VARIATIONS IN BREATHING 175 



of the chest. Just as the air pushes the rubber in, so 

 the air pushes its way through the air passages, dis- 

 tends the walls of the lungs, and presses them against 

 the receding chest wall. 



In expiration, the relaxing muscles permit the ribs to 

 drop, the chest cavity becomes smaller, the elastic lungs 

 contract, and the excess of air in them is pushed out. 



Rapidity of breathing. Owing to the large bulk of 

 air required to furnish the oxygen necessary for the 

 body's activity and to remove waste products, amounts 

 sufficient for more than a moment's use cannot be stored 

 up in the body. As a result, constant and fairly rapid 

 breathing is necessary. Its rapidity varies somewhat 

 with age and sex. Women breathe more rapidly than 

 men, and children more rapidly than either. 



Effects of exercise. Exercise causes a great increase 

 in both the depth and the rate of breathing, because the 

 additional amount of energy expended means a corre- 

 sponding increase in the body's demand for oxygen as 

 well as for the removal of its waste products. If the 

 exercise is mild, an increase in the depth of breathing 

 may at first give all the air necessary. If, however, 

 the exercise is continued and severe, the respiration 

 usually becomes rapid as well as deep, since to get all 

 the air necessary from mere depth of respiration would 

 require such extensive chest movements as would in- 

 terfere with the exercise itself. Thus, when exercise 

 becomes too severe, one has to stop to take breath, since 

 the demand for air becomes so imperative and gives 

 rise to so much distress that it cannot be disregarded. 

 The body is thus automatically protected from an in- 

 adequate supply of oxygen and from poisoning by an 

 accumulation of waste products, as a result of too great 

 activity. 



