Xlr.j THE EXCRETORY ORGANS. 463 



The two pleurae line the right and left halves of the thorax, 

 and are reflected over the two lungs at their roots respectively. 

 In this way the two adjacent (inner) sides of the two pleura 

 traverse the thorax from behind forwards. They are not, 

 however, in contact, but separated by two interspaces termed 

 inediastina. 



The anterior mediastinum contains the heart in its pericar- 

 dium, as described in our Tenth Lesson. 



The posterior mediastinum contains the oesophagus, the 

 aorta, the vena azygos, and the thoracic duct, together with 

 the pneumogastric nerves. 



FIG. 392. TRANSVERSE SECTION OF THE CHEST, WITH THE HEART AND 

 LUNGS (EACH LUNG INVESTED WITH ITS PLEURA) IN PLACE. 



DV, dorsal vertebra, or joint of the backbone ; Ao, Ao, aorta, the top of its arch 

 being cut away in this section; SC, superior vena cava ; PA, pulmonary 

 artery, divided into a branch for each lung ; LP, RP, left and right pulmonary 

 veins ; Br, bronchi ; RL, LL, right and left lungs; CE, oesophagus. 



Each lung is conical in shape, and has a broad, concave 

 "base, which rests upon the diaphragm. The inner surface of 

 each lung is turned towards the anterior mediastinum, and is 

 adapted to the convex pericardium. 



The right lung is shorter and wider than the left, and is 

 divided by two notches into three portions or lobes. The left 

 lung has only a single notch and two lobes. 



Beneath the serous coat, each lung has a layer of cellular 

 membrane, mixed with much elastic tissue, which passes in 

 "between the minute air-tubes and air-cells, as described in 

 8 of the Fourth Lesson of " Elementary Physiology." 



In development the lungs first make their appearance as 



