The shape of the diaphragm, when it is relaxed, resembles an open umbrella. When the diaphragm is flattened by contraction it no longer retains its dome-like shape, and thus gives greater depth to the thorax.
DIFFERENCE IN THE ACTION OF THE DIAPHRAGM DURING EXPIRATION OF THE BREATH AND DURING THE PRODUCTION OF TONE.
During expiration of breath the diaphragm is fully relaxed, while during the production of tone it should be somewhat contracted. In the proper adjustment of the diaphragm and abdominal muscles during voice production, the diaphragm by its contraction resists, to some extent, the pressure caused by the contraction of the abdominal muscles, and thus only gradually yields to the force brought against it by the contraction of these muscles, in consequence of which a firm and steady support is given to the voice.
The question is often asked, “Should one breathe through the nose or through the mouth?” Nature has so constructed the organs of respiration and determined their action that a person in health breathes through the nose, while a person in ill health often breathes through the mouth. By “breathing through the nose,” of course, is meant reposeful breathing. In extraordinary breathing some persons are obliged to breathe through the mouth, but this is always an indication of exhaustion or weakness. Every person should, if possible, maintain the habit of breathing through the nose.
[Illustration: Median Section of _Mouth_, _Nose_, _Pharynx_, and _Larynx_:—_a_, septum of nose; below it, section of hard palate; _b_, tongue; _c_, section of velum pendulum palati; _d_, _d_, lips; _u_, uvula; _r_, anterior arch or pillar of fauces; _i_, posterior arch; _t_, tonsil; _p_, pharynx; _h_, hyoid bone; _k_, thyroid cartilage; _n_, crycoid cartilage; _s_, epiglottis; _v_, glottis; 1, posterior opening of nares; 3, isthmus faucium; 4, superior opening of larynx; 5, passage into œsophagus; 6, mouth of right Eustachian tube.]
The organs that reinforce voice are its resonant chambers, viz.:—
Resonance means resounding or sounding again, and is caused by means of the air conveying the vibrations of one substance to another substance. This is familiarly illustrated in the echo.