240 THE RESPIRATORY APPARATUS 



entangled in the mucus toward the entrance of the 

 respiratory tract, where it is expectorated. The 

 submucosa contains numbers of mucous glands. 



The Bronchi. The bronchi enter the hilum of the 

 corresponding lung. The right is the shorter, wider, 

 and more horizontal, and enters the lung opposite the 

 fifth thoracic vertebra, the larger azygos vein arching 

 over it from behind, the right pulmonary artery below 

 and then in front of it. The left bronchus is about 

 2 inches long, and enters the lung opposite the sixth 

 dorsal vertebra. It passes under the arch of the aorta 

 and crosses in front of the esophagus, thoracic duct, 

 and descending aorta. The left pulmonary artery 

 lies at first above, then in front of it. 



Their structure resembles the trachea, only that 

 the cartilaginous rings become thinner and are replaced 

 by an increase in the muscular coat, as they approach 

 the terminal bronchioles. The alveoli, the termination 

 of the bronchioles, rest on a basement membrane of 

 elastic tissue, surrounded by a capillary plexus formed 

 by the pulmonary arteries and veins. 



The Pleurae. The pleurae are two separate serous 

 sacs which invest each lung to its root and are reflected 

 on to the thoracic walls and pericardium. That portion 

 of the serous membrane investing the surface of the 

 lung and extending into the fissures between the 

 lobes is called the visceral layer of the pleura (pleura 

 pulmonalis), while the portion lining the inner surface 

 of the thorax is called the parietal layer of the pleura 

 (pleura parietalis). The latter is subdivided into the 

 cervical, the costal, the diaphragmatic, and the 

 mediastinal portions. The space between the visceral 

 and parietal layers is the pleural cavity (cavum pleurae), 

 and contains a small amount of clear fluid. There is 

 no cavity when the pleura? are in a healthy condition, 

 the two layers being in contact. 



The two pleurse are distinct from each other, and 

 do not meet in the median line except behind the 



