RESPIRATION 517 



posterior. As the thorax expands, the air pressure inside the 

 lungs keeps them pressed against the chest wall, and the 

 lungs expand with the chest. As a result of this expansion 

 of the lungs, the pressure inside becomes less than the 

 atmospheric pressure, and air rushes m until the pressures 

 inside and outside again become equal. This can be shown by 

 placing a tube connected with a water manometer in the mouth, 

 closing the nostrils, and breathing (Practical Physiology). 



This expansion of the lungs can readily be determined in 

 the antero-posterior direction by percussion, and in the trans- 

 verse planes by measurement. By tapping the chest with 

 the finger over the lung in the intercostal spaces, a 

 resonant note is produced, while if the percussion is per- 

 formed below the level of the lung, a dull note is heard. 

 If the lower edge of this resonance be determined before an 

 inspiration, and again during it, the lung will be found to 

 have expanded backwards (Practical Physiology). The 

 change from before backwards cannot be seen directly, but 

 it is indicated by a downward movement of the wall of the 

 abdomen. It will be further described when considering the 

 mechanism by which it is brought about. 



The expansion of the chest in inspiration is a muscular act 

 and is carried out against the following forces : — 



1st. The Elasticity of the Lungs.— To expand the lungs 

 their elastic force has to be overcome, and the more they are 

 expanded the greater is their elasticity. This factor therefore 

 plays a smaller part at the beginning than towards the end of 

 inspiration. 



2nd. The Elasticity of the Chest Wall— The resting position 

 of the chest is that of expiration. To expand the chest the 

 costal cartilages have to be twisted. 



3rd. The Elasticity of the Abdominal Wall— As the cavity 

 of the thorax increases downwards, the abdominal viscera are 

 pushed against the muscular abdominal wall, which, in virtue 

 of its elasticity, resists the stretching force. 



The changes in inspiration are brought about by — 



1st. An Increase in the Thorax from before backwards. 



This is due to the contraction of the diaphragm (fig. 209) ; 

 see also fig. 168, p. 392. 



