 
of the side corresponding to the diseased tract 
diminished in size; and the experiments insti- 
tuted upon living animals with a view to deter- 
mine the functions of the several constituents 
of the brain by the successive removal of the 
different of the organ and careful obser- 
vation of the disturbance thereby produced, 
lead also to the belief that the optic nerves have 
an origin in the tubercula quadragemina. Of 
course great allowance must be made for inac- 
curacy in the result of such mutilations, but 
OPTIC: NERVES. 
Flourens, Magendie, Desmoulins, and Hertwig, 
all agree that destruction or mutilation of the 
nates and testis of one side invariably produces 
blindness of the opposite eye. 
The writer fully with Cruveilhier in — 
the belief that the optic nerves in the human 
subject can be rarely traced to the tuber- 
cula quadrigemina satisfactorily ; but never- — 
theless with the above facts before them, — 
anatomists can scarcely refuse to allow that 
the optic nerves in man derive a share of — 
a roots from these eminences. a 
he tubercula igemina probably fulfil 
other purposes leeides that of affording origin 
to the optic nerves. : : 
This may be inferred from the fact that the 
optic nerves are not invariably d in 
direct proportion: to the tubercles ; in 
certain mammals which are either devoid of 
optic nerves altogether, or in which they 
are so excessively diminutive as to be with 
difficulty discovered, the tubercula quadri- 
gemina are as large and perfect as in othe 
allied species possessed of well-marked org 
of vision. 
The nates and testes, ¢ 
bercula quadrigemina, 
of immense size ; 
bral hemispheres are sm 
The common mole, for example, has ¢ 
diminutive and imperfect in structure, and | 
subterranean habits bespeak so little neces 
for organs of vision, that many excellent ar 
mists believe it to have no optic nerves; ne 
theless the tubercula quadrigemina in | 
animal are of immense size. ( Fig. 414.) Ser 
never could satisfy himself that the mole 
sesses optic nerves, although he examined th 
or forty specimens for the express purpe 
if they do exist (as has been maintaine 
Carus and Treviranus) their minuteness 
be almost microscopic. (See Lysecrn 
vol. ii. fig. 453.) 
Other examples confirmatory of the | 
views are afforded by the mammalia; 
stated on the authority of Serres that it 
rat-mole of the Cape, and the Zanni, or 
rat-mole, there is no appearance whate 
proper optic nerves, (the rudimental é es 
supplied by the fifth pair,) and yet in 
animals the tubercula quadrigemina ex 
great perfection. ; Pe 
The human optic nerve probably de: 
JSrom the optic thalamus, 
The writer is of opinion that modern 
mists have fallen into error in supposil 
none of the roots of the second pair are dt 
from the optic thalamus, although the 
ments by which that supposition has 
pacar lly are sufficiently imposing, viz. + 
a 
