-posterior and lateral diameters.
There is, however, one muscle which forms the floor of the thoracic cage
called the diaphragm that requires more than a passing notice (_vide_ fig.
2), inasmuch as it is the most effective agent in the expansion of the
chest. It consists of a central tendinous portion, above which lies the
heart, contained in its bag or pericardium; on either side attached to the
central tendon on the one hand and to the spine behind, to the last rib
laterally, and to the cartilages of the lowest six ribs anteriorly, is a
sheet of muscle fibres which form on either side of the chest a dome-like
partition between the lungs and the abdominal cavity (_vide_ fig. 2). The
phrenic nerve arises from the spinal cord in the upper cervical region and
descends through the neck and chest to the diaphragm; it is therefore a
special nerve of respiration. There are two--one on each side supplying the
two sheets of muscle fibres. When innervation currents flow down these
nerves the two muscular halves of the diaphragm contract, and the floor of
the chest on either side descends; thus the vertical diameter increases.
Now the elastic lungs are covered with a smooth pleura which is reflected
from them on to the inner side of the wall of the thorax, leaving no space
between; consequently when the chest expands in all three directions the
elastic lungs expand correspondingly. But when either voluntarily or
automatically the nerve currents that cause contraction of the muscles of
expansion cease, the elastic structures of the lungs and thorax, including
the muscles, recoil, the diaphragm ascends, and the ribs by the force of
gravity tend to fall into the position of rest. During expansion of the
chest a negative pressure is established in the air passages and air flows
into them from without. In contraction of the chest there is a positive
pressure in the air passages, and air is expelled; in normal quiet
breathing an ebb and flow of air takes place rhythmically and
subconsciously; thus in the ordinary speaking of conversation we do not
require to exercise any voluntary effort in controlling the breathing, but
the orator and more especially the singer uses his knowledge and experience
in the voluntary control of his breath, and he is thus enabled to use his
vocal instrument in the most effective manner.

[Illustration: FIG. 2

Adapted from Quain's "Anatomy" by permission of Messrs. Longmans, Green &
Co.]

[Description: FIG