CHAPTER XXIX 



BREATHING IN MAN 



178. The lungs. Our 



lungs are two soft, rather 

 complex bags that are sus- 

 pended in the thorax, or 

 chest cavity, and are con- 

 nected with the pharynx 

 (Fig. 28, b) by means of 

 a tube called the trachea. 

 This windpipe branches 

 again and again, and ends 

 in thousands of tiny cham- 

 bers lined with a layer of 

 thin-walled cells in contact 

 with very fine blood vessels. 

 The gas exchange between 

 the outside air and the blood 

 takes place through the lin- 

 ing of these small air cham- 

 bers, which constitute the 

 working surface of the lungs. 

 The act of breathing is thus 

 a process of ventilating or 

 changing the gas contents 

 of the inside of these cham- 

 bers (see Fig. 47). 



179. The process of breathing. The lungs, consisting as they 

 do of air passages and air chambers, and having no muscular 

 tissue in their structure, are incapable of carrying on any 



148 



FIG. 47. The human lungs 



The arrows show the course of air from the 

 outside, m, mouth ; n, nostrils ; /, pharynx ; 

 /, larynx; /, trachea; , bronchi. The right 

 lung is shown cut open ; the bronchi branch 

 again and again, the last tubules ending in 

 delicate expansions, a, the air cells, or sacs ; 

 epi, the epiglottis, which closes over the air 

 pipe when food passes from the pharynx to 

 the esophagus e 



