THE NERVO-VASCULAR SUPPLY OF THE LUNGS. 469 



the artery breaks up into small radicles, which pass to the 

 central side of the sac in the sulci between the air-cells, and 

 are finally lost in the rich system of capillaries to which they 

 give rise. This net-work surrounds the whole air-sac and com- 

 municates freely with that of the surrounding sacs/ This 

 capillary net-work is exceedingly fine, and is shrunken into the 

 epithelium of the air-sacs; so that between the epithelium and 

 the capillary there is only the extremely delicate basement mem- 

 brane. The infundibula, the alveolar ducts and their alveoli, 

 and the alveoli of the respiratory bronchioles are supplied with 

 similar capillary net-works. The veins collecting the blood 

 from the lobules lie at the periphery of the lobules in the inter- 

 lobular connective tissue, and are as far distant from the in- 

 tralobular arteries as possible. These veins unite to form the 

 larger pulmonary veins. The bronchi, both large and small, 

 as well as the bronchioles, derive their blood-supply from the 

 bronchial arteries, which also partly supply the lung itself. 

 Capillaries derived from these arteries surround the bronchial 

 system, their caliber varying according to the structure they 

 supply: finer and more closely arranged in the mucous mem- 

 brane, and coarser in the connective-tissue walls. In the neigh- 

 borhood of the terminal bronchial tubes the capillary nets 

 anastomose freely with those of the respiratory capillary sys- 

 tem." To avoid confusion we may recall the fact that, while 

 the pulmonary artery and its branches contain venous blood, 

 and the bronchial arteries and their branches carry arterial 

 blood, the pulmonary veins, on the contrary, contain arterial 

 blood. When, therefore, bronchial capillaries are said to empty 

 into the pulmonary veins, it is not used, or venous, blood that 

 is transferred to the latter, but arterial blood originally derived 

 from the thoracic aorta or its primary branches. 



The innervation of the lungs, as in the extrapulmonary 

 respiratory structures, consists of vagal and the general motor 

 nerves. These unite to form plexuses, the anterior and posterior, 

 which enter the organs with the bronchial tubes and accompany 

 them along their ramifications. The anterior pulmonary plexus, 

 made up of vagal and sympathetic filaments, overlies the pul- 

 monary artery, while the richer posterior pulmonary plexus, 

 composed also of vagal filaments, intermixed with sympathetic 



