RESPIRATION. 



339 



border is convex; the anterior border is thin. The lung thus consists 



of bronchial tubes, pulmonary lobules or alveoli, blood-vessels, 



lymphatics, and nerves 



embedded in a stroma 



of fibrous and elastic 



tissue. 



In consequence of 

 the presence of the 

 elastic tissue, the 

 lungs are distensible 

 and elastic. After re- 

 moval from the body 

 the elastic tissue at 

 once recoils, forcing 

 out a portion of the 

 contained air. The 

 condition of the lung 

 is now one of collapse. 

 Under pressure, how- 



FIG. 161. SECTION OF SILVERED LUNG OF KITTEN, 

 INCLUDING PORTIONS OF INFUNDIBULUM AND 

 AIR-SAC, a. Small polyhedral epithelial cells 

 covering the wall of the infundibulum. b. Fibro- 

 elastic framework, c. Large flattened epithelial 

 plates lining air-sac, among which lie small groups 

 of small cells (d). (Pier sol.} 



ever, the lung can be 

 readily distended or 

 inflated. These prop- 

 erties endure for a 



long period after death, if not indefinitely, if the lungs are properly 

 preserved. The capacity of the lungs can be made to vary within 



rather wide limits in virtue of the 

 presence of the elastic tissue. 



The pulmonary blood-ves- 

 sels. The pulmonary artery 

 which conducts the venous blood 

 from the heart to the lungs di- 

 vides beneath the arch of the 

 into a right and a left 



-PA 



a 



branch. Each branch with its 

 subdivisions enters the lung at 

 the hilum in company with the 

 larger divisions of the bronchi. 

 Within the lung the arteries di- 

 vide and subdivide in a manner 

 corresponding to that of the 

 bronchial tubes, which they fol- 

 low to their ultimate termina- 

 tions. As the pulmonary lobules 



are approached, a small arterial branch plunges into the wall of 

 the lobule j (Fig. 162), in which it forms an elaborate capillary 



FIG. 162. THE RELATION OF THE PUL- 

 MONARY ARTERY, PA, AND THE PUL- 

 MONARY VEIN, PV, TO THE LOBULES, 

 A A. B. THE BRONCHIOLE. 



