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On The Generation Of Animals   

last to assume their form, for they have to wait till the time when
Nature has some residue to spare.

The bones, then, are made in the first conformation of the parts
from the seminal secretion or residue. As the animal grows the bones
grow from the natural nourishment, being the same as that of the
sovereign parts, but of this they only take up the superfluous
residues. For everywhere the nutriment may be divided into two
kinds, the first and the second; the former is 'nutritious', being
that which gives its essence both to the whole and to the parts; the
latter is concerned with growth, being that which causes
quantitative increase. But these must be distinguished more fully
later on. The sinews are formed in the same way as the bones and out
of the same materials, the Seminal and nutritious residue. Nails,
hair, hoofs, horns, beaks, the spurs of cocks, and any other similar
parts, are on the contrary formed from the nutriment which is taken
later and only concerned with growth, in other words that which is
derived from the mother, or from the outer world after birth. For this
reason the bones on the one hand only grow up to a certain point (for
there is a limit of size in all animals, and therefore also of the
growth of the bones; if these had been always able to grow, all
animals that have bone or its analogue would grow as long as they
lived, for these set the limit of size to animals. What is the
reason of their not always increasing in size must be stated later.)

Hair, on the contrary, and growths akin to hair go on growing as long
as they exist at all, and increase yet more in diseases and when the
body is getting old and wasting, because more residual matter is
left over, as owing to old age and disease less is expended on the
important parts, though when the residual matter also fails through
age the hair fails with it. But the contrary is the case with the
bones, for they waste away along with the body and the other parts.
Hair actually goes on growing after death; it does not, however, begin
growing then.

About the teeth a difficulty may be raised. They have actually the
same nature as the bones, and are formed out of the bones, but
nails, hair, horns, and the like are formed out of the skin, and
that is why they change in colour along with it, for they become
white, black, and all sorts of colours according to that of the
skin. But the teeth do nothing of the sort, for they are made out of
the bones in all animals that have both bones and teeth. Of all the
bones they alone go on growing through life, as is plain with the
teeth which grow out of the straight line so as no longer to touch
each other. The reason for their growth, as a final cause, is their
function, for they would soon be worn down if there were not some
means of saving them; even as it is they are altogether worn down in
old age in some animals which eat much and have not large teeth, their
growth not being in proportion to their detrition. And so Nature has
contrived well to meet the case in this also, for she causes the
failure of the teeth to synchronize with old age and death. If life
lasted for a thousand or ten thousand years the original teeth must
have been very large indeed, and many sets of them must have been
produced, for even if they had grown continuously they would still
have been worn smooth and become useless for their work. The final
cause of their growth has been now stated, but besides this as a
matter of fact the growth of the teeth is not the same as that of
the other bones. The latter all come into being in the first formation
of the embryo and none of them later, but the teeth do so later.
Therefore it is possible for them to grow again after the first set
falls out, for though they touch the bones they are not connate with
them. They are formed, however, out of the nutriment distributed to
the bones, and so have the same nature, even when the bones have their
own number complete.

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