a 
4 
NERVOUS SYSTEM. (Comparative Anatomy.) 625 
rous than in the preceding classes ; the fissure 
of Sylvius is a deep groove, marking the divi- 
sion into anterior and median lobes, and here, 
for the first time, are observed the posterior 
lobes (a* ), as yet but of small size, narrowed 
posteriorly, and free from convolutions. In 
the orang-outang they are altogether larger, and 
more approaching the form and character of 
the human brain, covering the cerebellum en- 
tirely, and convoluted on their posterior lobes.* 
_ These cerebral hemispheres are united by an 
important commissure, which makes its first 
appearance in mammiferious animals, the cor- 
; _ callosum ; in the lower orders, as in the 
t, 
rabbit (fig. 358, 6), rat, and mouse, it 
is very short,—shorter even than the tubercula 
quadrigemina ; in the pig, ass, and sheep, it is 
longer and broader; in the stoat, cat, and 
monkey (fig. 359, 6) it is increased in length 
and width, approaching the characters of the 
_ corpus callosum in the human adult brain. 
D cutting into the cerebral hemispheres, 
cavities are found in their interior, the lateral 
ventricles. In the bat and rodent animals, as 
in the rabbit (fig. 358, c), they are of small 
size, but large in proportion to the hemi- 
Spheres; in the pig, sheep, stoat, and cat they 
are larger and broader, but smaller in propor- 
tion to the hemispheres. In all these animals 
the anterior and descending cornua are obser- 
vable; the posterior are found only in the mon- 
key (fig. 359, c), where the lateral ventricles 
quite pepech the characters of the same parts 
in the human adult brain. In the interior of 
these ventricles are to be observed the corpora 
Striata, tenia (for the first time observable), 
Optic thalami, and fornix. In the bat genus 
and Rodentia, the corpora striata are very 
large, forming, indeed, the greater parts of the 
hemispheres of the brain, and the tenia very 
natrow (fig. 358, d, e); in the pig, sheep, and 
cat they are oblong and smooth; in the mon- 
key they were also oblong (fig. 359 d, e), and 
though in reality large, appear smaller, when 
compared with the hemispheres, than in the 
preceding classes, which apparent defects 
of relation Tiedemann considers evidently to 
depend on the greater augmentation of the 
hemispheres. The fornix, with its appendages, 
is for the first time observable in this class of 
animals, and exists in the brains of all the 
animals before mentioned ; in the lower orders, 
its relative size, particularly of the hippocampus 
major, is somewhat considerable. 
rom the anterior part of these cerebral 
hemispheres the olfactory nerves arise, which 
still possess many points of extreme interest. 
In the dolphin and other Cetacea, they are 
entirely wanting. In all the mammiferous 
animals before enumerated,-except the Quadru- 
mana, they consist of oblong or rounded me- 
dullary masses, situated on the cribriform plate 
of the ethmoid bone, from which filaments are 
given off to be distributed on the pituitary 
membrane. In the lower orders, as in the bat, 
* For the length, by measurement, of the cere- 
bral hemispheres in these different animals, see 
the table. 
VOL. ITI, 
rabbit, rat, and mouse, these masses o& zanglia 
of the olfactory nerves are situated ona plane 
directly anterior to the cerebral hemispheres, 
and may be seen on looking upon the superior 
face of the brain, these latter not being yet 
sufficiently developed anteriorly to cover them; 
in the pig they are nearly covered by the 
hemispheres; in the horse, ass, sheep, and 
deer, they are quite covered by them, and are 
only to be seen on the inferior surface of the 
brain; in the cat they are similarly situated, 
but the anterior edge of the hemispheres pro- 
jects still further beyond them. In all these 
animals a medullary band or tract (A) con- 
nects them with the median lobes of the hemi- 
spheres, and in all they contain cavities (i), 
which communicate with the lateral ventricles. 
In the monkey the olfactory nerves (processes) 
consist of free, flattened, medullary bands situ- 
ated on the inferior surface of the anterior 
lobes of the brain, precisely the same as in the 
human adult brain. 
2dly. The optic lobes, or second cerebral 
mass, or, as they are now to be called, the 
tubercula quadrigemina, consist of an anterior 
and posterior pair of ganglia, in which cavi- 
ties are no longer perceptible. They differ 
in size, relatively to each other as well as 
to the cerebral hemispheres, and in position. 
In the bat, rabbit (fig. 358, k), rat, and 
mouse, the anterior pair are the larger, 
and, compared with the cerebral hemispheres, 
are very voluminous; in the pig, horse, ass, 
sheep, and deer, the anterior pair are also the 
larger, but they are of less proportional size 
with the brain; in the cat and stoat the pos- 
terior pair are the larger; in the monkey they 
are nearly of equal size and present less relative 
volume, thus approaching very much the cha- 
racters of the tubercula quadrigemina in the 
human adult brain. With regard to their po- 
sition, as before observed, in the lower orders 
they are situated behind the cerebral hemi- 
spheres and are quite exposed, while in the 
higher orders they are situated underneath the 
hemispheres, and quite covered by them, as in 
the human adult brain. 
3dly. The cerebellum, or third cerebral 
mass (figs. 358, 359, 1), is remarkable for 
its great developement; nevertheless, it passes 
through many grades in the different orders. 
In the animals before enumerated it is marked 
externally by transverse striz and small con- 
volutions, and presents a division into me- 
dian and lateral lobes. The relative size of 
the mass itself, and of its different parts, and 
the number of external strie, differ accord- 
ing as the animal examined is high or low 
in the class. In the bat it is within half a 
line as long as the cerebral hemispheres, the 
popes being as 100: 125; the lateral 
obes are just observable, smooth on their sur- 
face, but on the large median portion there are 
two transverse strie. Inthe rabbit (fig. 358, 1) 
its proportional length in the median portion to 
that of the cerebral hemispheres is as 100: 207; 
in the rat, as 100: 166. The lateral lobes in 
both are more distinctly developed, and the 
strie are better marked. In the horse its pro- 
25s 
