RESPIRATION. 381 



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 pulmonary veins. The blood 

 in the pulmonary 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, p v. lii^V PA 



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 Pulmonary Blood-vessels. 

 The pulmonary artery which conducts the 



venous blood from the heart to the lungs FIG .^.-THE RELATION _ 

 divides beneath the arch of the aorta into PULMONARY ARTERY, PA, AND THE 

 a right and a left branch. Each branch PULMONARY VEIN, PV, TO THE LOBULES, 

 with its subdivisions enters the lung at AA ' B - THE BRONCHIOLE - 

 the hilum in company with the larger divisions of the bronchi. 'Within 

 the lung the arteries divide and subdivide in a manner corresponding 

 to that of the bronchial tubes, which they follow to their ultimate ter- 

 minations. As the pulmonary lobules are approached, a small arterial 

 branch plunges into the wall of the lobule (Fig. 183), in which it forms 

 an elaborate capillary network which surrounds and embraces the air- 

 sacs on all sides. As this network is to subserve the respiratory exchange 

 of gases it lies nearer the inner than the outer surface of the lobule and in 

 close relation to the respiratory epithelium. The air and blood are thus 

 brought into intimate relationship, being separated only by the respiratory 

 epithelium and the wall of the capillary vessel. The blood emerging from 

 the capillary vessels is conducted by a corresponding converging system of 

 vessels, the pulmonary veins, out of the lungs and into the left auricle of the 

 heart. The main function of the pulmonary apparatus and the pulmonary 

 division of the circulatory apparatus is to afford a ready means for the 

 exhalation of the carbon dioxid and the absorption of oxygen. In conse- 



OF THE 



