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_.

Now every beat of the heart sends two ounces of blood into the minute,
hair-like blood-vessels, called capillaries, that line these air-cells,
where the air in the air-cells gives its oxygen to the blood, and in
its place receives carbonic acid. This gas is then expired by the lungs
into the surrounding atmosphere.

Thus, by this powerful little organ, the heart, no less than
twenty-eight pounds of blood, in a common-sized man, is sent three
times every hour through the lungs, giving out carbonic acid and watery
vapor, and receiving the life-inspiring oxygen.

Whether all this blood shall convey the nourishing and invigorating
oxygen to every part of the body, or return unrelieved of carbonic
acid, depends entirely on the pureness of the atmosphere that is
breathed.

Every time we think or feel, this mental action dissolves some particles
of the brain and nerves, which pass into the blood to be thrown out
of the body through the lungs and skin. In like manner, whenever we
move any muscle, some of its particles decay and pass away. It is in
the capillaries, which are all over the body, that this change takes