On The Parts Of Animals   

part, that is in virtue of its flesh, or of the substance analogous to
flesh. But the blood, as before stated, is not sensitive; as therefore
is neither lard nor suet, seeing that they are nothing but concocted
blood. Were then the whole body composed of these substances, it would
be utterly without sensation. Such animals, again, as are excessively
fat age rapidly. For so much of their blood is used in forming fat,
that they have but little left; and when there is but little blood the
way is already open for decay. 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.
Part 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.
The consistency of the marrow agrees with that of the fat. For when
the fat consists of lard, then the marrow also is unctuous and
lard-like; but when the blood is converted by concoction into suet,
and does not assume the form of lard, then the marrow also has a suety
character. In those animals, therefore, that have horns and are
without upper front teeth, the marrow has the character of suet; while
it takes the form of lard in those that have front teeth in both jaws,
and that also have the foot divided into toes. What has ben said
hardly applies to the spinal marrow. For it is necessary that this
shall be continuous and extend without break through the whole
backbone, inasmuch as this bone consists of separate vertebrae. But
were the spinal marrow either of unctuous fat or of suet, it could not
hold together in such a continuous mass as it does, but would either
be too fluid or too frangible.
There are some animals that can hardly be said to have any marrow.
These are those whose bones are strong and solid, as is the case with
the lion. For in this animal the marrow is so utterly insignificant
that the bones look as though they had none at all. However, as it is
necessary that animals shall have bones or something analogous to
them, such as the fish-spines of water-animals, it is also a matter of
necessity that some of these bones shall contain marrow; for the
substance contained within the bones is the nutriment out of which
these are formed. Now the universal nutriment, as already stated, is
blood; and the blood within the bone, owing to the heat which is
developed in it from its being thus surrounded, undergoes concoction,
and self-concocted blood is suet or lard; so that it is perfectly
intelligible how the marrow within the bone comes to have the
character of these substances. So also it is easy to understand why,
in those animals that have strong and compact bones, some of these
should be entirely void of marrow, while the rest contain but little
of it; for here the nutriment is spent in forming the bones.
Those animals that have fish-spines in place of bones have no other
marrow than that of the chine. For in the first place they have
naturally but a small amount of blood; and secondly the only hollow
fish-spine is that of the chine. In this then marrow is formed; this
being the only spine in which there is space for it, and, moreover,