RESPIRATION AND THE VOICE. 133 



all of the chest, except the space filled by the heart. 

 They are attached to its walls and floor. They are elas- 

 tic; and so, when the chest enlarges, the lungs enlarge 

 with it, and the air-cells open w r ide. When the chest 

 sinks back to its former size, the elastic lung shrinks too ; 

 and the air is driven out. 



6. The lungs are held to the chest-wall by the pleurae. 

 These are two empty, air-tight sacs, one for each lung. 

 One layer covers the lung; and the other lines the chest- 

 wall, and is attached to it. The insides of these two 

 sacs are moistened by a fluid, so that there is no friction 

 between the lung and the chest : and, as the sacs are air- 

 tight, the layers can not be separated ; and the chest- wall, 

 in its movements, pulls the lung with it, just as the circle 

 of wet leather, which the boys call a sucker, lifts a stone. 

 The pleura is like the inner portion of the pericardium. 



7. To recapitulate briefly. The lungs, consisting largely 

 of little air-cells, with the air-tubes leading to them, are 

 elastic. They nearly fill the chest, and cling closely to 

 its walls and floor. When the chest expands by the de- 

 scent of its floor and the ascent of its walls, the lungs 

 expand with it. The air-cells open, and the outside air is 

 pushed in to fill them, as the air enters a bellows when 

 we separate its walls. When the inspiratory muscles stop 

 pulling, the chest settles back to its former size, the expi- 

 ratory muscles helping it a little, and sometimes a good 

 deal. 



8. We breathe without thinking of it in sleep as well 

 as when awake. We can not refrain from it for many 

 seconds. Constant breathing is necessary to life, so it 

 must be independent of our wills. 



