PULMONARY AND BRONCHIAL CAPILLARIES. 903 



are scarcely wider than the vessels themselves. Those which lie nearest to 

 the mouths of the alveoli are observed arching and coiled over and through 

 the elastic fibres found in the interalveolar septa (Luschka and Badoky). 

 The coats of the capillaries are also exceedingly thin, and thus more readily 

 allow the free exhalation and absorption of which the pulcnouary cells are 

 the seat. 



The branches of the pulmonary artery accompany the bronchial tubes, but they sub- 

 divide more frequently, and are much smaller, especially in their remote ramifications. 

 They ramify without anastomoses, and at length terminate upon the walls of the 

 air-cells and on those of the bronchia in a fine and dense capillary network, from 

 which the radicles of the pulmonary veins arise. The smaller branches of these veins, 

 especially near the surface of the lung, frequently do not accompany the bronchia and 

 arterial branches (Addison, Bourgery), but are found to run alone for a short distance 

 through the substance of the organ, and then to join some deeper vein which passes 

 by the side of a bronchial tube, uniting together, and also forming, according to Ros- 

 signol, frequent lateral communications. The veins coalesce into large branches, 

 which at length accompany the arteries, and thus proceed to the root of the lung. In 

 their course through the lung, the artery is usually found above and in front of a 

 bronchial tube, and the vein below. 



The pulmonary vessels differ from the systemic in regard to their contents, inas- 

 much as the artery conveys dark blood, whilst the veins carry red blood. The pul- 

 monary veins, unlike the other veins of the body, are not more capacious than their 

 corresponding arteries ; indeed, according to Winslow, Santorini, Haller, and others, 

 they are somewhat less so. These veins have no valves. Lastly, it may be remarked 

 that, whilst the arteries of different lobules are independent, their veins freely anas- 

 tomose together. 



The bronchial vessels. The bronchial arteries and veins, which are much 

 smaller than the pulmonary vessels, carry blood for the nutrition of the 

 lung, and are doubtless also the principal source of the mucous secretion 

 found in the interior of the air tubes, and of the thin albuminous fluid 

 which moistens the pleura pulmonalis. 



The bronchial arteries, from one to three in number for each lung, arise 

 from the aorta, or from an intercostal artery, and follow the divisions of the 

 air-tubes through the lung. They are ultimately distributed in three ways : 

 (1) many of their branches ramify in the bronchial lymphatic glands, the 

 coats of the large blood-vessels, and in the fibrous and muscular walls of 

 the large and small air-tubes, and give supply to a copious capillary plexus 

 iu the bronchial mucous membrane, which in fine bronchial tubes is con- 

 tinuous with that supplied by the pulmonary artery ; (2) others form plexuses 

 in the interlobular areolar tissue ; (3) branches spread out upon the surface 

 of the lung beneath the pleura, forming plexuses and a capillary network, 

 which may be distinguished from those of the pulmonary vessels of the 

 superficial air-cells by their tortuous course and open arrangement, and also 

 by their being outside the investing membrane of tho lobules, and by ulti- 

 mately ending iu the branches of the superficial sat of bronchial veins. 



The bronchial veins have not quite so large a distribution in the lung as 

 the bronchial arteries, since part of the blood carried by the bronchial 

 arteries is returned by the pulmonary veins. The superficial and deep 

 bronchial veins unite at the root of the lung, opening on the right side into 

 the vena azygos, and on the left usually into the superior intercostal vein. 



The absorbent vessels of the lungs have been already sufficiently described 

 (p. 496). 



Nerves. The lungs are supplied with nerves from the anterior and posterior 

 pulmonary plexuses (pp. 023, 093). These are formed chiefly by branches 



