ill i.l M KM. STRUCTURE OF THE BODY 25 



over the whole surface of the lung, forming a kind of bag for 

 it ; in fact, it is chiefly the pleura which in this way fixes the 

 lung to tin- vertebral column. This layer of the pleura over 

 each lung is closely adherent to the tissue of the lung, and cannot 

 be separated from it ; it is this which gives the lung its smooth 

 and shining ap|>earance. Between the layer of pleura on the lung 

 or visceral pleura and the layer of pleura on the wall of the 

 thorax or parietal pleura there would be a space if each lung 

 'did not entirely fill each lateral half of the thorax. This it 

 does, for during life each lung is distended with air till the 

 pleura on its surface meets the pleura on the wall of the 

 thorax, leaving only enough room for a very thin layer of fluid, 

 just enough to keep the surfaces of each layer of the pleura 

 moist so that they can slide easily on each other. 



The heart is placed obliquely across the front of the thorax. 

 Its base is just underneath the sternum, at the level of the 

 third rib, and it extends downwards and towards the left, so 

 that the apex is situated between the fifth and sixth ribs on 

 the left side. The apex touches the chest wall, and the 

 beating of the heart can be easily felt through the intercostal 

 muscles between these two ribs. It is generally most distinct 

 about one inch below and half an inch to the inner side of the 

 left nipple. 



