as that of the heart, although it may be
transformed into a semi-voluntary muscle by an effort of the will.
When it expands, it increases the size of the chest and lungs, and the
air rushes into the vacuum thus created. When it relaxes the chest and
lungs contract and the air is expelled from the lungs.

Now, before considering what happens to the air in the lungs, let us
look a little into the matter of the circulation of the blood. The
blood, as you know, is driven by the heart, through the arteries, into
the capillaries, thus reaching every part of the body, which it
vitalizes, nourishes and strengthens. It then returns by means of the
capillaries by another route, the veins, to the heart, from whence it
is drawn to the lungs.

The blood starts on its arterial journey, bright red and rich, laden
with life-giving qualities and properties. It returns by the venous
route, poor, blue and dull, being laden down with the waste matter of
the system. It goes out like a fresh stream from the mountains; it
returns as a stream of sewer water. This foul stream goes to the right
auricle of the heart. When this auricle becomes filled, it contracts
and forces the stream of blood through an opening in the right
ventricle of the heart, which in turn sends it on to the lungs, where
it is distributed by millions of hair-like blood vessels to the air
cells of the lungs, of which we have spoken. Now, let us take up the
story of the lungs at this point.

The foul stream of blood is now distributed among the millions of tiny
air cells in the lungs. A breath of air is inhaled and the oxygen of
the air comes in contact with the impure blood through the thin walls
of the hair-like blood vessels of the lungs, which walls are thick
enough to hold the blood, but thin enough to admit the oxygen to
penetrate them. When the oxygen comes in contact with the blood, a
form of combustion takes place, and the blood takes up oxygen and
releases carbonic acid gas generated from the waste products and
poisonous matter which has been gathered up by the blood from all
parts of the system.

The blood thus purified and oxygenated is carried back to the heart,
again rich, red and bright, and laden with life-giving properties and
qualities. Upon reaching the left auricle of the heart, it is forced
into the left ventricle, from whence it is again forced out through
the arteries on its mission of life to all parts of the system. It is
estimated that in a sin