368 



COMPARATIVE PHYSIOLOGY. 



great extent, lined with flattened cells and supporting innu- 

 merable capillary blood-vessels. The air is admitted to the com- 



FiG. 290. — Bronchia and lungs, posterior view (Sappey). 1, 1, summit of lungs; 2, 2, 

 base of lungs; 3, trachea; 4, right bronchus; 5, division to upper lobe of ning; 6, 

 division to Tower lobe; 7, left bronchus; 8, division to upper lobe; 9, division to 

 lower lobe; 10, left branch of pulmonary artery; 11, right branch; 13, left auricle 

 of heart; 13, left superior pulmonary vein; 14, left inferior pulmonary vein; 15, 

 right superior pulmonary vein; 16, right inferior pulmonary vein; 17, inferior vena 

 cava; 18, left ventricle of heart; 19, right ventricle. 



plicated foldings of this membrane by tubes which remain, 

 throughout the greater part of their extent, open, being com- 

 posed of cartilaginous rings, completed by soft tissues, of which 

 plain muscle-cells form an important part, serving to main- 

 tain a tonic resistance against pulmonary and bronchial press- 

 ure, as well as serving to aid ia the act of coughing, etc., 

 so important in expelling foreign bodies or preventing their 

 ingress. 



The bronchial tubes are lined with a mucous membrane, 

 kept moist by the secretions of its glands, and covered with 

 ciliated epithelium, as are also the nasal passages, which, by 

 the outward currents they create, favor diffusion of gases and 

 removal of excess of mucus. The thoracic walls and the lungs 



