of the whole thorax.  Between the ribs and the cartilages the space is filled by the intercostal muscles (vide fig. 2), the action of which, in conjunction with other muscles, is to elevate the ribs.  It is, however, unnecessary to enter into anatomical details, and describe all those muscles which elevate and rotate the ribs, and thereby cause enlargement of the thorax in its antero-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.]