by them in a high degree, and others much more imperfectly. Of the
latter kind are the senses of touch and taste, which are believed to be
very slightly developed. On the other hand, those of hearing, seeing,
and smelling, are ascertained to be acute, but the first in a lesser
degree than both the second and third. Their possession of an auditory
organ was long doubted, and even denied by some physiologists; but it
has been found placed on the sides of the skull, or in the cavity which
contains the brain. It occupies a position entirely distinct and
detached from the skull, and, in this respect, differs in the local
disposition of the same sense in birds and quadrupeds. In some fishes,
as in those of the ray kind, the organ is wholly encompassed by those
parts which contain the cavity of the skull; whilst in the cod and
salmon kind it is in the part within the skull. Its structure is, in
every way, much more simple than that of the same sense in those animals
which live entirely in the air; but there is no doubt that they have the
adaptation suitable to their condition. In some genera, as in the rays,
the external orifice or ear is very small, and is placed in the upper
surface of the head; whilst in others there is no visible external
orifice whatever. However perfect the _sight_ of fishes may be,
experience has shown that this sense is of much less use to them than
that of smelling, in searching for their food. The optic nerves in
fishes have this peculiarity,--that they are not confounded with one
another in their middle progress between their origin and their orbit.
The one passes over the other without any communication; so that the
nerve which comes from the left side of the brain goes distinctly to the
right eye, and that which comes from the right goes distinctly to the
left. In the greater part of them, the eye is covered with the same
transparent skin that covers the rest of the head. The object of this
arrangement, perhaps, is to defend it from the action of the water, as
there are no eyelids. The globe in front is somewhat depressed, and is
furnished behind with a muscle, which serves to lengthen or flatten it,
according to the necessities of the animal. The crystalline humour,
which in quadrupeds is flattened, is, in fishes, nearly globular. The
organ of _smelling_ in fishes is large, and is endued, at its entry,
with a dilating and contracting power, which is employed as the wants of
the animal may require. It is mostly by the acuteness of their smell
that fishes are enabled to discover their food; for their tongue is not
designed for nice sensation, being of too firm a cartilaginous substance
for this purpose.