THE LUNGS 179 



The larynx contains some thickened cartilage, popularly called 

 Adam's apple, which projects slightly on the front of the neck. The 

 air passes down the trachea or windpipe into the two branches, 

 the bronchi, which subdivide again and again into the bronchial 

 tubes, which in turn end in small pouchlike sacs called the air 

 sacs. All of the air tubes are lined with mucous membrane 

 vvhich warms and moistens the air. The trachea is lined with 

 specialized epithelial cells bearing hairlike processes or cilia. These 

 cilia are in constant motion. Their function is to push or move any 

 solid particles, in the air, into the throat, where they are expelled. 



There are numerous capillaries in the air sacs. The walls of 

 the capillaries are membranous and air easily diffuses through 

 them into the blood. The red corpuscles take the oxygen from 

 the air. Thus blood becomes oxygenated. At the same time, the 

 blood gives out its carbon dioxide. The air sacs may be com- 

 pared with tiny balloons. The fact that they are so numerous and 

 are capable of such inflation affords a tremendous surface for the 

 absorption of oxygen even though the lungs themselves are fairly 

 small. The large absorbing surface of the air sacs makes possible 

 the presence of a tremendous number of capillaries running through 

 them. Therefore, a great deal of oxygen quickly passes into the 

 blood and carbon dioxide passes out. The lungs consist of mil- 

 lions of air sacs and bronchial tubes held together by connective 

 tissue. The moist membranous pleurae cover the lungs, and pre- 

 vent friction during the breathing movements. 



Diseases of the respiratory tract. The mucous membrane 

 lining the pharynx contains many lymph glands and at the back 

 and upper part of the cavity there is a large mass of this lym- 

 phoid tissue. During infancy and childhood this tissue may 

 increase greatly, and the child is then said to have adenoids. 

 Adenoids are a menace to health. They may obstruct the openings 

 to the Eustachian tubes, and in some cases cause deafness. They 

 usually interfere with the passage of air through the nose and 



