RESPIRATORY CHAMBERS 141 



be one hundred times the area of the skin which covers 

 his body. This membrane, if inflated, would form a 

 balloon 10 feet in diameter, the cover being little thicker 

 than the film of a soap bubble. Every breath we take 

 is distributed over the entire surface of this extensive 

 membrane. 



We cannot empty the air sacs of our lungs completely, 

 however hard we may squeeze or compress our chests. 

 Nature has taken care to render that impossible — for a 

 reason we are now to explain. It is not so with the 

 engine of a motor cycle ; it clears all the air from the 

 cylinder with each exhaust stroke. The minute air sacs 

 of the two lungs of a grown man, as we have seen, can 

 hold rather less than 5 litres (4700 c.c.) of air when the 

 deepest breath is taken, but in ordinary breathing they 

 contain only about 3 litres. With each expiration about 

 half a litre (500 c.c.) is expelled from the two lungs ; 

 from each air sac only one-fifth is discharged with each 

 breath ; with the next inspiration a fifth is again added, 

 but in all phases of the respiratory tide the air sacs retain 

 at least four-fifths of their charge. If we breathe out very 

 hard we can reduce the amount in the air sacs to one- 

 third of the normal charge, but the remaining or residual 

 amount cannot be expelled by the greatest effort we can 

 make. The mouths of the respiratory chambers are so 

 framed that they automatically close or collapse when their 

 charge of air is reduced to one-third of the normal. 



To understand why the air sacs should be thus 

 controlled and safeguarded we must look at the nature 

 of the operations carried on in them. At one time it was 

 supposed that their cavities were combustion chambers 

 like the cylinders of engines, and that the heat which 

 warmed the body was really produced in the lungs. 

 Presently it will be shown how chemists discovered that 

 combustion is carried out elsewhere in the body. The 

 air sacs are, however, connected with combustion. We 

 have now to see how the red blood discs are brought to 

 them in order that they may pick up the loads of oxygen 

 which they carry off to feed the myriads of vital com- 



