a delicately constructed but strong sac, known as the pleural sac, one
wall of which closely adheres to the lung, and the other to the inner
wall of the chest, and which secretes a fluid which allows the inner
surfaces of the walls to glide easily upon each other in the act of
breathing.

The Air Passages consist of the interior of the nose, pharynx, larynx,
windpipe or trachea, and the bronchial tubes. When we breathe, we draw
in the air through the nose, in which it is warmed by contact with the
mucous membrane, which is richly supplied with blood, and after it has
passed through the pharynx and larynx it passes into the trachea or
windpipe, which subdivides into numerous tubes called the bronchial
tubes (bronchia), which in turn subdivide into and terminate in minute
subdivisions in all the small air spaces in the lungs, of which the
lungs contain millions. A writer has stated that if the air cells of
the lungs were spread out over an unbroken surface, they would cover
an area of fourteen thousand square feet.

The air is drawn into the lungs by the action of the diaphragm, a
great, strong, flat, sheet-like muscle, stretched across the chest,
separating the chest-box from the abdomen. The diaphragm's action is
almost as automatic 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