Take these lungs, made up within of branching tubes, these in turn formed
by myriads of air-cells, and each air-cell owning its network of minute
cells called _capillaries_. To every air-cell is given a blood-vessel
bringing blood from the heart, which finds its way through every capillary
till it reaches another blood-vessel that carries it back to the heart. It
leaves the heart charged with carbonic acid and watery vapor. It returns,
if pure air has met it in the lung, with all corruption destroyed, a
dancing particle of life. But to be life, and not slow death, thirty-three
hogsheads of air must pass daily into the lungs, and twenty-eight pounds
of blood journey from heart to lungs and back again three times in each
hour. It rests wholly with ourselves, whether this wonderful tide, ebbing
and flowing with every breath, shall exchange its poisonous and clogging
carbonic acid and watery vapor for life-giving oxygen, or retain it to
weigh down and debilitate every nerve in the body.
With every thought and feeling some actual particles of brain and nerve
are dissolved, and sent floating on this crimson current. With every
motion of a muscle, whether great or small, with every process that can
take place in the body, this ceaseless change of particles is going on.
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