Such an instrument is better adapted for use by adults than by children; and if it should ever come to be introduced into the schools, it should not in any case be used below the elementary grades.
The person who is going to measure the capacity of his lungs by means of the spirometer, begins by drawing in an unusually deep or forced inhalation; then, after holding his breath for a moment, he proceeds to expel into the rubber tube all the air in his lungs, in a forced exhalation. In an exercise of this sort, all the difficulties of respiratory gymnastics are successively surmounted—inspiration, respiratory pause, expiration.
In fact, in accomplishing the forced inspiration, all the pulmonary alveoli must be dilated to the maximum extent, and at the same time the thorax must reach its maximum dilation. This is a very different matter from normal inspiration, which does not completely dilate the alveoli. As a matter of fact, the tidal air or air of respiration, i.e., the air taken in and expelled in each normal respiration, is about 500 cubic centimetres; but the sum total of air habitually contained in the lungs is made up of two quantities: first, that which may be emitted by a forced expiration, the supplemental or reserve air, amounting to 1,600 cubic centimetres; and secondly, the air which cannot ever be emitted, because no amount of effort could completely expel all the air from the lungs; residual air or respiratory residuum amounting to 1,200 cubic centimetres. To recapitulate, the average pulmonary capacity is the sum of the following average quantities of air:
| Residual air, or respiratory residuum (which can never be expelled from the lungs) | = 1200 cu. cm. |
| Respiratory reserve (which can be expelled by a forced expiration) | = 1600 cu. cm. |
| Tidal air | = 500 cu. cm. |
| Complementary air (which can be drawn in by a forced inspiration) | = 1670 cu. cm. |
Accordingly, the total pulmonary capacity is about 5,000 cubic centimetres, or five litres. But in normal respiration, the capacity is less, i.e., about 3,300 cubic centimetres, the air due to a forced inspiration not being included.
Therefore, in each normal respiration a half litre of pure air (assuming that it is pure) is introduced and mingled with the vitiated air already within the lungs; and since, in expiration, a third only of this 500 cubic centimetres is eliminated, it follows that 166 cubic centimetres are mingled with the 3,300 cubic centimetres; in other words, that only one-tenth of the air is renewed in each normal act of respiration.
A very energetic forced inspiration may draw into the lungs, in addition to the customary 500 cubic centimetres, an additional 1,670 cubic centimetres of pure air, complementary air. In this case the lungs contain upward of 5,000 cubic centimetres of air.
The forced expiration which follows upon this extra deep inhalation purges the lungs of the vitiated air which has formed there. In this way we complete an exercise that is eminently hygienic.
Now, these spirometric movements are fraught with difficulties: 1. The forced inspiration, deep enough to extend the alveoli, may be more or less complete. If a cloth wrung out in cold water is laid across the shoulders, the inspiration which follows as a result of reflex action is far deeper than that produced by an act of will; this proves that the lungs can be dilated to a point beyond that which seems to us to be the extreme limit, and therefore that with practice we may learn to dilate our lungs still further.