46 MECHANISM OF BREATHING. 



vital organs. This division is effected by a broad and some- 

 what bell-shaped muscle, the diaphragm or midriff, which is 

 attached, round its margin, to the ends of the last twelve ribs, 

 to the rearmost extremity of the breast-bone, and to the spine 

 under the loins ; while its apex or centre projects forwards. 

 When it contracts, it tends to become flat, and thus enlarges 

 the capacity of the chest by pushing back the contents of the 

 abdomen. Its action, especially during forced breathing, is 

 aided by that of another muscle, which is attached by one 

 end to the last four neck vertebrae, and by the other to the 

 first rib, which it pulls forward on contracting, and in this 

 manner helps to increase the size of the chest. 



Air is taken into, and expelled from, the lungs by means 

 of the alternate increase and diminution of the capacity of 

 the chest. When the former act occurs, the air contained in 

 the lungs becomes rarefied, and consequently the external air 

 rushes in through the animal's nostrils to restore the balance 

 of pressure. When the latter takes place, a portion of the 

 air which is already in the lungs is forced out. It is evident, 

 therefore, that the power of taking a large volume of air 

 into the lungs at each breath, is more dependent on the 

 difference between the respective capacities of the chest when 

 expanded, and contracted, than on the actual size of the chest 

 itself. 



The act of breathing is called respiration ; that of ex- 

 pelling air from the lungs, expiration ; and that of drawing it 

 in, inspiration. 



The chest is enlarged by the diaphragm, as we have seen ; 

 and also by muscles which pull the middle pieces of the 

 ribs forward, so as to make the cavity wider from side to side, 

 and consequently to increase its size. The ribs are drawn 



