ternum, and there it divides into two
branches, called bronchi. Each branch enters the lung of its own
side, and breaks up into a great number of smaller branches, called
bronchial tubes. These divide into smaller tubes, which continue
subdividing till the whole lung is penetrated by the branches, the
extremities of which are extremely minute. To all these branches the
general name of bronchial tubes is given. The smallest are only about
one-fiftieth of an inch in diameter.

[Illustration: Fig. 86.--Relative Position of the Lungs, Heart, and its
Great Vessels.

A, left ventricle;
B, right ventricle;
C, left auricle;
D, right auricle;
E, superior vena cava;
F, pulmonary artery;
G, aorta;
H, arch of the aorta;
K, innominate artery;
L, right common carotid artery;
M, right subclavian artery;
N, thyroid cartilage forming upper portion of the larynx;
O, trachea.
]

Now the walls of the windpipe, and of the larger bronchial tubes would
readily collapse, and close the passage for air, but for a wise
precaution. The horseshoe-shaped rings of cartilage in the trachea and the
plates of cartilage in the bronchial tubes keep these passages open.
Again, these air passages have elastic fibers running the length of the
tubes, which allow them to stretch and bend readily with the movements of
the neck.

205. The Cilia of the Air Passages. The inner surfaces of the
windpipe and bronchial tubes are lined with mucous membrane, continuous
with that of the throat, the mouth, and the nostrils, the secretion from
which serves to keep the parts moist.

Delicate, hair-like filaments, not unlike the pile on velvet, called
cilia, spring from the epithelial lining of the air tubes. Their
constant wavy movement is always upwards and outwards, towards the mouth.
Thus any excessive secretion, as of bronchitis or catarrh, is carried
upwards, and finally expelled by coughing. In this way, the lungs are kept
quite free from particles of foreign matter derived from the air.
Otherwise we should suffer, and often be in danger from the accumulation
of mucus and dust in the air passages. Thus these tiny cilia act as
dusters which Nature uses to keep the air tubes free and clean (Fig. 5).

[Illustration: Fig. 87.--Bronchial tube, with its Divisions and
Subdivisions. (Showing groups of air cells at the termination of minute
bronchial tubes.)]

206. The Lungs. The lungs, the organs of respiration, are two
pinkish gray