92 RESPIRATION 



the right being shorter, larger and more nearly horizontal than the left. 

 These tubes, provided, like the trachea, with imperfect cartilaginous 

 rings, enter the lungs, divide and subdivide, until the minute ramifica- 

 tions of the bronchial tree open directly into the air-cells. After pene- 

 trating the lungs, the cartilages become irregular and are in the form 



Fig;. 29. Lungs, anterior view (Sappey). 



I, upper lobe of the left lung ; 2, lower lobe ; 3, fissure; 4, notch corresponding to the apex of the 

 heart; 5, pericardium; 6, upper lobe of the right lung; 7, middle lobe ; 8, lower lobe ; 9, fissure ; 10, 

 fissure ; n, diaphragm ; 12, anterior mediastinum ; 13, thyroid gland ; 14, middle cervical aponeurosis ; 

 15, process of attachment of the mediastinum to the pericardium ; 16, 16, seventh ribs; 17, 17, trans- 

 versales muscles ; 18, linea alba. 



of oblong angular plates, which are so disposed as to completely en- 

 circle the tubes. In tubes of very small size, these plates are fewer 

 than in the larger bronchia, until in tubes of a diameter less than -fa of 

 an inch (0.5 millimeter), they are lost. 



The walls of the trachea and bronchial tubes are composed of two 

 distinct membranes ; an external membrane, between the layers of which 



