On The Parts Of Animals   

texture. In the vivipara it is large and rich in blood, because of
their natural heat; while in the ovipara it is small and dry but
capable of expanding to a vast extent when inflated. Among terrestrial
animals, the oviparous quadrupeds, such as lizards, tortoises, and the
like, have this kind of lung; and, among inhabitants of the air, the
animals known as birds. For in all these the lung is spongy, and like
foam. For it is membranous and collapses from a large bulk to a small
one, as does foam when it runs together. In this too lies the
explanation of the fact that these animals are little liable to thirst
and drink but sparingly, and that they are able to remain for a
considerable time under water. For, inasmuch as they have but little
heat, the very motion of the lung, airlike and void, suffices by
itself to cool them for a considerable period.
These animals, speaking generally, are also distinguished from others
by their smaller bulk. For heat promotes growth, and abundance of
blood is a sure indication of heat. Heat, again, tends to make the
body erect; and thus it is that man is the most erect of animals, and
the vivipara more erect than other quadrupeds. For no viviparous
animal, be it apodous or be it possessed of feet, is so given to creep
into holes as are the ovipara.
The lung, then, exists for respiration; and this is its universal
office; but in one order of animals it is bloodless and has the
structure described above, to suit the special requirements There is,
however, no one term to denote all animals that have a lung; no
designation, that is, like the term Bird, applicable to the whole of a
certain class. Yet the possession of a lung is a part of their
essence, just as much as the presence of certain characters
constitutes the essence of a bird.
Part 7
Of the viscera some appear to be single, as the heart and lung; others
to be double, as the kidneys; while of a third kind it is doubtful in
which class they should be reckoned. For the liver and the spleen
would seem to lie half-way between the single and the double organs.
For they may be regarded either as constituting each a single organ,
or as a pair of organs resembling each other in character.
In reality, however, all the organs are double. The reason for this is
that the body itself is double, consisting of two halves, which are
however combined together under one supreme centre. For there is an
upper and a lower half, a front and a rear, a right side and a left.
This explains why it is that even the brain and the several organs of
sense tend in all animals to consist of two parts; and the same
explanation applies to the heart with its cavities. The lung again in
Ovipara is divided to such an extent that these animals look as though
they had actually two lungs. As to the kidneys, no one can overlook
their double character. But when we come to the liver and the spleen,
any one might fairly be in doubt. The reason of this is, that, in
animals that necessarily have a spleen, this organ is such that it
might be taken for a kind of bastard liver; while in those in which a
spleen is not an actual necessity but is merely present, as it were,
by way of token, in an extremely minute form, the liver plainly
consists of two parts; of which the larger tends to lie on the right
side and the smaller on the left. Not but what there are some even of
the Ovipara in which this condition is comparatively indistinctly
marked; while, on the other hand, there are some Vivipara in which the
liver is manifestly divided into two parts. Examples of such division
are furnished by the hares of certain regions, which have the
appearance of having two livers, and by the cartilaginous and some
other fishes.
It is the position of the liver on the right side of the body that is
the main cause for the formation of the spleen; the existence of which
thus becomes to a certain extent a matter of necessity in all animals,
though not of very stringent necessity.
The reason, then, why the viscera are bilateral is, as we have said,
that there are two sides to the body, a right and a left. For each of