Although the elastic recoil of the lungs
and the structure of the expanded thorax is the main force employed in
normal breathing and to some extent in vocalisation (for it keeps up a
constant steady pressure), the mind, by exercising control over the
continuance of elevation of the ribs and contraction of the abdominal
muscles, regulates the force of the expiratory blast of air so as to employ
the bellows most efficiently in vocalisation. Not only does the contraction
of the abdominal muscles permit of control over the expulsion of the air,
but by fixing the cartilages of the lowest six ribs it prevents the
diaphragm drawing them upwards and _inwards_ (_vide_ fig. 2). The greatest
expansion is just above the waistband (_vide_ fig. 3). We are not conscious
of the contraction of the diaphragm; we are conscious of the position of
the walls of the chest and abdomen; the messages the mind receives relating
to the amount of air in the bellows at our disposal come from sensations
derived from the structures forming the wall of the chest and abdomen, viz.
the position of the ribs, their degree of elevation and forward protrusion
combined with the feeling that the ribs are falling back into the position
of rest; besides there is the feeling that the abdominal muscles can
contract no more--a feeling which should never be allowed to arise before
we become conscious of the necessity of replenishing the supply of air.
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