THE RESPIRATORY SYSTEM 115 



external surface is convex and adapted to the ribs and the inter- 

 costal muscles. The internal surface is irregular and less exten- 

 sive. In the living animal the lung is of a pinkish color, due to 

 the blood in its tissues. It becomes pale gray in the bled subject, 

 while the lung from an unbled subject is dark red. Lung tissue 

 is soft, spongy, elastic and floats in water. Consequently the 

 lungs are colloquially called "the lights." 



Fig. 34. — Diagram of a longitudinal section of two alveoli with their com- 

 mon bronchiole, and showing in black the larger blood-vessels in the walls. 

 (From Hough and Sedgwick's The Human Mechanism, by permission of Ginn 

 and Company, Publishers.) 



A microscopic examination of the trachea shows it to be lined 

 with a mucous membrane composed of ciliated epithelium 

 interspersed with mucous cells. The bronchi and bronchial 

 tubes have flakes of cartilage instead of rings of this tissue in 

 their walls. This gives them a flexibility, and yet sufficient 

 rigidity, so that they are extremely efficient as tubes for conduct- 

 ing the air. Each bronchial tube divides and subdivides until 

 the extremely small tubes termed bronchioles are formed. Bron- 

 chioles end in minute blind compartments, about 3^5 inch in 

 diameter, known as alveoli. Each 'air alveolus has several air 

 cells, as shown in Fig. 34. The air cells average only J^oo inch 



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