CHAPTER V. 



MECHANISM OF BREATHING. 



The body of the horse (Fig. 19), viewed apart from his head, neck, 

 limbs, and tail, may be divided into chest and abdomen (belly), the 

 former containing the lungs and heart ; the latter, the stomach, in- 

 testines, liver, bladder, and other vital organs. This division is 

 effected by a broad and somewhat 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 

 forward. 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 vertebrse, and bj^ 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 expelling air 

 from the lungs, expiration ; and that of drawing it in, inspiration. 



The capacity of 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 forward 

 by muscles which cover them, and by others which are situated 



