As fishes respire through the intervention of water alone, that is, as
they can scarcely avail themselves, in rendering their blood _arterial_,
of anything more than the small portion of oxygen contained in the air
which is suspended in the water, their blood is necessarily cold, and
the general energy and activity of their senses are by no means so great
as those of quadrupeds and birds. Their brain also, though of similar
composition, is proportionally much smaller, whether as compared with
the total size of the body, with the mass of nerves which proceed from
it, or with the cavity of the cranium in which it is contained. In the
turbot (_Gadus lota_) for example, the weight of the brain to that of
the spinal marrow is ascertained to be as 8 to 12, and to that of the
whole body as 1 to 720; and it has been ascertained that the brain of a
pike, weighed in proportion to the whole body, is as 1 to 1305. Now, in
many small birds, the brain, viewed in relation to the rest of the body,
is equal to a twentieth part. In the generality of fishes, the spinal
cord extends along the whole of the caudal vertebræ, and it is thus that
it preponderates over the brain; but the fishing frog, or sea devil
(_Lophius piscatorius_), the moon fish (_Lampris guttatus_), and a few
others, form exceptions to this rule, the spinal marrow disappearing
before it reaches the eighth vertebra. The brain of fishes by no means
fills up the cavity of the cranium; and the interval between the _pia
mater_, which envelopes the brain itself, and the _dura mater_, which
lines the interior of the skull, is occupied only by a loose
cellulosity, frequently impregnated by an oil, or sometimes, as in the
sturgeon and thunny, by a more compact fatty matter. It has also been
remarked that this void between the cranium and the brain is much less
in young subjects than in adults; from which it may be inferred that the
brain does not increase in an equal proportion with the rest of the
body. Cuvier, in fact, has found its dimensions nearly the same in
different individuals—of the same species—of which the general size of
one was double that of the other.
Although we should be sorry to lower the subjects of our present
observation in the estimation of society, we think it undeniable that,
of all vertebrated animals, fishes exhibit the smallest apparent
symptoms of refined sensibility. Having no elastic air to act upon, they
are necessarily mute, or nearly so, and all the sweet sensations which
the delightful faculty of voice has called into being among the higher
tribes, are to them unknown. Their glazed, immovable eyes, their fixed
and bony faces, admit of no playful range in their physiognomical
expression, of no variation connected with emotion. Their ears,
surrounded on every side by the bones of the cranium, destitute of
external conch, without any internal cochlea, and composed merely of
certain sacks and membranous canals, scarcely suffice for the perception
of the loudest sounds. Yet they will sink affrighted into the darksome
depths of lakes, beneath the banks of rivers, or in oceans blue
profound, when the “sky lowers and mutters thunder,” and with elemental
fierceness the sheeted lightning flashes broad and bright above their
liquid dwellings.
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