130 THE HEART. 



from neighbouring cells by partitions of parenchyma, and thus are 

 formed lobules. These subdivisions are evident upon the external 

 surface of the lung. The air-vesicles are formed of ligamentous 

 tissue lined by a mucous membrane. 



The root of the lung, consists of the bronchus, pulmonary artery, 

 two pulmonary veins, bronchial artery and vein, nerves, and lym- 

 phatics. These enter upon its internal surface, and in some degree 

 retain the lung in its proper position. The branches of the pulmo- 

 nary artery terminate in capillaries, upon the walls of the air ves- 

 sels, from which arise the branches of the pulmonary veins. 



The bronchial artery is the nutritious artery of the lung. Its 

 nerves are derived from the sympathetic and eighth pair. 



PLEURA. 



The pleura is a serous membrane investing each lung, and then 

 reflected upon the parietes of the chest. That portion in contact 

 with the chest is called pleura costalis; that covering the lung, 

 pleura pulvionalis. It also covers the diaphragm and the root of 

 the lung, between which there exists a fold called ligamentum pul- 

 monis. 



MEDIASTINUM. 



The space between the two pleurae of the lungs is called the 

 mediastinum, and it is divided, by the position of the heart, into the 

 anterior, posterior, and superior mediastinum. 



The anterior 'inediastiniiin is that space between the sternum and 

 the heart ; it contains some loose cellular tissue, and a portion of 

 the remains of the thymus gland. 



The posterior mediastinum is that portion between the vertebral 

 column and the heart, and through it passes the oesophagus, par va- 

 gum nerve, aorta, vena azygos, and the thoracic duct. 



The superior mediastinum is above the heart, and surrounded by 

 the first ribs and sternum ; it contains the roots of the large vessels, 

 and the remains of the thymus gland. 



THYMUS GLAND. 



This is a triangular body of a pinkish hue, occupying a portion 

 of the superior and anterior mediastina. It grows until the end 

 of the second year, after which it diminishes, and almost entirely 

 disappears. It has a large lymphatic vessel, which has been sup- 

 posed to be an excretory duct. Its structure is lobulated, and sur- 

 rounded by loose cellular tissue. 



THE HEART. 



The heart is a hollow muscular organ, situated between the two 



