552 BRONCHIAL TUBES. PLEURAE. 



lobules again consist of smaller lobules, and the latter are formed by a 

 cluster of air cells, in the parietes of which the capillaries of the pul- 

 monary artery and pulmonary veins are distributed. 



Bronchial tubes. The two bronchi proceed from the bifurcation of 

 the trachea to their corresponding lungs. The right takes its course 

 nearly at right angles with the trachea, and enters the upper part of 

 the right lung ; while the left, longer and smaller than the right, passes 

 obliquely beneath the arch of the aorta, and enters the lung at about 

 the middle of its root. Upon entering the lungs they divide into two 

 branches, and each of these divides and subdivides dichotomously to 

 their ultimate termination in small dilated sacs, the bronchial or pul- 

 monary cells. The fibro-cartilaginous rings which are observed in the 

 trachea become incomplete and irregular in shape in the bronchi, and 

 in the smaller bronchial tubes are lost altogether. At the termination 

 of these tubes the fibrous and muscular coats become extremely thin, 

 and are probably continued upon the lining mucous membrane of the 

 air cells. 



The Pulmonary artery, conveying the dark and impure venous blood 

 to the lungs, terminates in capillary vessels, which form a minute net- 

 work upon the parietes of the bronchial cells, and then converge to 

 form the pulmonary veins, by which the arterial blood, purified in its 

 passage through the capillaries, is returned to the left auricle of the 

 heart. 



The Bronchial arteries, branches of the thoracic aorta, ramify upon 

 the bronchial tubes and in the tissue of the lungs, and supply them 

 with nutrition, while the venous blood is returned by the bronchial 

 veins to the vena azygos. 



The Lymphatics, commencing upon the surface and in the substance 

 of the lungs, terminate in the bronchial glands. These glands, very 

 numerous and often of large size, are placed at the roots of the lungs, 

 around the bronchi, and at the bifurcation of the trachea. In early 

 life they resemble lymphatic glands in other situations ; but in old 

 age, and often in the adult, are quite black, and filled with carbon- 

 aceous matter, and occasionally with calcareous deposits. 



The Nerves are derived from the pneumogastric and sympathetic. 

 They form two plexuses, anterior pulmonary plexus, situated upon 

 the front of the root of the lungs, and composed chiefly of filaments 

 from the great cardiac plexus ; and posterior pulmonary plexus on the 

 posterior aspect of the root of the lungs, composed principally of 

 branches from the pneumogastric. The branches from these plexuses 

 follow the course of the bronchial tubes, and are distributed to the 

 bronchial cells. 



Each lung is enclosed, and its structure maintained, by a serous 

 membrane, the pleura, which invests it as far as the root, and is 

 then reflected upon the parietes of the chest. That portion of the 



