254 



CHORDATE ANATOMY 



which contains the heart, esophagus, and the great blood-vessels which 

 leave the heart. In childhood, the color of the lungs is pinkish, but may 

 become slaty grey in the adult as the result of the accumulation of soot. 

 The structure of the lungs is admirably adapted to the need of exposing 

 to the air a large amount of surface, estimated to equal that of a balloon 



S-ALVEOLAR SAC 



Fig. 236. — Diagram of a lung lobule showing the subdivision of a bronchiolus into 

 alveolar ducts, sacs and alveoli. Respiratory epithelium may extend into the bronchioli. 

 (Redrawn after Bremer.) 



ten feet in diameter, and a section of the lungs shows that the volume of 

 air space greatly exceeds that of solid tissue. The required moisture is 

 supplied by mucous glands. 



The trachea or wind-pipe subdivides into bronchi, both structures 

 having cartilaginous supports. The bronchi divide into bronchioli, the 



