THE RESPIRATION 153 



lining, the first one. Next imagine the lungs to be found 

 inside the inner sac. Here we have the lungs within the 

 two membranes, called the plenras. The heart is in the 

 chest, but is not inclosed by the pleuras. It is in a deep 

 indentation in the lower surface, such as you might make 

 in a paper bag by pressing your fist up into the bottom 

 without breaking the bag. 



FIG. 135. The Dome-shaped Diaphragm. 



A, aorta; B, esophagus; C, vena cava inferior; Z>, muscular pillars of the diaphragm 

 arising from the spinal column E; F, ribs, and G, sternum, sawed through so as to allow 

 removal of the front of the thorax ; H, hind, and K t front, muscular sheet, and /, central 

 tendinous part of diaphragm. 



256. Use of the Pleuras. The outer pleura lines the 

 chest wall, the inner pleura covers the lungs. The two 

 membranes form between them a closed sac, a serous cavity 

 which is air-tight and aids the lungs in following the chest 

 wall without friction when the chest expands. The two 

 pleural surfaces are in contact, and secrete just enough 

 fluid to enable them to glide smoothly upon each other. 



