140 THE ENGINES OF THE HUMAN BODY 



biggest breath we are capable of, the lungs can hold 

 a little under five litres of air — about one gallon. Of 

 that amount 140 c.c. — about ^th of a litre, or a quarter 

 of a pint — is contained in the air passages, the rest is 

 distributed among the millions of air sacs. The walls 

 of" the air sacs are not uniform in outline, but show 

 saucer-like elevations on their outer surfaces, correspond- 

 ing to recesses or depressions within. These recesses 



ironchiolc 



Artery 



blood ) 



Fig. 34. 



Fig. 35. 



Fig. 34. — A cluster of air or respiratory chambers, with a bronchiole 

 leading to it. 



Fig. 35. — A cluster of air chambers with blood-vessels applied to them. 



of the air sacs are known as alveoli or air cells, and 

 they help to make the inner surfaces of the air sacs 

 more extensive. The interior of an air sac, thus in- 

 creased by the presence of the alveoli, is lined by a 

 delicate, thin, transparent membrane. If the lining 

 membranes could be stripped off from all the sacs of 

 both lungs and spread out as a continuous sheet, it has 

 been calculated that it would cover the floor of a room 

 which measures 30 feet square. The membrane which 

 lines the lungs of a man of middle size is supposed to 



