THE ATMOSPHERE 89 



The lungs are composed of combinations of air sacs or 

 air cells which are grouped around the numerous small 

 subdivisions of the bronchial tubes. These air sacs 

 are composed of a thin elastic outer layer of connective 

 tissue and a lining of mucous membrane. Between 

 these two layers the minute thin-walled capillaries are 

 located. The blood here is separated from the air only 

 by thin walls of the capillaries and the mucous mem- 

 brane lining the air sacs. 



The Air Passages. Air enters the body through 

 the mouth or nose and passes through the pharynx and 

 larynx into the windpipe or trachea. The trachea is 

 a tube about three fourths of an inch in diameter and 

 about four inches long. Its walls are strengthened by 

 rings of cartilage. The trachea divides at its lower 

 end into two branches called the bronchial tubes. These 

 subdivide and finally terminate in the tiny air sacs. The 

 tubes and lungs are lined throughout with a mucous 

 membrane. The entrance to the larynx and trachea 

 is guarded by the epiglottis, which stands open to admit 

 air. As food is forced back in the act of swallowing it 

 strikes the epiglottis and closes it over the trachea, 

 forming a bridge over which the food passes into the 

 gullet. 



Mechanism of Breathing. Breathing involves two 

 processes, inhaling or inspiration and exhaling or expira- 

 tion. Inhaling consists of drawing the air through the 

 various parts of the lungs to the air sacs. Exhaling is 

 simply the reverse of this process. The diaphragm 

 forms a movable floor to the chest cavity and draws 

 air into the lungs. The air is really forced into the lungs 

 by the air pressure, to relieve the partial vacuum that 

 has been created. When the diaphragm relaxes, it returns 



