MECHANISM AND CHEMISTRY OF RESPIRATION 193 



the size of that cavity. The center of the diaphragm is a 

 tough membrane, from the edges of which muscles radiate 

 to the walls of the cavity. When these muscles contract, 

 the center of the diaphragm is drawn downward and the 

 whole structure takes a more nearly flat position. In doing 

 this the diaphragm presses upon the stomach and other ab- 

 dominal organs, forcing them downward and outward. Thus 

 the chest cavity is increased (Fig. 102) , and air is sucked into 

 <he lungs, which swell and fill the enlarged space. 



At each expiration the muscles of the diaphragm relax, 

 and the muscles of the abdomen which were stretched some- 

 what when the organs in it were pressed downward, now 

 shorten, and the diaphragm is thus carried up into its former 

 dome-like shape; the room in the thoracic cavity is conse- 

 quently lessened, and air is forced out of the lungs. Breathing 

 may thus be accompanied by a rise and fall of the abdominal 

 walls, but this does not mean, of course, that air is taken into 

 that cavity; merely that the contained organs are displaced 

 at the descent of the diaphragm, and that the abdominal 

 muscles force them back again when the diaphragm relaxes. 



The reason that air rushes into the lungs when the thoracic 

 cavity is enlarged is that air is constantly exerting a pressure 

 upon our bodies equivalent to fifteen pounds to the square 

 inch. As soon as the thoracic cavity is enlarged, air naturally 

 enters it, since otherwise there would be a vacuum. 



EXTERNAL AND INTERNAL RESPIRATION 



The alternate inflow and outflow of air through the passages 

 is the most superficial part of the breathing process. All the 

 air in these passages is still connected from the outside world, 

 and is not yet a part of the body in any sense whatever. This 

 tidal flow inward and outward is called external respiration. 



Through the thin walls of the air sacs of the lungs the oxygen 

 of the air passes, following the law of osmosis of gases, and com- 

 bines with the haemoglobin of the red blood corpuscles, as de- 



