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. 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
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