5 24 THE POPULAR SCIENCE MONTHLY. 



pend 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 hun- 

 dred 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 horizon- 

 tal 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 quan- 

 tity of oxygen for haematosis. 



It is to the more active respiratory need, to the " thirst for air," 

 that the instinctive movement by which the ribs are more ener- 

 getically 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, devel- 

 oping the thorax, and expanding the chest, consists in the per- 

 formance 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. 



