but as soon as they become inflated with air, they spread over a larger
surface, and are therefore more buoyant. Each lung is invested, as far
as its root, with a membrane, called the _pleura_, which is then
continuously extended to the cavity of the chest, thus performing the
double office of lining it, and constituting a partition between the
lungs. The part of the membrane which forms this partition is termed the
_mediastinum_. Inflammation of this membrane is called _pleurisy_. The
lungs are held in position by the root, which is formed by the pulmonary
arteries, veins, nerves, and the bronchial tubes. Respiration is the
function by which the venous blood, conveyed to the lungs by the
pulmonary artery, is converted into arterial blood. This is effected by
the elimination of carbonic acid, which is expired or exhaled from the
lungs, and by the absorption of oxygen from the air which is taken into
the lungs, by the act of inspiration or inhalation. The act of
expiration is performed chiefly by the elevation of the diaphragm and
the descent of the ribs, and inspiration is principally effected by the
descent of the diaphragm and the elevation of the ribs.

[Illustration: Fig. 44.
A representation of the heart and lungs. 4. The
heart. 5. The pulmonary artery. 8. Aorta. 9, 11.
Upper lobes of the lungs. 10, 13. Lower lobes. 12.
Middle lobe of the right lung. 2. Superior vena
cava. 3. Inferior vena cava.]

When the muscles of some portions of the air-passages are relaxed, a
peculiar vibration follows, known as snoring. Coughing and sneezing are
sudden and spasmodic expiratory efforts, and generally involuntary.
Sighing is a prolonged deep inspiration, followed by a rapid, and
generally audible expiration. It is remarkable that laughing and