RESPIRATION AND ENERGY IN MAN 127 



the throat with a quick movement, and then more slowly 

 recover their former position (Fig. 18). In this way any dust 

 particles that have passed the barrier of hairs at the nostril 

 openings and the mucus secreted by the membrane are 

 moved steadily upward until they reach a point where they 

 can be coughed out into the mouth cavity. 



180. The lungs.^ — When the finest branches of the 

 bronchial tubes are traced, we find that each one ends in 

 a branching air sac with extremely thin walls of elastic 

 tissue (Fig. 41). When air comes into these sacs, they are 

 expanded; but as expiration begins, their elastic walls 

 help to force back through the branches of the windpipe the 

 air that has been taken into the lungs. 



^ 181. Blood supply to the lungs. — The artery supplying 

 the lungs, as we learned (167), arises from the right 

 ventricle and soon divides into two branches, one for the 

 right and one for the left lung. Within the lung tissue each 

 artery divides into small branches that follow the course of 

 the bronchial tubes to the air sacs. Here the arteries com- 

 municate with a maze of capillaries that run just beneath 

 the thin lining of the air sacs. , It is here that the exchange 

 of material takes place between the blood and the inhaled 

 air, for the two are separated only by the extremely thin 



1 One can get a good idea of the structure of the human air 

 passages and lungs by securing from the butcher the chest organs 

 of a sheep or calf. These consist of the larynx, windpipe, and its 

 branches, and the two lungs, between which Kes the heart. A piece 

 of the diaphragm should also be secured if possible. The lungs 

 are composed of soft, pink tissue, easily compressed by the hands. 

 If air is forced through a tube inserted in the glottis opening, the 

 lungs swell, and when fully distended occupy a space several times 

 their size when collapsed. Just as soon as one ceases to blow into 

 the lungs, these organs begin to collapse, and soon reach their 

 former condition. The characteristics of the lungs and air pas- 

 sages should be demonstrated before 180 is assigned for study. 



