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,
through the valves marked V, V, into the lower part. When the lower
portion contracts, it closes the valves and forces the impure blood into
the lungs on one side, and also on the other side forces the purified
blood through the aorta and arteries to all parts of the body.
As before stated, the lungs consist chiefly of air-cells, the walls
of which are lined with minute blood-vessels; and we know that in every
man these air-cells number _eighteen millions_.
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