THE PLEURA AND THE LYMPHATICS OF THE LUNG. 



223 



to the walls of the large blood-vessels (vasa vasorum), the pulmonary pleura, 

 the bronchial walls, and the interlobular septa. The blood which issues from 

 their capillaries is returned partly by the pulmonary veins hence, any con- 

 siderable interference with the pulmonary circulation causes congestion of the 

 bronchial mucous membrane, resulting in a catarrhal condition of that membrane. 

 The greater part of the blood is returned by the bronchial veins which open into 

 the vena azygos, intercostal vein, or superior vena cava. The veins of the smaller 

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

 bronchial also communicate with the pulmonary veins (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 pari- 

 etal. The visceral pleura 

 covers the lung ; the pari- 

 etal portion lines the 

 wall of the chest, and the 

 two layers of the corre- 

 sponding pleura are con- 

 tinuous 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 mem- 

 brane, and consists of a 

 thin layer of fibrous tissue 

 covered by a layer of en- 

 dothelium. Under this 

 layer, or the pleura pro- 

 per, is a deep or sub-serous 

 layer of looser areolar 

 tissue, containing many 

 elastic fibres. This layer 

 of the pleura pulmonalis 

 of some animals, as the 

 guinea - pig, contains a 

 net- work of non- striped 

 muscular fibres (Klein). 

 Over the lung it is also 

 continuous with the in- Normal human lung (x50 and reduced |) a, small 



bronchus; b b, branches of the pulmonary artery; 



c, branch of the pulmonary vein; e, interlobular 



septa, continuous with the deep layer of the 



pleura, p. 



Fig, 100. 



terlobular septa. The 

 interlobular septa (Fig. 

 100, e) consist of bands 

 of fibrous tissue separat- 

 ing adjoining lobules, 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 through- 

 out 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 im- 

 portant 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 



