Page:Popular Science Monthly Volume 36.djvu/540

524 much force in a short time. One arm may use its whole strength without its work representing, in the unit of time, a great number of kilogrammetres. So, whatever form the exercise takes, if the arm alone is working, we shall not find that the breathing is much quickened. The exercise may induce local fatigue before the intensity of the respiratory need has increased. It may even happen that the work of both arms together does not, after a given time, amount to enough to demand more ample respirations.

In general, the exercises which are performed with the legs represent more work than those which are performed with the arms. The muscles of the upper limbs could not support, without extreme fatigue, an expenditure of force which will cause no effort to the lower limbs. It is not tiring to any one to walk five hundred metres in five minutes: what gymnast could traverse the same distance in the same time hanging by his hands from a stretched rope? The total mechanical work would be, however, the same—displacing the same weight through the same horizontal distance.

We must not, then, trust to the muscles of the arms to expand the chest. Muscular exercise can only lead to the development of the thorax in an indirect manner, and in no way by a direct effect comparable to the increase in size of a muscle which works. The muscle which contracts often becomes larger because its nutrition is more active. But the chest only expands when the surcharge of the blood with carbonic acid creates a need of a greater quantity of oxygen for hæmatosis.

It is to the more active respiratory need, to the "thirst for air," that the instinctive movement by which the ribs are more energetically raised is due, in order to draw into the lungs a greater quantity of air.

The thirst for air, carried too far, produces breathlessness, which is nothing else than a powerless struggle of the system seeking in vain to satisfy a need. When breathlessness is very moderate, it causes very ample respiratory movements; but when it is excessive, the breathing becomes very shallow as well as very rapid. So that exercise has no longer any effect in expanding the chest when breathlessness reaches an extreme degree.

To sum up, the most profitable way of dilating the lungs, developing the thorax, and expanding the chest, consists in the performance of exercises capable of increasing the respiratory need, without pushing them so as to produce an extreme degree of breathlessness.

If we pass from physiological explanation to observation of facts, we see that practice gives a striking confirmation of theory.