620 
the mass of nervous matter, it is greatly in 
favour of the spinal marrow, though, as regards 
complexity of structure, the brain preponde- 
rates. Again, the extreme smallness of this 
latter compared to the rest of the body, the 
simple formation of the different masses com- 
posing it, and the predominance of the median 
one, (which in the lower animals is the only 
one developed,) are points that also mark its 
low degree of developement. Still the ground- 
work of the most important structures has been 
laid, and we shal! trace these identical parts in 
the succeeding classes of animals through vari- 
ous modifications of form and phases of deve- 
fopement. 
2and 3. Amputpra anp Reptiria.—We 
now proceed to the Amphibia, the Batrachia of 
Cuvier, which, in a system of arrangement, 
must be considered as a class distinct from the 
true Reptilia; but their nervous system pre- 
senting so great a similarity in structure and 
conformation to that class, and, indeed, differ- 
ing only in an inferiority of developement, we 
will, to save time and space, notice the two 
classes of Amphibia and true Reptilia con- 
jointly. The nervous system in these animals 
bears a great similarity in structure and deve- 
lopement to the fishes. 
The spinal cord presents much the same cha- 
racter as in the class just described, with regard 
to its relative size, its extent, (excepting in the 
frog,) and its physical conformation. Ina 
species of Triton weighing 39 grains, the spinal 
marrow weighed } grain, and the brain only 4 
grain, the proportion being as 100 to 180. We 
thus observe that the weight of the spinal mar- 
row preponderates over that of the brain, al- 
though not to so great an extent as in the fishes, 
in consequence of the increased developement 
of the latter. In most of the Amphibia, and 
in all the Reptilia, the spinal cord passes down 
the whole length of the caudal vertebree, as in 
the fishes, but to this the frog forms an exce 
tion. In that animal it descends no lower in 
its adult state than barely midway between the 
anterior and posterior extremities, and termi- 
nates by a few nervous filaments, which pass 
downwards towards the sacrum; in the young 
and tadpole state, however, it is prolonged 
into the coccygeal vertebre, and terminates in 
a point. The form and structure of the spinal 
cord, and of the medulla oblongata, differ but 
little from what has been described in the 
fishes. In the triton and frog there is a lon- 
gitudinal fissure on its anterior and posterior 
aspects and a central canal communicating with 
the cavity of the fourth ventricle which is very 
large, covered over by a vascular plexus, is 
formed in the same manner, and bears great 
resemblance to the fourth ventricle described 
in the lamprey: in the lumbar region the 
spinal cord is thickened where the nerves of 
e extremities are given off; in the tadpole state, 
however, no such enlargement is visible. 
Amongst the true Reptilia, in the ringed snake 
( Coluber natrix ), lizard ( Lacerta viridis), and 
turtle ( Testudo mydas, fig. 353), the spinal cord 
has an anterior and posterior longitudinal fis- 
sure, and a central canal (g) communicating 
NERVOUS SYSTEM. (Comparative Anatomy.) 
with the fourth ventricle (/), which in the 
ringed snake and lizard is small, but deep ; in 
the turtle, large, but shallow, and partly €o- — 
vered in by the cerebellum. According to~ 
Bojanus,* the spinal cord in the Chelonia be- 
comes enlarged where the nerves for the ante- 
rior and posterior extremities are given off, and 
very thin between those enlargements. Carusf 
has observed the same enlargements, but ii 
less degree, in a young crocodile. 
The brain is composed of a suite of gangliz 
approaching very much in form and charac 
to the fishes, especially the Rays and Sharks. 
In the triton (Triton cristata), frog (Ran 
temporaria), 
sae 
viper (Coluber verus), rin, 
snake (Coluber natrix), lizard (Lacerta viridis, 
fig. 354), and turtle (Testudo mydas, fig. 353) 
ig. 353. a, first cerebral mass 
| 
| 
or cerebral hemisph 
__ b*, first cerebral mass ex 
open, shewing its inte 
nal cavity and tub 
b, seconu 
or opticlobe, c*, 
cerebral mass cut op 
shewing the small inte 
nal cavity. c¢, third ce: 
bral mass or cerebellui 
ing cavit 
l, bristle shewing t 
communication betw 
the cavity of the olfacto 
nerve and the cer 
hemisphere. m, 
shewing the commu 
tion between the ¢ 
of the optic lobe and 
fourth ventricle. m,t 
raise it upware 
the fourth vent: 
distinctly, 
Brain and portion of spinal marrow o; 
gr pet fos: natural  & el 
it fills the cranial cavity destined to rece? 
though that cavity is very small when con 
with the whole head ; thus the size of thé 
is no criterion for the size of the brain 
weight, too, when compared with the bor 
another proof of its small size. In a 
weighing npwenia of 50 pounds, the 
(with the olfactory nerves, and a very 
portion of the spinal marrow), weighe 
77 grains, the proportions beingas 100:4¢ 
and, as before observed, in a triton weighi 
grains, the brain weighed only } grain, the 
portions being as 100 : 27,300. a 
On taking a general review of its struct 
we find, as before, three principal parts to 
* Anatome testudinis Europea. 
+ Op. cit. vol. i, p. 78. 
