For decay
may be said to be deficiency of blood, the scantiness of which renders
it liable, like all bodies of small bulk, to be injuriously affected
by any chance excess of heat or cold. For the same reason fat
animals are less prolific than others. For that part of the blood
which should go to form semen and seed is used up in the production of
lard and suet, which are nothing but concocted blood; so that in these
animals there is either no reproductive excretion at all, or only a
scanty amount.
6
So much then of blood and serum, and of lard and suet. Each of these
has been described, and the purposes told for which they severally
exist. The marrow also is of the nature of blood, and not, as some
think, the germinal force of the semen. That this is the case is quite
evident in very young animals. For in the embryo the marrow of the
bones has a blood-like appearance, which is but natural, seeing that
the parts are all constructed out of blood, and that it is on blood
that the embryo is nourished. But, as the young animal grows up and
ripens into maturity, the marrow changes its colour, just as do the
external parts and the viscera. For the viscera also in animals, so
long as they are young, have each and all a blood-like look, owing
to the large amount of this fluid which they contain.
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