RESPIRATION 



189 



the neck (Fig. 47, E, and 48). Within the chest this tube is 

 divided into two branches, one entering each lung; these in 

 turn give rise to numerous branches, or bronchial tubes, as 

 they are called, which gradually diminish in size until they 

 are about one twenty-fifth of an inch in diameter. Each of 

 these terminates in a cluster of little pouches, or " air-cells," 

 having very thin walls, and covered with a capillary network, 

 the most intricate in the body (Fig. 49). 



4. These tubes are somewhat flexible, sufficiently so to bend 

 when the parts in which they are situated move j but they are 



FIG. 48. LARYNX, TRACHEA, AND 

 BRONCHIAL TUBES 



FIG. 49. DIAGRAM AND SECTION OF 

 THE AIR-CELLS 



greatly strengthened by bands or rings of cartilage which keep 

 the passages always open; otherwise there would be a con- 

 stantly-recurring tendency to collapse after every breath. The 

 lung-substance essentially consists of these bronchial tubes 

 and terminal air-cells, with the blood-vessels ramifying about 

 them (Fig. 50). At the top of the trachea is the larynx, a 

 sort of box of cartilage, across which are stretched the vocal 



4. Office of the bronchial tubes ? What further can you state of thiu 1 



