The capacity of the lungs for air is a very variable quantity:
1. The quantity of air taken in with a single inspiration in quiet breathing (tidal air) is about 20-30 cubic inches.
2. The quantity taken in with the deepest possible inspiration (complemental air) is about 100 cubic inches.
3. The quantity that may be expelled by the most forcible expiration (supplemental air) is about 100 cubic inches.
4. The quantity that can under no circumstances be expelled (residual air) is about 100 cubic inches.
5. The quantity that can be expelled after the most forcible inspiration—i.e., the amount of air that can be moved—indicates the vital capacity. This varies very much with the individual, and depends not a little on the elasticity of the chest walls, and so diminishes with age. It follows that youth is the best period for the development of the chest, and the time to learn that special breath-control so essential to good singing and speaking.
When the ribs have been raised by inspiration and the abdominal organs pressed down by the diaphragm, the chest, on the cessation of the act, tends to resume its former shape, owing to elastic recoil quite apart from all muscular action; in other words, inspiration is active, expiration largely passive. With the voice-user, especially the singer, expiration becomes the more important, and the more difficult to control, as will be shown later.
It must now be apparent that such use of the voice as is necessitated by speaking for the public, or by singing, still more, perhaps, must tend to the general welfare of the body—i.e., the hygiene of respiration is evident from the physiology. Actual experience proves this to be the case. The author has known the greatest improvement in health and vigor follow on the judicious use of the voice, owing largely to a more active respiration. It also follows, however, that exhaustion may result from the excessive use of the respiratory muscles, as with any others, even when the method of chest-expansion is quite correct. Before condemning any vocal method one does well to inquire in regard to the extent to which it has been employed, as well as the circumstances of the voice-user. A poor clergyman worried with the fear of being supplanted by another man, or a singer unable to secure employment, possibly from lack of means to advertise himself, is not likely to grow fat under any method of vocal exercise, be it ever so physiological; while the prima donna who has chanced to please the popular taste and become a favorite may "wax fat and kick."
Figs. 14, A and B, are to be compared: that on the left shows the position of the diaphragm, abdominal walls, etc., during expiration; the one on the right, during inspiration. The relative quantities of air in the chest in each case are approximately indicated by the shaded areas.