Thus they are of a bloody character, and in substance resemble
each other while they differ from other parts.
11
The viscera are enclosed each in a membrane. For they require some
covering to protect them from injury, and require, moreover, that this
covering shall be light. To such requirements membrane is well
adapted; for it is close in texture so as to form a good protection,
destitute of flesh so as neither to attract humour nor retain it,
and thin so as to be light and not add to the weight of the body. Of
the membranes those are the stoutest and strongest which invest the
heart and the brain; as is but consistent with reason. For these are
the parts which require most protection, seeing that they are the main
governing powers of life, and that it is to governing powers that
guard is due.
12
Some animals have all the viscera that have been enumerated;
others have only some of them. In what kind of animals this latter
is the case, and what is the explanation, has already been stated.
Moreover, the self-same viscera present differences in different
possessors. For the heart is not precisely alike in all animals that
have one; nor, in fact, is any viscus whatsoever. Thus the liver is in
some animals split into several parts, while in others it is
comparatively undivided.
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