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NERVOUS SYSTEM. (Comparative Anatomy.) 
cupy our attention,—the olfactory tubercles, 
situated most anteriorly, the optic lobes, situated 
posteriorly to these, and the cerebellum. 
ist. The olfactory tubercles, or first cere- 
bral mass (figs. 352, 353, a, a, a), now be- 
come obviously the cerebral hemispheres, are 
of an increased proportional size, are com- 
Mencing to cover the tubercula quadrige- 
mina, and contain a cavity which was first 
developed in the Plagiostome fishes ; they are 
Very various as to form. Amongst the Am- 
phibia, in the triton they are elongated and 
oblong; in the frog, more oval they are united 
at their anterior parts by a commissure, but 
posteriorly they are separated. Amongst the 
true Reptilia, in the viper and ringed snake 
poy are of a rounded form, and extended late- 
lly; in the lizard and turtle they are oval 
(figs. 353, 354, a,a); in the crocodile they 
are more extended laterally. On cutting into 
them, in the turtle there is found an oblong 
tubercle analogous to the corpus striatum (fig. 
353, b*), on the inner side of which is a plexus 
choroides. From the anterior part of these 
hemispheres in the different animals mentioned, 
the olfactory nerves arise, and run forwards to 
the cribriform plate of the ethmoid bone, on 
the upper surface of which, in the viper and 
lizard, they form a bulbous enlargement (fig. 
354, g): in the turtle this is wanting, but at 
their origin they form a large round hollow 
Swelling, situated immediately anterior to the 
_ cerebral hemispheres, and communicating with 
the cavities in their interior (fig. 353, i, k, /). 
Fig. 354. 
Brain and portion of spinal cord of La- 
certa viridis (lizard ), slightly magnified. 
a, first cerebral mass or cerebral 
hemispheres. 6, second cerebral mass 
or optic lobes. c, third cerebral mass 
or cerebellum. d, spinal cord, with 
its posterior longitudinal fissure. e, 
fourth ventricle. f, Pineal gland. g, 
olfactory nerves, with their bulbous 
enlargements, 
2d. The optic lobes, or second cerebral mass 
(figs. 353, 354, 6, b, 6), are of small size, 
and are more solid than the same parts in the 
fishes, the internal cavity being smaller: we 
thus see them gradually approaching to the 
form and character of the tubercula quadrige- 
mina of the Mammalia, and of wd Cf In the 
triton, frog viper, ringed snake, lizard (fig.354), 
and er Cig. 353); they are of a rounded 
form, and situated on a plane immediately pos- 
terior to the cerebral hemispheres. In all these 
species there is found also, immediately anterior 
to them, and partly covered by the cerebral © 
hemispheres, a pair of small ganglia, analogous 
’ to the optic thalami of the human brain, on the 
superior surface of which was situated the pineal 
gland (fig. 354). These different eminences 
give origin to the fibres of the optic nerves. 
3d. The third cerebral mass, or cerebellum 
(figs. 353, 354, c), presents some inte- 
resting grades of developement in these two 
621 
classes of animals. In all of them it is small, 
in most of them extremely small, an@:overs 
in the fourth ventricle in a similar manner to 
what has been described in the fishes. In 
the triton and frog it consists of a thin 
transverse band of medullary matter, pre- 
cisely analogous to the cerebellum of the lam- 
prey, and, as in that animal, leaving the fourth 
ventricle quite open and exposed : in the viper 
and lizard (fig. 353) it presents a similar ap- 
pearance, but the band of medullary matter is 
rather thicker; in the turtle (fig. 352) it con- 
sists of a tongue-shaped lobe, very similar to 
the cerebellum of the cod: there are very dis- 
tinct lateral appendages, the rudiments of 
which we first observed in the Plagiostome 
fishes, and which we shall trace in the suc- 
ceeding classes to increased degrees of deve- 
lopement: these lateral appendages are found 
also in the crocodile; they lead us, by strict 
analogies, to the cerebellum of the birds. 
On reviewing these statements of the ner- 
vous system of the Reptilia, we observe that 
the equality and horizontality of the brain and 
spinal marrow again claim our attention as 
marks of low organization. Still, however, 
the preponderance of the spinal marrow over 
the brain is less, while the weight of this latter, 
compared with the body, is greater. The first 
cerebral mass has increased in size; cavities 
are developed in its interior, and it is united 
into two portions, which are divided by a com- 
missure. The second cerebral mass, or tuber- 
cula quadrigemina, has decreased in size, and 
the cavities are much smaller. The third cere- 
bral mass, or cerebellum, is in the lower Rep- 
tilia imperfectly developed, but in the higher 
ones it is of some size, and marked by ex- 
ternal strie. 
4. Aves.—In the class Aves, or birds, the 
nervous centre has acquired a high degree of 
developement in all its parts, but particularly 
as regards the cerebellum, and the different 
portions composing the cerebral mass are ar- 
ranged more above and less behind each other. 
The spinal cord (fig. 355, d) is of less re- 
lative size, and of less extent, than in the fishes 
and reptiles, but it still is traversed by an 
anterior and posterior longitudinal fissure, and 
still contains a central canal. In a pigeon 
weighing (according to Carns) eight ounces, 
(360 grains) the brain weighed 37 grains, and 
the spinal marrow only 11 grains, the propor- 
tion bene as 100:30. We thus observe that 
the brain now preponderates in size over the 
spinal cord for the first time; this at once 
marks its increased developement. Where the 
nerves supplying the anterior and posterior ex- 
tremities are given off, the spinal cord presents 
distinct enlargements, the inferior of which is 
the largest, and is placed in the sacrum : this 
may be considered as the termination of the 
spinal cord, for Carus considers that portion 
passing through the coccygeal vertebra to be 
only a large terminal filament. <A canal passes 
through the whole extent of the spinal cord, 
which, at the inferior enlargement, forms a 
large and remarkable excavation, called the 
