PLEUBISY. 321 



tissue of the mediastinum, and is supplied by the mediastinal 

 and pericardiac branches of the aorta and by branches of the 

 bronchial and internal mammary arteries. A similar plexus lies 

 in the connective tissue beneath the costal pleura, being formed 

 by branches of all the arteries of the thoracic wall. These 

 plexuses establish a communication between the pulmonary and 

 pleural vessels on the one hand and the vessels of the surface of 

 the chest on the other ; and this may explain the beneficial 

 effects of counter-irritation applied to the chest in pleural and 

 lung diseases. 



In pyaemic pleurisy the infection is carried by branches of the 

 pulmonary artery, having been brought to the right auricle by 

 the systemic veins. Infarctions of the lung and superjacent 

 pleurisy result. 



The visceral, mediastinal, diaphragmatic, costal and apical 

 regions of the pleura may all be invaded by the extension of 

 inflammation from the structures which lie in immediate relation 

 to them. 



The intimate relations of the visceral pleura to the connective 

 tissue of the lung ; and of the parietal pleura to the connective 

 tissue of the mediastina, chest walls, diaphragm and root of the 

 neck explain the occurrence of pleurisy by extension of inflam- 

 matory mischief from these regions, and also account for the 

 association of pleural and pericardial adhesions with mediastinitis. 

 Extension of infection to the visceral pleura is common in abscess, 

 infarction and gangrene of the lung. Pleurisy also frequent!} 7 

 accompanies pneumonia. The lymphatics of the lung and of 

 the visceral pleura pass into the same efferent trunks ; and the 

 latter also receive the lymphatics of the mediastinal tissues and 

 of the mediastinal pleura. Lymphatic convection, as well as mere 

 continuity of tissue, may therefore play a part in the production 

 of secondary pleurisy. In phthisis, small aggregations of miliary 

 tubercles may often be recognised on the visceral pleura close to 

 the older lung lesions. These miliary deposits are probably 

 situated in the lymphatics. 



The structures which lie beneath the mediastinal pleura and 



C.A.A. 21 



