A more important difference between the bird and the reptile is
that the heart of the bird is completely divided into four
chambers, but, as we saw, this probably occurred also in the
other flying reptiles. It may be said to be almost a condition of
the greater energy of a flying animal. When the heart has four
complete chambers, the carbonised blood from the tissues of the
body can be conveyed direct to the lungs for purification, and
the aerated blood taken direct to the tissues, without any
mingling of the two. In the mud-fish and amphibian, we saw, the
heart has two chambers (auricles) above, but one (ventricle)
below, in which the pure and impure blood mingle. In the reptiles
a partition begins to form in the lower chamber. In the turtle it
is so nearly complete that the venous and the arterial blood are
fairly separated; in the crocodile it is quite complete, though
the arteries are imperfectly arranged. Thus the four-chambered
heart of the bird and mammal is not a sudden and inexplicable
development. Its advantage is enormous in a cold climate. The
purer supply of blood increases the combustion in the tissues,
and the animal maintains its temperature and vitality when the
surrounding air falls in temperature.
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