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.

But the heart has a most important agency in this operation. Fig. 27
is a diagram of the heart, which is placed between the two lobes of
the lungs. The right side of the heart receives the dark and impure
blood, which is loaded with carbonic acid. It is brought from every
point of the body by branching veins that unite in the upper and the
lower _vena cava_, which discharge into the right side of the heart.
This impure blood passes to the capillaries of the air-cells in the
lungs, where it gives off carbonic acid, and, taking oxygen from the
air, then returns to the left side of the heart, from whence it is sent
out through the _aorta_ and its myriad branching arteries to every part
of the body. When the upper portion of the heart contracts, it forces
both the pure blood from the lungs, and the impure blood from the body,