112 THE WONDERFUL HOUSE THAT JACK HAS 



in our lungs can hold on the average more than a gallon 

 of air. All the vast membrane-surface of these air- 

 cells, penetrated by its net-work of capillaries, would, 

 if spread out flat, cover far more space than the out- 

 side surface of the entire body. These facts show us 

 how wisely the Creator has planned for the complete 

 purification of blood in the lungs. 



Although the capacity of the lungs is from two hun- 

 dred fifty to three hundred fifty cubic inches, about 

 two hundred cubic inches usually remain in them. 

 On the average, then, only one-tenth of the entire 

 capacity, or thirty cubic inches (a little less than a 

 pint), is taken in at each ordinary breath, one-fifth 

 of this, or six cubic inches, being oxygen. It is esti- 

 mated that about one-fifth of this oxygen is given to 

 the blood through the membrane of the air-cells. 

 A little less carbonic acid gas, together with some 

 watery vapor, is thought to be received from the blood 

 and breathed out at every breath. Taking air into 

 the lungs is often called inspiring or inhaling, and 

 forcing the air out, expiring or exhaling. Respiration 

 is the name that includes both acts. When a person's 

 respiration is said to be twenty-five, the meaning is 

 that he breathes both in and out twenty-five times a 

 minute. An exchange of oxygen for carbonic acid 

 gas and watery vapor takes place every time men or 

 other animals breathe. 



All animals are exchanging carbonic acid gas for 

 oxygen in this way. Everything out-of-doors con- 



