The two lungs are the essential organs of respiration; the right lung
has three lobes, the left, two. The base of each lung rests upon the
convex surface of the diaphragm.
[Illustration: =FRONT VIEW OF THE HEART AND LUNGS, AND LARGE
BLOOD-VESSELS.=]
The root of each lung is formed by the bronchus and blood-vessels, which
enter the lung a little above the middle of its inner surface, and
connect it to the heart and trachea. With the exception of the root, the
surface of each lung is free and moves in the cavity of the thorax. The
bronchus is one of two tubes which arise from the bifurcation of the
trachea. It conducts the air from the trachea to either lung. The
bronchial tubes are sub-divisions, or ramifications, of the bronchus and
terminate in the air-cells.
_MUSCLES OF RESPIRATION._
Inasmuch as voice is vocalized breath, it is important to give attention
to respiration.
The principal muscles used in the ordinary movements of inspiration
are:—
I. Diaphragm.
II. Levatores costarum.
III. External intercostals.