On the other hand, in such animals as have a bladder, and whose lung
contains blood, the spleen is watery, both for the reason already
mentioned, and also because the left side of the body is more watery
and colder than the right. For each of two contraries has been so
placed as to go together with that which is akin to it in another pair
of contraries. Thus right and left, hot and cold, are pairs of
contraries; and right is conjoined with hot, after the manner
described, and left with cold.
The kidneys when they are present exist not of actual necessity, but
as matters of greater finish and perfection. For by their special
character they are suited to serve in the excretion of the fluid which
collects in the bladder. In animals therefore where this fluid is very
abundantly formed, their presence enables the bladder to perform its
proper office with greater perfection.
Since then both kidneys and bladder exist in animals for one and the
same function, we must next treat of the bladder, though in so doing
we disregard the due order of succession in which the parts should
be enumerated. For not a word has yet been said of the midriff,
which is one of the parts that environ the viscera and therefore has
to be considered with them.
8
It is not every animal that has a bladder; those only being
apparently intended by nature to have one, whose lung contains
blood.
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