314 EESPIKATION 



by following the course of the air as it passes through 

 them. 



Air enters the body by way of the nostrils or mouth and 

 passes backward into the pharynx or throat cavity. Here, 

 t^n'o passages are open to it: one, the gullet, leads to the 

 stomach; the other, the trachea, or windpipe, leads to the 

 lungs. As we inhale, the walls of the esophagus contract, 

 and the air is sucked down the windpipe and enters the 

 lungs. Just how this suction is produced we shall investi- 

 gate later. For the present let us examine these passages 

 a little more in detail. The figure (see Fig. 145) shows the 

 arrangement of these parts with relation to one another. 

 If we examine it carefully we note that the two external 

 nostrils are merely openings into a large cavity (the nasal 

 cavity) located just above the roof of the mouth and con- 

 necting at the back with the pharynx. The nostril en- 

 trance to this cavity is guarded by many hairs which serve 

 to catch and remove dust particles from the air. The 

 cavity itself is lined with a layer of mucous membrane, the 

 secretion from which aids in catching particles of dust and 

 germs that have escaped the , hairs. This membrane is 

 also lined with many blood vessels, and the heat which 

 escapes from them serves to warm the air before it comes 

 in contact with the dehcate membranes of the lungs. 

 These facts show clearly that the nose is adapted for the 

 entrance of air to the body, and that while mouth breath- 

 ing permits the air to reach the lungs, it has none of the 

 safeguards against impurities and cold that are found in 

 the nasal passage. Mouth breathing is a habit to be 

 avoided. 



The location of the top of the windpipe may be easily 

 found by applying the fingers to the outside of the throat 



