RESPIRATION 



79 



each other and to the outside atmosphere will be seen 

 through a study of the 



Air Passages. The air passages consist of a system of 

 tubes which form a continuous passageway between the 

 outside atmosphere and the different divisions of the lungs. 

 The air passes through them as it enters and leaves the 

 lungs, a fact which accounts for the name. 



The incoming air first enters the nostrils. These consist 

 of two narrow passages lying side by side in the nose, and 

 connec'ting with the pharynx 

 behind. The lining of the 

 nostrils, called mucous mem- 

 brane, is quite thick, and has 

 its surface much extended 

 by reason of being spread 

 over some thin, scroll-shaped 

 bones that project into the 

 passage. This membrane 

 is well supplied with blood 

 vessels and secretes a con- 

 siderable quantity of liquid. 

 Because of the nature and 

 arrangement of the mem- 

 brane, the nostrils are able p IG> 34. Model of section 

 to warm and moisten the through the head, showing upper air 

 incoming air, and to free it passages and other parts, i. Left nos- 

 .. , . . ,., . tril. 2. Pharynx. 3. Tongue and cav- 



from dust particles, preparing ityofmouth> 4 . Larynx . 5 . Trachea. 

 it, in -this way, for entrance (,. Esophagus, 

 into the lungs (Fig. 34). 



The nostrils are separated from the mouth by a thin 

 layer of bone, and back of both the mouth and the nostrils 

 is the pharynx. The pharynx and the month serve as 

 parts of the food canal, as well as air passages, and are 



