Fig. 23 shows the outside of a cluster of these
air-cells, and Fig. 24 is the inside view. The lining membrane of each
air-cell is covered by a network of minute blood-vessels called
_capillaries_ which, magnified several hundred times, appear in the
microscope as at Fig. 25. Every air-cell has a blood-vessel that brings
blood from the heart, which meanders through its capillaries till it
reaches another blood-vessel that carries it back to the heart, as
seen in Fig. 26. In this passage of the blood through these capillaries,
the air in the air-cell imparts its oxygen to the blood, and receives
in exchange carbonic acid and watery vapor. These latter are expired
at every breath into the atmosphere.
By calculating the number of air cells in a small portion of the lungs,
under a microscope, it is ascertained that there are no less than
eighteen million of these wonderful little purifiers and feeders of
the body. By their ceaseless ministries, every grown person receives,
each day, thirty-three hogsheads of air into the lungs to nourish and
vitalize every part of the body, and also to carry off its impurities.
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