THE CHEST. 25 



muscular tissue, and stretches across from side to side 

 in such a way as completely to separate the body into 

 two gi'eat cavities. By its motion backwards and for- 

 wards, either of these cavities can be lessened or enlarged 

 as circumstances may require. 



The large cavity called the chest, which interv^enes be- 

 tween the boundaries specified, is occupied almost entirely 

 by the heart and lungs. The size of the heart alters little 

 at different periods in a state of health. Not so, how- 

 ever, with the lungs. They are veiy elastic in their 

 natm-e, and consequently are capable of being gi-eatly 

 collapsed, or enormously distended, at certain periods, 

 diu-ing the process of respiration. 



When the demand for breath arises, the ribs are 

 elevated, and the diaphragm is drawn back, for the 

 purpose of enlarging the cavity within. The lungs, being 

 elastic, immediately start up to fill the expanded chest ; 

 and the atmospheric air mshes into the now open air- 

 cells of the lungs, from whence it passes into, and 

 purifies, the blood. So soon as the inhaled air has been 

 exhausted by the blood of its \atahsing principle, it is 

 thrown out again by the pressm-e of the diaphragm 

 and ribs (whose action ceases) against the lungs. The 

 lungs are then reduced to their original size, and the ca- 

 pacity of the chest is diminished. This constitutes the 

 process of respu-ation or breathing, and on its due per- 

 formance will chiefly depend the health and usefulness 

 of the animal. A horse calling " bellows -to-mend " is 



