The external
cooling agent must be either air or water. In fishes the agent is
water. Fishes therefore never have a lung, but have gills in its
place, as was stated in the treatise on Respiration. But animals
that breathe are cooled by air. These therefore are all provided
with a lung.
All land animals breathe, and even some water animals, such as the
whale, the dolphin, and all the spouting Cetacea. For many animals lie
half-way between terrestrial and aquatic; some that are terrestrial
and that inspire air being nevertheless of such a bodily
constitution that they abide for the most time in the water; and
some that are aquatic partaking so largely of the land character, that
respiration constitutes for them the man condition of life.
The organ of respiration is the lung. This derives its motion from
the heart; but it is its own large size and spongy texture that
affords amplitude of space for entrance of the breath. For when the
lung rises up the breath streams in, and is again expelled when the
lung collapses. It has been said that the lung exists as a provision
to meet the jumping of the heart. But this is out of the question. For
man is practically the only animal whose heart presents this
phenomenon of jumping, inasmuch as he alone is influenced by hope
and anticipation of the future.
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