THE LUNGS 



195 



bound together with tough connective tissue and lined with 

 mucous membrane. 



336. The bronchi. Within the chest the trachea divides 

 into two tubes, the bronchi. Each bronchus divides again 

 and again, until the finest 



divisions are about -fa 

 inch in diameter. Like 

 the trachea, each bron- 

 chus is composed of hoops 

 of cartilage lined with 

 mucous membrane. 



337. Cilia. The sur- 

 face of the epithelium of 

 the mucous membrane of 

 most of the nose and 

 larynx and the whole of 

 the trachea and bronchi 

 is covered with micro- 

 scopic hairs, the cilia. 

 Each cilium is slightly 

 curved upward and waves 

 continually in a rapid up 

 and down motion which 



and Diagram of trachea and bronchi. 



the a l ar y nx> ^ trachea. c bronchi. 



* d air sacs of the lung. 



lungs. 



338. The lungs. The ends of the bronchi are studded 

 with numerous cup-shaped depressions called air sacs, each 

 about Yj-g- inch in diameter. Upon the inner surface of 

 each air sac is a close network of capillary blood tubes. 

 The collection of bronchi, air sacs, and blood tubes forms 

 two spongy bodies called lungs. Between the air sacs 

 is a thin layer of connective tissue. The lungs can be 



tends to force dust 

 mucus away from 



