326 THE VITAL FUNCTIONS. 



the side next to the abdomen. This muscle is 

 called the Diaplwagm : it is j^erforated, close to 

 its origin from the spine, by four tubes, namely, 

 the oesophagus, the aorta, the vena cava, and the 

 thoracic duct. Its surface is not flat, but convex 

 above, or towards the chest ; and the direction of 

 its fibres is such that when they contract they 

 bring down the middle part, which is tendinous, 

 and render it more flat than before, (the passage 

 of the four tubes already mentioned, not inter- 

 fering with this action,) and thus the cavity of 

 the thorax may be considerably enlarged. It is 

 obvious that if, upon the descent of the dia- 

 phragm, the lungs were to remain in their ori- 

 ginal situation, an empty space would be left 

 between them and the diaphragm. But no 

 vacuum can take place in the body ; the air 

 cells of the lungs must always contain, even in 

 their most compressed state, a certain quantity 

 of air : and this air will tend, by its elasticity, to 

 expand the cells ; the lungs will consequently 

 be dilated, and will continue to fill the chest ; and 

 the external air will rush in through the trachea 

 in order to restore the equilibrium. This action 

 is termed inspiration. The air is again thrown 

 out when the diaphragm is relaxed, and pushed 

 upwards, by the action of the large muscles of 

 the trunk ; the elasticity of the sides of the 

 chest, concurring also in the same effect; and 

 thus expiration is accomplished. 



