152 The Vertebrate Organ Systems 



Finally the walls of these bronchi consist of thin layers of smooth 

 muscle and mucosa. These smaller divisions are all within the lung 

 substance. Each division of a bronchiole finally terminates in a tiny air 

 sac, the alveolus. 



The epithelium lining the respiratory tract is of the columnar type 

 with the bases of the cells resting on a special connective tissue mem- 

 brane, the basement membrane. Most of the cells have cilia at the free 

 end. Beneath the basement membrane are many mucous glands which 

 keep the walls of the tube moist. This mucus catches the dust particles 

 which may enter with the air, and the waving cilia carry the mucus 

 and contained dust up to the throat where it can be expectorated. 



UARYMX 



Fig. 45. — The respiratory tract. (After Dalton, from Zoethout 

 and Tuttle: Textbook of Physiology.) 



The Lungs. — The lobated lungs occupy the greater part of the 

 thoracic cavity of mammals. In human beings, the right lung is divided 

 into three lobes and the left into two. Surrounding each lung is a 

 double-walled sac, the pleura. One portion of this sac is adherent to 

 the walls of the chest and diaphragm. This is the parietal pleura. The 



