and
extend to the air-cells themselves, which may be considered as built
up around them in some such fashion as a toy balloon on its wooden
stem, but with many infoldings, etc. (Fig. 10). The air-cells are
composed of a membrane which may be compared to the walls of the
balloon, but we are of course dealing with living tissue supplied by
countless blood-vessels of the most minute calibre, in which the blood
is brought very near to the air which passes over them.

Throughout, the respiratory tract is lined with mucous membrane.
Mucous membranes are so named because they secrete mucus, the fluid
which moistens the nose, mouth, and all parts of the respiratory
tract. When one suffers from a cold the mucous membrane, in the early
stages, may become dry from failure of this natural secretion; hence
sneezing, coughing, etc., as the air then acts as an irritant.

At no time do we breathe pure oxygen, but "air"--_i.e._, a mixture of
21 parts of the former with 79 parts of an inert gas, nitrogen; and
there is always in the air more oxygen than the blood actually takes
from it in the air-cells.

The intaking of air is termed by physiologists _inspiration_, and its
expulsion _expiration_, the whole process being _respiration_.
Expiration takes a very little longer than inspiration, and the
rapidity of respiration depends on the needs of the body. The more
active the exercise, the more rapidly vital processes go on, the more
ventilation of the tissues is required and the more is actually
effected. When one is at rest breathing takes place at the rate of
from 14 to 18 inspirations and expirations in the minute; but of all
the processes of the body none is more variable than respiration,
and of necessity, for every modification of action, every movement,
implies a demand for an increased quantity of oxygen. It is not
surprising, therefore, that the very exercise of singing tends in
itself to put one out of breath.

[Illustration: FIG. 12. In the above, the shaded outlines indicate the
shape of the bony cage of the chest during inspiration, and the
lighter ones the same during expiration. The alterations in the
position of the ribs and in the diameters of the chest, giving rise to
its greater capacity during inspiration, are evident.]

[Illustration: FIG. 13. This figure is intended to indicate, in a
purely diagrammatic way, by dotted lines, the position of the
diaphragm (1) when inspiration is moderate, and (2) when very deep.