3. The quantity that can be expelled by a forcible expiration is about 100 cubic inches.
4. The quantity that cannot be expelled at all is about 100 cubic inches.
The above are named: (1) The tidal air; (2) complemental air; (3) supplemental air; (4) residual air. The quantity that can be expelled by the most forcible expiration after the most forcible inspiration, that is, the air that can be moved, indicating the "vital capacity," is about 225-250 inches.
The chest is enlarged by the muscles of inspiration, the principal of which is the diaphragm or midriff. This muscle (tendinous in the centre) is attached to the spinal column (behind) and to the last six or seven ribs. When it contracts it becomes less domed upward, and is pressed down more or less on the contents of the abdomen; hence the walls of the latter move outward. During ordinary inspiration the lower ribs are steadied by other muscles, so that no indrawing of these ribs takes place, but a very forcible expiration makes such indrawing very noticeable. In addition to the enlargement of the chest by the descent of the diaphragm, the ribs are elevated and everted by the muscles attached to them, with the total result that the chest cavity is enlarged in all its three diameters during inspiration. The first rib is fixed by muscles from above. During extremely forced inspiration a large proportion of all the muscles of the body may act. Ordinary expiration is the result largely of the elastic recoil of the chest walls, only a few muscles taking part. The diaphragm ascends and becomes more domed. During forced expiration many other muscles are called into action. It is of importance for the singer and speaker to note: (1) That the chest cavity should be increased in all its directions; (2) that the muscular action should be easy and under perfect control, but also vigorous when required; (3) that the breath be taken through the nostrils when the individual is not actually vocalizing or about to do so; (4) that the breath be kept in or let out in the proportion required.
Breathing is a reflex or involuntary act. The respiratory centre, consisting of an expiratory and inspiratory division, is situated in the bulb, or medulla oblongata, the portion of the brain just above the spinal cord. All the ingoing nervous impulses affect respiration through the outgoing impulses that pass along the nerves to the muscles; that is, the ingoing impulses pass up by the nerves from the lungs to the centre, and thence along other nerves to the respiratory muscles. The condition of the blood determines the activity of the respiratory centre, but the incoming impulses regulate this activity. The respiratory centre can be approached from every part of the body.
Hygiene.
Every thing that favors the full and free expansion of the chest in a pure atmosphere is favorable, and the reverse unfavorable. Corsets are against the laws of beauty, are unnecessary for support, and may by compression injure and displace important organs, as the liver, stomach, etc.; and must interfere with the fullest expansion of the chest. They have militated against the physical, and indirectly the moral and mental advancement of the race.
Practical Exercises.
I. Measurements of the chest.
II. Exercises to strengthen muscles, promote complete expansion, regulate inflow and outflow of air, etc.