The reason of this is that a serpent,
like an insect, has a body that admits of being curled up, its
vertebrae being cartilaginous and easily bent. The faculty in question
belongs then to serpents simply as a necessary consequence of this
character of their vertebrae; but at the same time it has a final
cause, for it enables them to guard against attacks from behind. For
their body, owing to its length and the absence of feet, is ill-suited
for turning round and protecting the hinder parts; and merely to
lift the head, without the power of turning it round, would be of no
use whatsoever.
The animals with which we are dealing have, moreover, a part which
corresponds to the breast; but neither here nor elsewhere in their
body have they any mammae, as neither has any bird or fish. This is
a consequence of their having no milk; for a mamma is a receptacle for
milk and, as it were, a vessel to contain it. This absence of milk
is not peculiar to these animals, but is common to all such as are not
internally viviparous. For all such produce eggs, and the nutriment
which in Vivipara has the character of milk is in them engendered in
the egg. Of all this, however, a clearer account will be given in
the treatise on Generation. As to the mode in which the legs bend, a
general account, in which all animals are considered, has already been
given in the dissertation on Progression.
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