RESPIRATORY SYSTEM 349 



by incomplete rings of cartilage. These can be easily 

 felt on the ventral side of the neck. Attention has 

 already been called to the fact that the trachea 

 divides at the base of the lungs into a right and a left 

 bronchus. 



The lungs. Each bronchus divides and subdivides a 

 great many times. As these bronchial tubes become 

 smaller, their walls become thinner, each tube ending 

 finally in a branching air sac with extremely thin walls 

 of elastic tissue. The walls of these sacs are richly 

 filled with blood capillaries. The bronchial tubes and 

 air sacs are all bound together with a soft, pink, elastic 

 tissue. Over the whole is stretched an elastic mem- 

 brane known as the pleura. 



The pleura. This membrane covers both lungs. At 

 the point where the bronchi enter the lungs the pleura 

 is turned back and lines the interior of the chest cavity. 

 This is another membrane which secretes a fluid to 

 reduce friction. When the lungs are filled with air 

 the two layers of pleura rub against each other. 



Mechanism of breathing 1 . We are all familiar with 

 the fact that certain movements of the body accompany 

 breathing. As a result of those movements the chest 

 cavity is enlarged. This causes the air in the lungs to 

 expand ; in this way the pressure is reduced. The greater 

 atmospheric pressure outside of the body then forces 

 more air into the lungs. Usually about one pint of air 

 is taken in at 'each inhalation. In exhaling, the chest 

 again becomes smaller. This pressure upon the lungs, 



