On The Parts Of Animals   

being the only one which owing to its division into parts requires a
connecting bond. This too is the reason why the marrow of the chine,
as already mentioned, is somewhat different from that of other bones.
For, having to act the part of a clasp, it must be of glutinous
character, and at the same time sinewy so as to admit of stretching.
Such then are the reasons for the existence of marrow, in those
animals that have any, and such its nature. It is evidently the
surplus of the sanguineous nutriment apportioned to the bones and
fish-spines, which has undergone concoction owing to its being
enclosed within them.
Part 7
From the marrow we pass on in natural sequence to the brain. For there
are many who think that the brain itself consists of marrow, and that
it forms the commencement of that substance, because they see that the
spinal marrow is continuous with it. In reality the two may be said to
be utterly opposite to each other in character. For of all the parts
of the body there is none so cold as the brain; whereas the marrow is
of a hot nature, as is plainly shown by its fat and unctuous
character. Indeed this is the very reason why the brain and spinal
marrow are continuous with each other. For, wherever the action of any
part is in excess, nature so contrives as to set by it another part
with an excess of contrary action, so that the excesses of the two may
counterbalance each other. Now that the marrow is hot is clearly shown
by many indications. The coldness of the brain is also manifest
enough. For in the first place it is cold even to the touch; and,
secondly, of all the fluid parts of the body it is the driest and the
one that has the least blood; for in fact it has no blood at all in
its proper substance. This brain is not residual matter, nor yet is it
one of the parts which are anatomically continuous with each other;
but it has a character peculiar to itself, as might indeed be
expected. That it has no continuity with the organs of sense is plain
from simple inspection, and is still more clearly shown by the fact,
that, when it is touched, no sensation is produced; in which respect
it resembles the blood of animals and their excrement. The purpose of
its presence in animals is no less than the preservation of the whole
body. For some writers assert that the soul is fire or some such
force. This, however, is but a rough and inaccurate assertion; and it
would perhaps be better to say that the soul is incorporate in some
substance of a fiery character. The reason for this being so is that
of all substances there is none so suitable for ministering to the
operations of the soul as that which is possessed of heat. For
nutrition and the imparting of motion are offices of the soul, and it
is by heat that these are most readily effected. To say then that the
soul is fire is much the same thing as to confound the auger or the
saw with the carpenter or his craft, simply because the work is
wrought by the two in conjunction. So far then this much is plain,
that all animals must necessarily have a certain amount of heat. But
as all influences require to be counterbalanced, so that they may be
reduced to moderation and brought to the mean (for in the mean, and
not in either extreme, lies the true and rational position), nature
has contrived the brain as a counterpoise to the region of the heart
with its contained heat, and has given it to animals to moderate the
latter, combining in it the properties of earth and water. For this
reason it is, that every sanguineous animal has a brain; whereas no
bloodless creature has such an organ, unless indeed it be, as the
Poulp, by analogy. For where there is no blood, there in consequence
there is but little heat. The brain, then, tempers the heat and
seething of the heart. In order, however, that it may not itself be
absolutely without heat, but may have a moderate amount, branches run
from both blood-vessels, that is to say from the great vessel and from
what is called the aorta, and end in the membrane which surrounds the
brain; while at the same time, in order to prevent any injury from the
heat, these encompassing vessels, instead of being few and large, are
numerous and small, and their blood scanty and clear, instead of being