RESPIRATION 



183 



muscular fiber, whose function is to draw the ends of the 

 rings together and reduce the caliber of the tube. The 

 whole tube is inclosed in a fibrous membrane, and lined, 

 like the rest of the passages, with mucous membrane. The 

 superficial layer of the epithelium of the trachea is ciliated, 

 that is, supplied with minute 

 hairlike prolongations. 



256. The Bronchi resemble 

 the trachea in structure, but 

 have a distinct layer of plain 

 muscle running around them. 



257. The Bronchial Tubes 

 and Alveoli. The bronchi 

 have numerous branches called 

 bronchial tubes reaching to 

 all parts of the lungs, each 

 branch ending finally in a 

 wider, funnel-shaped passage, 

 the alveolus, surrounded by 

 clusters of short, somewhat 



Fig. 101. Two alveoli, show- 

 ing the clusters of air cells. 



dilated sacs, the air cells (Fig. 101). All the tubes are lined, 

 like the trachea, with ciliated epithelium. 

 The cilia being continually in motion 

 drive out the mucus which is constantly 

 secreted, and along with it the dust 

 brought into the passages with the air. 

 258. The Air Cells are the hollow ex- 

 pansions of the alveolus (Fig. 102). 

 They have a lining of very fine epi- 



Fi 102 s f f thelium, without cilia, and within that 

 an alveolus, show- a close network of capillaries. In these 



mg air cells open- ^he ar teries of the lungs make connec- 



ing into the central 



cavity. tion with the veins. 



