RESPIRATION 395 



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. 



Bronchial Innervation. The bronchial muscles are presumably in a 

 state of tonic contraction and impart to the bronchial tubes a certain average 

 caliber best adapted for respiratory purposes. Experimental investigations 

 indicate that they are innervated by efferent fibers of the vagus nerve (broncho- 

 constrictors and possibly broncho-dilatators) inasmuch as stimulation of this 

 nerve is usually followed by a contraction of the muscles and a narrowing of 

 the lumen of the bronchial system. These muscles may also be thrown into 

 increased activity by the inhalation of irritating gases and into a tetanus by 

 pathologic causes as seen in the certain forms of asthma. 



The vaso-motor nerves distributed to the muscle-fibers of the bronchial 

 arteries are derived from the sympathetic and enter the substance of the 

 lung at and around its root. The vagus nerve imparts sensibility to 

 mucous membrane of the larynx, trachea and bronchial tubes. 



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

 sible and retractile and therefore elastic. After removal from the body 

 the elastic tissue at once recoils, forc- 

 ing out a portion of the contained air. 

 The condition of the lung now is one 

 of collapse. Under pressure, however, 

 the lung can be readily distended or in- 

 flated. These properties 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 Pulmonic Blood-vessels. 

 The pulmonic artery which conducts the 



venous blood from the heart to the lungs FlG> l82 .-TH E RELATION OF THE 

 divides beneath the arch of the aorta into PULMONIC ARTERY, PA, AND THE PUL- 

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



lit? *. i T ' 11 B. THE BRONCHIOLE. 



with . its subdivisions enters the lung at 



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 pulmonic lobules are approached, a small arterial 

 branch plunges into the wall of the lobule (Fig. 182), 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 



