THE BLOOD-VESSELS OF THE LUNGS. 



163 



veins of the smaller bronchi (fourth order onwards) open into the pulmonary veins, and the 

 anterior bronchial also communicate with the pulmonary vein (Zuckerkandl). 



[The Pleura. Each pleural cavity is distinct, and is a large serous sac, which really belongs 

 to the lymphatic system of the lung. The pleura consists of two layers, visceral and parietal. 

 The visceral pleura covers the lung ; the parietal portion lines the wall of the chest, and the 

 two layers of the corresponding pleura are continuous with one another at the root of the lung. 

 The visceral pleura is the thicker, and may readily be separated from the inner surface of the 

 chest. Structurally, the pleura resembles a serous membrane, and consists of a thin layer of 

 fibrous tissue covered by a layer of endothelium. Under this layer, or the pleura proper, is a 

 deep or sub-serous layer of looser areolar tissue, containing many elastic fibres. The layer of 

 the pleura pulmonalis of some animals, as the guinea-pig, contains a network of non-striped 

 muscular fibres (Klein). Over the lung it is also continuous with the interlobular septa. The 

 interlobular septa (fig. 133, e) consist of bands of fibrous tissue separating adjoining lobules, 



Uiife- 



Fig. 132. 

 Semi-diagrammatic representation of the air-vesicles of the lung, v, v, blood-vessels at the 

 margins of an alveolus ; c, c, its blood-capillaries ; E, relation of the squamous epithelium 

 of an alveolus to the capillaries in its wall ; /, alveolar epithelium shown alone ; e, e, elastic 

 tissue of the lung. 



and they become'continuous with the peri-bronchial connective-tissue entering the lung at its 

 hilum. Thus the fibrous framework of the lung is continuous throughout the lung, just as in 

 other organs. The connection of the sub-pleural fibrous tissue with the connective-tissue 

 within the substance of the lung has most important pathological bearings. The interlobular 

 septa contain lymphatics and blood-vessels. The endothelium covering the parietal layer is of 

 the ordinary squamous type, but on the pleura pulmonalis* the cells are less flattened, more 

 polyhedral, and granular. They must necessarily vary in shape with changes in the volume 

 of the lung, so that they are more flattened when the lung is distended, as during inspiration. 

 The pleura contains many lymphatics, which communicate by means of stomata with the 

 pleural cavity.] 



[The Lymphatics of the lung are numerous, and are arranged in several systems. The various 

 air-cells are connected with each other by very delicate connective-tissue, and, according to J. 

 Arnold, in some parts this interstitial tissue presents characters like those of adenoid tissue ; so 

 that the lung is traversed by a system of juice-canals or "Saft-canalchen."] [In the deep 

 layer of the pleura there is a (a) sub-pleural plexus of lymphatics partly derived from the pleura, 



