RESPIRATION 3gi 



wall of the alveolus is extremely thin and consists of fibro-elastic tissue sup- 

 porting a very elaborate capillary network of blood-vessels. The bronchial 

 system as far as the alveolar passages is lined by ciliated epithelium. The 

 air-sacs are lined by flat epithelial plates of irregular shape, termed the re- 

 spiratory epithelium. The alveoli are united one to another by fibro-elastic 

 tissue. 



The bronchial arteries which supply nutritive material to the pulmonary 

 structures arise from the aorta as a rule, though sometimes from an inter- 

 costal artery. Each lung receives two arteries which accompany the bronchi 

 as far as the distal ends of the alveolar passages. From the capillary net- 

 work formed out of the terminals of these arteries, two systems of veins arise, 

 one of which returns the blood from the larger tubes and empties it into the 

 azygos vein; the other of which returns the blood from the smaller tubes and 

 the alveolar passages, and empties it into the pulmonic veins. The blood 

 in the pulmonic veins, though largely arterialized, nevertheless contains 

 some venous blood derived from the veins arising from the capillary network 

 of the bronchial arterioles. 



The nerves distributed to the muscle-fibers of the bronchial arteries, and 

 of the bronchial tubes and to the mucous membrane, are derived from the 

 vagus and the sympathetic and enter the substance of the lung at and around 

 its root. 



In consequence of the presence of the elastic tissue, the lungs are disten- 

 sible and elastic. After removal from the body the elastic tissue at once 

 recoils, forcing out a portion of the con- 

 tained air. The condition of the lung 

 is now one of collapse. Under pressure, 

 however, the lung can be readily dis- 

 tended or inflated. These properties 

 endure for a long period after death, 

 if not indefinitely, if the lungs are prop- 

 erly 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 Pulmonic Blood-vessels. 

 The pulmonic artery which conducts the 

 venous blood from the heart to the lungs 

 divides beneath the arch of the aorta into 

 a right and a left branch. Each branch *o 

 with its subdivisions enters the lung at 



the hilum in company with the larger divisions of the bronchi 

 the lung the arteries divide and subdivide ma manner com 

 to that of the bronchial tubes, which they follow to their ultimat 

 minations. As the pulmonic lobules are approached; small artei 

 branch plunges into the wall of the lobule i xwS iTr 



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it lies nearer the inner than the 



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