EESPIEATION AND ENERGY IN MAN 131 



When the diaphragm is at rest, it forms a dome-shaped par- 

 tition between the organs of the chest and those of the abdo- 

 men (Fig. 42). During inspiration, the muscles of which 

 the diaphragm is largely composed, are made to contract, 

 the dome of this organ becomes flattened, and so presses 

 down upon the stomach, liver, and other abdominal organs, 

 and these in turn force outward the wall of the abdomen. 

 By the action just described, the size of the chest cavity 

 is increased in its third dimension; namely, from top to 

 bottom. 



Thus, by the combined movements of the ribs and dia- 

 phragm, the chest cavity is enlarged in all three of its dimen- 

 sions. The walls of the chest cavity would, therefore, tend to 

 move away from the lungs ; but the air already in the lungs 

 expands the many air sacs in the lung tissue, and so keeps these 

 organs in close contact with the chest walls. The moment, 

 however, that the air sacs begin to enlarge, the air expands 

 to fill the larger space, and so the pressure of the air on every 

 square inch inside the lungs is diminished, and therefore 

 becomes less than the air pressure outside the body. At 

 once more air is forced in through the air passages until the 

 pressure within and outside the body becomes equalized. 

 This process we have described is called inspiration. Every 

 inspiration requires 'muscular action in elevating the ribs and 

 flattening the diaphragm. 



189. To determine the breathing capacity of the lungs. 



Laboratory demonstration. 



Fill a large tray half full of water. Mark on a gallon bottle 

 the level of 1, 2, 3, and 4 quarts; completely fill the bottle 

 with water, and invert it in the tray, just as was done in 

 collecting oxygen and other gases (P. B., 10). Beneath 

 the mouth of the bottle insert one end of a glass or rubber 

 tube. Now take in a deep inspiration, filling all parts of the 



