uped a number of honeycomb-like sacs, the air cells[35] or
alveoli of the lungs. These communicate freely with the passage, and
through it with the bronchial branches, but have no other openings. The
whole arrangement of passages and air cells springing from the end of a
bronchial tube, is called an ultimate lobule. Now each lobule is a
very small miniature of a whole lung, for by the grouping together of
these lobules another set of larger lobules is formed.

[Illustration: Fig. 89.

A, diagrammatic representation of the ending of a bronchial tube in air
sacs or alveoli;
B, termination of two bronchial tubes in enlargement beset with air sacs
(_Huxley_);
C, diagrammatic view of an air sac.

a lies within sac and points to epithelium lining wall;
b, partition between two adjacent sacs, in which run capillaries;
c, elastic connective tissue (_Huxley_).
]

In like manner countless numbers of these lobules, bound together by
connective tissue, are grouped after the same fashion to form by their
aggregation the lobes of the lung. The right lung has three such
lobes; and the left, two. Each lobule has a branch of the pulmonary artery
entering it, and a similar rootlet of the pulmonary vein leaving it. It
also receives lymphatic vessels, and minute twigs of the pulmonary plexus
of nerves.

[Illustration: Fig. 90.--Diagram to illustrate the Amounts of Air
contained by the Lungs in Various Phases of Ordinary and of Forced
Respiration.]

The walls of the air cells are of extreme thinness, consisting of delicate
elastic and connective tissue, and lined inside by a single layer of thin
epithelial cells. In the connective tissue run capillary vessels belonging
to the pulmonary artery and veins. Now these delicate vessels running in
the connective tissue are surrounded on all sides by air cells. It is
evident, then, that the blood flowing through these capillaries is
separated from the air within the cells only by the thin walls of the
vessels, and the delicate tissues of the air cells.

This arrangement is perfectly adapted for an interchange between the
blood in the capillaries and the air in the air cells. This will be more
fully explained in sec. 214.

208. Capacity of the Lungs. In breathing we alternately take into and
expel from the lungs a certain quantity of air. With each quiet
inspiration about 30 cubic inches of air enter the lungs, and 30 cubic
inches pass out with each expiration. The