Bauhaus-Universität Weimar

Titel:
The Cyclopaedia of Anatomy and Physiology, vol. 3: Ins-Pla
Person:
Todd, Robert Bentley
PURL:
https://digitalesammlungen.uni-weimar.de/viewer/image/lit29464/628/
620 
NERVOUS SYSTEM. (Comparative Anatomy.) 
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 shall trace these identical parts in 
the succeeding classes of animals through vari¬ 
ous modifications of form and phases of deve- 
iopement. 
2 and 3. Amphibia and Reptilia.—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. In a 
species of Triton weighing 39 grains, the spinal 
marrow weighed J grain, and the brain only ^ 
gram, the proportion being as 100 to 180. We 
thus observe that the weight of the spinal mar¬ 
row preponderates over that of the brair , 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 vertebræ, as in 
the fishes, but to this the frog forms an excep¬ 
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 vertebræ, 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 
the 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 viridisj, and 
turtle ( Testudo my das, fig. 353), the spinal cord 
has an anterior and posterior longitudinal fis¬ 
sure, and a central canal (g) communicating 
with the fourth ventricle ([h), which in tire 
ringed snake and lizard is small, but deep ; in 
the turtle, large, but shallow, and partly co¬ 
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 in a 
less degree, in a young crocodile. 
The brain is composed of a suite of ganglia 
approaching very much in form and character 
to the fishes, especially the Rays and Sharks. 
In the triton (Triton cristata), frog (Rana 
temporaria), viper (Coluber verus), ringed 
snake (Coluber natrix), lizard (Lacerta viridis, 
fig. 354), and turtle (Testudo mydas,^%. 353), 
a, first cerebral mass 
or cerebral hemispheres. 
b*, first cerebral mass cut 
open, shewing its inter¬ 
nal cavity and tubercle. 
b, second cerebral mass 
or optic lobe, c*, second 
cerebral mass cut open, 
shewing the small inter¬ 
nal cavity, c, third cere¬ 
bral mass or cerebellum, 
turned slightly upwards. 
f, posterior longitudinal 
fissure of spinal cord, g, 
central canal of spinal 
cord, h, fourth ventri¬ 
cle. i, olfactory nerve. 
k, bulbous enlargement 
at the origin of the olfac¬ 
tory nerve cut open,she w- 
ing its internal cavity. 
l, bristle shewing the 
communication between 
the cavity of the olfactory 
nerve and the cerebral 
hemisphere, m, bristle 
shewing the communica¬ 
tion between the cavity 
of the optic lobe and the 
fourth ventricle, n, bris¬ 
tle passed along the cen¬ 
tral canal of the spinal 
cord, o, bristle passed 
under the cerebellum to 
raise it upwards, to shew 
the fourth ventricle more 
distinctly. 
Brain and portion of spinal marrow of Testudo my das 
( turtle), about natural size. _ 
it fills the cranial cavity destined to receive it, 
though that cavity is very small when compared 
with the whole head ; thus the size of the head 
is no criterion for the size of the brain. Its 
weight, too, when compared with the body, is 
another proof of its small size. In a turtle 
weighing upwards of 50 pounds, the brain 
(with the olfactory nerves, and a very small 
portion of the spinal marrow), weighed only 
77 grains, the proportions being as 100:454,500 ; 
and, as before observed, in a triton weighing 39 
grains, the brain weighed only ± grain, the pro¬ 
portions being as 100 : 27,300. 
On taking a general review of its structure, 
we find, as before, three principal parts to oc- 
* Anatome testudinis Europeæ. 
f Op. cit. vol. i. p. 78. 
Fig. 353.
        

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