structures of a light, spongy appearance, that fill the chest
cavity, except the space taken up by the heart and large vessels. Between
the lungs are situated the large bronchi, the oesophagus, the heart
in its pericardium, and the great blood-vessels. The base of the lungs
rests on the dome-like diaphragm, which separates the chest from the
abdomen. This partly muscular and partly tendinous partition is a most
important factor in breathing.

Each lung is covered, except at one point, with an elastic serous membrane
in a double layer, called the pleura. One layer closely envelops the
lung, at the apex of which it is reflected to the wall of the chest cavity
of its own side, which it lines. The two layers thus form between them a
Closed Sac a serous cavity (see Fig. 69, also note, p. 176).

[Illustration: Fig. 88.--The Lungs with the Trachea, Bronchi, and Larger
Bronchial Tubes exposed. (Posterior view.)

A, division of left bronchus to upper lobe;
B, left branch of the Pulmonary artery;
C, left bronchus;
D, left superior pulmonary vein;
E, left inferior pulmonary vein;
F, left auricle;
K, inferior vena cava;
L, division of right bronchus to lower lobe;
M, right inferior pulmonary vein;
N, right superior pulmonary vein;
O, right branch of the pulmonary artery;
P, division of right bronchus to upper lobe;
R, left ventricle;
S, right ventricle.
]

In health the two pleural surfaces of the lungs are always in contact, and
they secrete just enough serous fluid to allow the surfaces to glide
smoothly upon each other. Inflammation of this membrane is called
_pleurisy_. In this disease the breathing becomes very painful, as the
secretion of glairy serum is suspended, and the dry and inflamed surfaces
rub harshly upon each other.

The root of the lung, as it is called, is formed by the bronchi, two
pulmonary arteries, and two pulmonary veins. The nerves and lymphatic
vessels of the lung also enter at the root. If we only remember that all
the bronchial tubes, great and small, are hollow, we may compare the whole
system to a short bush or tree growing upside down in the chest, of which
the trachea is the trunk, and the bronchial tubes the branches of various
sizes.

207. Minute Structure of the Lungs. If one of the smallest bronchial
tubes be traced in its tree-like ramifications, it will be found to end in
an irregular funnel-shaped passage wider than itself. Around this passage
are gro