RESPIRATION 273 



the mouth. Fine particles which may lodge in the 

 trachea are therefore carried outside instead of being al- 

 lowed to accumulate in the lungs. Nature has furnished 

 us with a number of admirable contrivances by which the 

 lungs are shielded from injury. The broad nasal cavity 

 for warming the air and collecting dust, the epiglottis 

 which closes the opening to the air passages at the very 

 moment when materials are apt to pass into them, the 

 tracheal cartilages to keep open the trachea and thus to 

 insure access of air to the lungs, and the fine tracheal cilia 

 beating in the right direction to carry away offending par- 

 ticles all these structures act so as to pro- 

 vide the lungs with air devoid of solid 

 matter. 



At its lower end the trachea divides into 

 two bronchi, one for each lung, and these 

 two tubes subdivide into smaller and 

 smaller ones. The final subdivisions lead 

 to minute pockets, the air cells, the walls of 

 which are exceedingly thin and abundantly 

 furnished with capillary blood vessels. A 

 large surface is thus provided in which the open showing the 



-,,,., , , . . . , air cells, C: bron- 



blood is brought into intimate contact with c hi a i tubes, r. 

 air, the thin walls by which the two are 

 separated facilitating the exchange of oxygen and carbon 

 dioxide which, as we have seen, is the essential function 

 of organs of respiration. The total surface of the numerous 

 air cells is estimated to be about 15,000 square feet, an area 

 equal to the floor space of a fair-sized dwelling. 



The lungs are fairly large organs, pinkish in color and 

 of very spongy texture, and they fill most of the chest which 

 is not occupied by the heart. They are surrounded by a 

 double membrane, the pleura, one layer of which is closely 

 applied to the lungs while the other forms the inner lining 



18 



