10
We must now go back to the animals that have blood, and consider
such of their parts, already enumerated, as were before passed over.
We will take the viviparous animals first, and, we have done with
these, will pass on to the oviparous, and treat of them in like
manner.
The parts that border on the head, and on what is known as the
neck and throat, have already been taken into consideration. All
animals that have blood have a head; whereas in some bloodless
animals, such as crabs, the part which represents a head is not
clearly defined. As to the neck, it is present in all the Vivipara,
but only in some of the Ovipara; for while those that have a lung also
have a neck, those that do not inhale the outer air have none. The
head exists mainly for the sake of the brain. For every animal that
has blood must of necessity have a brain; and must, moreover, for
reasons already given, have it placed in an opposite region to the
heart. But the head has also been chosen by nature as the part in
which to set some of the senses; because its blood is mixed in such
suitable proportions as to ensure their tranquillity and precision,
while at the same time it can supply the brain with such warmth as
it requires. There is yet a third constituent superadded to the
head, namely the part which ministers to the ingestion of food.
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