BRONCHIAL TUBES PLEURAE. 495 



the bronchial tubes. The air-cells composing a lobule also communicate 

 freely \vith each other. 



The bronchial tubes, after the cessation of the fibro-cartilaginous plates, 

 are composed of fibrous membrane (constructed of longitudinal and cir- 

 cular fibres) and mucous membrane, the latter being invested by a ciliated 

 columnar epithelium. At the termination of these tubes the mucous mem- 

 brane and epithelium cease abruptly, and the fibrous membrane, very thin 

 and transparent, is continued onwards through the intercellular passages 

 and air-cells. The same change takes place in the structure of the parie- 

 tes, where the air-cells open directly into the bronchial tubes. 



The capillaries of the lungs form plexuses which occupy the w r alls and 

 septa of the air-cells and the walls of the intercellular passages, but are 

 not continued into the bronchial tubes. The septa between the cells con- 

 sist of a single layer of the capillary plexus enclosed in a fold of the 

 fibrous lining membrane. The cells of the central parts of the lung are 

 .nost vascular, and at the same time smallest, w r hile those of the periphery 

 are less vascular and larger. 



The pigmentary matter of the lungs is contained in the air-cells, as well 

 as in the areolo-fibrous tissue of the inter-lobular spaces and of the blood- 

 vessels. 



The Pulmonary artery, conveying the dark and impure venous blood 

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

 in the parieties of the intercellular passages and air-cells, and then con- 

 verge 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 on the 

 parieties of the bronchial tubes, while the venous blood is returned by the 

 bronchial veins to the vena azygos. 



The Lymphatics, commencing on 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 carbonaceous matter, and occasion- 

 ally 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 pneu 

 mogastric. The branches from these plexuses follow the course of the 

 bronchial tubes, and are distributed to the intercellular passages and air 

 cHls. 



PLEU R^E. 



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

 brane, the pleura, which invests it as far as the root, and is then reflected 

 upon the parietes of the chest. That portion of the membrane which is 

 in relation with the lung is called pleura pulmonalis, and that in contact 

 with the parietes, pleura costalis. The reflected portion, besides forming 

 the internal lining to the ribs and intercostal muscles, also covers the dia- 

 phragm and the thoracic surface of the vessels at the root of the neck 



