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.
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