K KS I >I RATION 117 



inside and outside the alveoli is equal ; there is therefore no 

 force to keep them distended. The lungs at once shrink to 

 less than half their natural size, and the elastic walls of the 

 alveoli are no longer on the stretch, as they constantly are 

 before the thorax is opened. 



When the thoracic cavity is increased in size the atmospheric- 

 pressure can distend the lungs still more, and so more air goes 

 into the lungs ; but however much the thoracic cavity is 

 increased, each lung always fills its corresponding half. When 

 the thoracic cavity is decreased in size the lungs have less 

 room, and so some of the air is driven out of them, so 

 that they are less distended than before. The size of the 

 lungs depends therefore on the size of the thorax. Thus more 

 air will go into the lungs if the thorax is increased in size, 

 because the lungs can then be further distended by the pressure 

 of the atmosphere. It is the pressure of the atmosphere that 

 drives the air into the lungs when the thorax is being increased 

 in size. The pressure of the air in the lungs has no power of 

 itself to increase the size of the thorax, because the atmosphere 

 is pressing with equal force on the outside of the thorax as 

 well. Thus in order that air may be drawn into the lungs, 

 the thorax has to be increased in size by some force of the 

 living body. The thorax is increased in size by the flattening 

 of the arch of the diaphragm and by the elevation of the ribs, 

 both being brought about by muscular contraction. Drawing 

 air into the lungs is called inspiration, forcing air out of the 

 lungs is called expiration, and the two together constitute 

 respiration. 



The Respiratory Movements of the Walls of the Chest 



The Action of the Diaphragm Inspiration. The 

 diaphragm, which separates the thorax from the abdomen, is 

 arched upwards, so that it is convex to the thorax. It is 

 covered by peritoneum below and by pleura above. The 

 centre of the diaphragm consists of a sheet of tendon, to which 

 1 uinls of striated muscle are attached all round. These 

 muscular bands form the greater part of the diaphragm and 

 make it in reality a muscular partition, tendinous in the centre 

 only. By means of these muscular bands the diaphragm is 



