Now the cartilages of these land animals are without marrow,
that is without any distinctly separate marrow. For the marrow,
which in bones is distinctly separate, is here mixed up with the whole
mass, and gives a soft and mucilaginous consistence to the
cartilage. But in the Selachia the chine, though it is
cartilaginous, yet contains marrow; for here it stands in the stead of
a bone.
Very nearly resembling the bones to the touch are such parts as
nails, hoofs, whether solid or cloven, horns, and the beaks of
birds, all of which are intended to serve as means of defence. For the
organs which are made out of these substances, and which are called by
the same names as the substances themselves, the organ hoof, for
instance, and the organ horn, are contrivances to ensure the
preservation of the animals to which they severally belong. In this
class too must be reckoned the teeth, which in some animals have but a
single function, namely the mastication of the food, while in others
they have an additional office, namely to serve as weapons; as is
the case with all animals that have sharp interfitting teeth or that
have tusks. All these parts are necessarily of solid and earthy
character; for the value of a weapon depends on such properties. Their
earthy character explains how it is that all such parts are more
developed in four-footed vivipara than in man.
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