THE RESPIRATORY ORGANS 



179 



Air Passages in the Lungs. In the lungs each bronchial 

 tube, or bronchus, divides into smaller branches, called bron- 

 chioles, and these divide, until finally they become minute 

 twigs, as shown in Figure 98. At the end of every twig, the 

 air passage is swollen into a chamber, or series of chambers, 

 somewhat larger than the 

 passage itself. These little 

 air spaces are known as 

 alveoli, and each is partially 

 subdivided into smaller 

 compartments, "air-sacs" 

 or "air-cells." There are 

 thousands of them in each 

 lung, and they are the 

 places where blood and air 

 exchange gases. This con- 

 struction provides a very 

 large amount of surface, it 

 being estimated that the 

 area so provided amounts 

 to about 960 square feet; 

 this is over one hundred 

 times the skin surface of the 

 body. The walls of the alveoli are very thin and elastic, so that 

 they are expanded when filled with air, and would become 

 shrunken and nearly collapsed if emptied. 



In spite of the filtering that the air receives in its passage 

 through the nose, much fine dust is constantly passing into 

 the windpipe and lungs. This would produce trouble were 

 there no means for its removal; but the whole series of pas- 

 sages, the trachea and all the bronchioles in the lungs, are 

 lined with the tiny, waving, hair-like bodies we have called 

 cilia; Fig. 3 d. These cilia are in constant motion, creating 

 a current upward toward the pharynx, and any dust taken in 

 with the air and caught on the moist surfaces is carried up- 



FIG. 



'esieb 



99. TWO OF THE AIR SACS IN 



THE LUNGS 



Highly magnified. The black lines are sec- 

 tions of the capillaries that fill the walls of 

 the air sacs. The arrows show the direc- 

 tion of the exchange of gas. 



