THE DIAPHRAGM 143 



person who is breathing. Again, when the ribs are raised, 

 each rib must evidently, by its upward motion, tend to 

 occupy the position previously held by the rib next above 

 it ; but the arched curve of each rib increases in size from 

 the first to the seventh pair of ribs, so that this upward 

 movement makes a rib with a larger arch take the place of 

 one with a smaller curve. This must clearly result in an 

 increase in toidth of the thorax from side to side, an increase 

 which may, as before, be i-eadily felt by placing the hands 

 on the opposite sides of the chest. 



The floor of the thorax is formed by the diaphragm, a 

 great partition situated between the thorax and the 

 abdomen, and always concave to the latter and convex to 

 the former (Fig. 1, D). Frcflii its middle, which is 

 tendinous, muscular fibres extend downwards and outwards 

 to the ribs, and two, especially strong masses, which ai-e 

 called the pillars of the diaphragm, to the spinal column 

 (Fig. 47) . When these muscular fibres contract, therefore, 

 they tend to make the diaphragm flatter, and to increase 

 the capacity of the thorax at the expense of that of the 

 abdomen, by pulling down the bottom of the thoracic box 

 (Fig. 48, A), or in other words when the diaphragm is 

 flattened, the size of the thorax is increased from above 

 downwards. 



By means then of the movements of the ribs and of the 

 diaphragm the size of the thorax may be increased in all 

 its dimensions. Let us now consider what must happen 

 to the lungs when the thorax becomes larger. The lungs, 

 as we have said (p. 137), are kept distended by the 

 pressure of the atmosphere acting down the trachea and 

 keeping the outer walls of each lung firmly pressed against 

 the inner wall of the chest. This being so, if the wall of 

 the thorax tends to move away from the wall of the lung, 

 as it must do when the thorax is enlarged, then the wall 

 of the lung must follow the wall of the thorax, air rushing 

 in through the trachea to increase the distension of the 



