* Brain of a Turtle. ( From nature.) Lateral view. 
aa, optic nerves ; b, chiasma ; ¢, optic lobes; d, tractus opticus, 
; coanecting the right optic lobe to the chiasma. 
immense in birds of prey, and much smaller in 
NERVES. 
765 
what parts of the mammal’s brain 
are analogous to the optic lobes of 
the lower classes. 
The tubercu drigemina in 
man and the mammalia are iden- 
tical with the optic lobes of the 
lower vertebrata; they occur as 
four small tubercles arranged in 
pairs, of which the anterior are 
called the nates, and the posterior 
the testes. In some of the class, 
as for example, Ruminantia, Soli- 
peda, and Rodentia, the nates are of 
larger dimensions than the testes; in 
others, as for instance, Carnivora, the testes 
other birds not equally remarkable for perfec- predominate in size over the nates, and in 
» 
tion of sight (fig. 412). ° 
; Fig. 412. 
in of an Eagle. ( From nature. ) Seen from below. 
_ @, a, optic nerves ; c, chiasma, of immense size ; 
6, b, optic lobes of large dimensions, placed at the 
inferior and lateral aspect of the encephalon, 
_ The situation of these bodies in the brain of 
the bird, so different from their position in 
eptiles and fish, created at one period some 
doubts as to their true analogies ; but Serres 
as shown that during the early stages of deve- 
ement the optic lobes occupy precisely the 
@ position in the encephalon of the chick 
h they hold permanently in the brain of 
@ reptile and fish, and he has thereby divested 
is subject of much of its obscurity. Thus 
efore the tenth day of incubation the optic 
Tobes of the chick are placed between the cere- 
bellum and the cerebral hemispheres, and are 
| then best seen at the dorsal aspect of the brain : 
ut after this epoch the hemispheres and cere- 
ellum approach each other at the expense of 
(the optic lobes—the hemispheres extending 
backwards, and the cerebellum inclining for- 
| wards. By this double movement the optic 
lobes are soon overlapped behind, separated 
from each other, and at length pushed down- 
‘wards and outwards to their permanent si- 
‘tuation (fig. 413). _ 
~ In Man the optic nerves derive some roots 
— _ from the tubercula quadrigemina. 
In birds, reptiles, and fish, the optic lobes 
lonstitute the principal sources of the optic 
erves, and therefore in any attempt to ascer- 
ain the true origin of the second pair in man, 
a necessary preliminary will be to determine 
Man and Quadrumana the two pairs are nearly 
equal. ‘ 
Brain of achick. (After Serres.) At three different 
stages of incubation. 
A, at sixth day; B, at tenth day; C, at four- 
teenth day. 
A, a, a, optic lobes ; 6, rudimental cerebellum 3; 
¢, ¢, cerebral hemispheres. 
» @, a, optic lobes separated from each other in 
front, and here slightly depressed; 5, cerebellum 
inclining upwards and forwards between optic lobes ; 
c, c, cerebral hemispheres growing backwards so as 
to overlap the optic lobes. 
C, a, a, optic lobes still farther separated from 
each other and depressed towards base of brain; 
b, cerebellum growing upwards between the optic 
lobes ; ¢, c, cerebral hemispheres carried backwards 
so as to come nearly into contact with the cere- 
bellum. Reference to fig. 412 will shew the brain 
of the bird in its full-grown condition. 
The tubercles in question have but little 
apparent similarity to the optic lobes of the 
lower Vertebrata : they occur as four eminences, 
while the optic lobes of birds, reptiles, and 
fish, are but ¢wo in number: they are of dimi- 
nutive size; the optic lobes of birds, reptiles, 
and fish are of large dimensions in proportion 
to the brain: they are solid; the optic lobes of 
birds, reptiles, and fish are Aollow: and in 
Man and most Mammalia they are covered 
upon the upper surface by the cerebral hemi- 
spheres, while the optic lobes in reptiles and 
fish are not so covered. Such obvious dissimi- 
larity tended materially to obscure the real 
nature of the tubercula quadrigemina, but a 
careful study of the developement of these 
bodies in the fcetal brain led anatomists at 
length to discover their true analogies; and 
the researches of Tiedemann and Serres have 
chiefly contributed to establish the following 
particulars. 
“‘ In the earlier stages of uterine life the tuber- 
cula quadrigemina of Man and Mammalia 
