82 



APPLIED PHYSIOLOGY 



always in motion. The hairs vibrate in such a way as to 

 drive anything out of the air passages. In the finest 

 bronchi they reach a considerable way across the tube and 

 entangle any dust which may reach them. So they pro- 

 tect the delicate air sacs from dust. They also force 

 mucus towards the mouth, so that we can get rid of it 



a 



A slice from the trachea (X 200). 



a cartilage. b glands in the mucous membrane. 



c lining of epithelial cells. 



d cilia upon the surface of the epithelium. 



when we have a cold. Finally, the cilia make air currents 

 in the tiny air tubes, and so hasten the exchange of pure 

 for impure air. 



148. Breathing sounds. As air goes in and out of the 

 lungs, it makes a low, blowing sound. Listen to each 

 other's chests while breathing deeply and notice these 

 sounds of breathing. Listen, also, while a person counts 

 three. You will feel the chest tremble and hear the voice 

 through the chest. The physician listens to the chest 

 when he thinks a person has lung disease. By the 



