The Diaphragm is the most active agent in breathing.  It is a muscular structure which forms the floor of the thoracic cage and the roof of the abdominal cavity.  Its outer rim of muscle fibres, attached on the inner side of the rib-cage, arises from the lower border of the rib-cage in front, on the sides, and from the lumbar spine in the back.  The whole structure is in the shape of a lop-sided mushroom with its stem nearer to the back margin than to the front.  The heart rests on the top of the diaphragm, and the liver, stomach and spleen are immediately beneath its under surface.  All of these viscera are in close contact with it and even directly connected with its tissues.

The diaphragm, then, is so situated and attached to parts of the skeleton and to parts of the viscera, as to be able to function in close relationship with both the visceral rhythms and the skeletal rhythms, is affected by them and has an effect upon them.

Through deepening of the thoracic cavity, the movement of the diaphragm affects the rhythms of the thoracic viscera.  By stimulating peristalsis and pressure on other abdominal viscera, it affects the digestive rhythms; and through internal respiration and the associated muscle attachments of its long fibres, it affects the rhythms of the skeletal musculature.  So it is closely tied to the organic rhythms and to the skeletal rhythms.

It is the only muscle having such a strategic position between the rhythms of the two systems.  To a degree, it responds to voluntary directions – but only to a degree.  One may hold one’s breath for but a limited time, usually less than a minute.  One may voluntarily starve by denying oneself food; it is physically impossible for one to starve voluntarily for the want of oxygen.

Next to the heart, the diaphragm is the most continuously active of all body structures.  It does not become fatigued, partly because like the heart, its rest periods are longer than its working periods; that is, the phase of relaxation is longer than that of contraction.  Expiration normally lasts from 1.3 to 1.4 times the length of inspiration, and there is, moveover, a slight pause after expiration.

Like the “rest” that the musician employs in composition, to enhance and amplify appreciation of musical tone quality and variation in phrasing, so Nature employs frequent rest periods.  It is as though nature must re-establish the potential energy balance before allowing it to be employed again in kinetic form.

Extracted from The Thinking Body – Mabel Elsworth Todd

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