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This triple movement of the ribs, especially in the combined outward and upward direction, the latter at right angles to the spine, causes a great enlargement of the chest-cavity and gives the lungs a great amount of space in which to expand.
The forward poise of the body also favors many of the muscles employed in inspiration, because many of these extend upward and forward so that the forward inclination aids them in assisting the horizontal lifting of the ribs and the resultant enlargement of the chest-cavity.
But it was probably in the Permian sauro-mammals that the two originally separate parts were united, and the diaphragm became a complete partition between the thoracic and abdominal cavities in the mammals; as it considerably enlarges the chest-cavity when it contracts, it becomes an important respiratory muscle.
Sometimes the blending of the two diaphragmatic structures, and consequently the severance of the one pleural duct from the abdominal cavity, is not completed in man. The two cavities then remain in communication by an open pleural duct, and loops of the intestine may penetrate by this "rupture opening" into the chest-cavity.
For of the three methods mentioned the expansion of the ribs creates the largest chest-cavity, within which the lungs will have room to become inflated, so that more air can be drawn into them by this method than by either of the others.
In breathing, the correct method of inspiration is to provide the room required for the inflation of the lungs by enlarging the chest-cavity to its greatest possible extent, which is accomplished by expanding the whole framework of the ribs and allowing the diaphragm to descend, the clavicle rising passively while the wall of the abdomen at first extends and then, as to its lower anterior portion, slightly sinks in.
Correct breathing results, with each intake of breath, in as great an enlargement of the chest-cavity as is necessary to make room for the expansion of the lungs when inflated. But as clavicular breathing acts only on the upper ribs, it causes only the upper part of the chest to expand, and so actually circumscribes the space within which and the extent to which the lungs can be inflated.
I have stated that the upward and outward movement of the ribs greatly enlarges the chest-cavity, and with this slight forward poise of the body it is not necessary for the ribs to move all the way upward to the natural horizontal position in order to stand at right angles to the spine.
In breathing a singer is required to take in, on an average, from 100 to 150 cubic inches of air, and one of the purposes of artistic breathing is to provide room in the chest-cavity for the expansion of the lungs due to this intake.
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