THE RESPIRATORY SYSTEM 



91 



than rings of this tissue in their walls. This gives them a flexi- 

 bility, and yet sufficient rigidity, so that they are extremely efficient 

 as tubes for conducting the air. Each bronchial tube divides and 

 subdivides until the extremely small tubes termed bronchioles 

 are formed. Bronchioles end in minute compartments, about ts 

 inch in diameter, known as alveoli. Each air alveolus has sev- 

 eral air cells, as shown in Fig. 26. The air cells average only ttt 

 inch in diameter. Their walls are composed of a single layer of 



Fig. 26. — Diagram of a longitudinal section of two alveoli with their com- 

 mon bronchiole, and showng in black the larger blood-vessels in the connective 

 tissue. (From Hough and Sedgwick's The Human Mechanism, by permission 

 of Ginn and Company, Pubhshers.) 



epithelial cells, supported and given elasticity by fibers of yellow 

 elastic tissue. The interchange of gases between the external air 

 and the blood takes place through these cells. The alveoli are 

 bound together in groups bj^ connective tissue to form larger masses, 

 termed lobules. These lobules, in turn, are joined to form the dif- 

 ferent lobes of the lung. 



The pleura is the serous membrane which lines the thoracic 

 cavity. It is arranged to form two sacs, and is reflected at the roots 

 of the lungs to invest these and other organs in the cavity. Its 



