RESPIRATION. 



which, the trachea, or windpipe, runs along the front of tl 

 neck (Fig. 34, E, and 35) .With- 

 in the chest this tube divides 

 into two branches, one entering 

 each lung; these in turn give 

 rise to numerous branches, or 

 bronchial tubes, as they are 

 called, which gradually dimin- 

 ish in size until they are about 

 one- twenty-fifth of an inch in 

 diameter. Each of these ter- 

 minates 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. 36). 



4. These tubes are some- 

 what flexible, sufficiently so to 

 bend when the parts move in 

 which they are situated ; but 

 they are greatly strengthened 

 by bands or rings of cartilage 

 which keep the passages always 

 open; otherwise there would 

 be a constantly-recurring ten- 

 dency to collapse after every 

 breath. The lung-substance 

 essentially consists of these 

 bronchial tubes and terminal 

 air-cells, with the blood-ves- 

 sels ramifying . about them 

 (Fig. 37). At the top of the 



v . ,! , , FIG. 36. DIAGRAM AND SECTION OP 



trachea is the larynx, a sort of THE AIR-CELLS. 



PIG. 35. LARYNX, TBACHEA, AND 

 BRONCHIAL TUBES. 



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



