260 MANUAL OF MODERN FARRIERY. 



cells and blood-vessels are connected together by an inter- 

 vening substance of a fibrous and cellular texture. 



The blood circulating through the capillaries of the body 

 contribute to the nourishment of the animal system, and 

 furnishing all the secretions, becomes changed, and is no 

 longer capable of supporting life : it becomes of a poisonous 

 quality, in consequence of having in it a too large portion 

 of carbon. This must be expelled before the blood can 

 again be rendered subservient to the purposes of life. That 

 portion of the atmospheric air called oxygen having a strong 

 attraction for carbon, unites with it whenever they come in 

 contact. The chest enlarges by means of the diaphragm 

 and the muscles between the ribs, called the intercostal 

 muscles, and others, and the lungs expand with the chest in 

 order to fill up the vacuum which would otherwise exist 

 between them and the sides of the chest. These cells 

 enlarge, and a sort of vacuum is formed in each of them, 

 as the air rushes down and fills them ; and being divided 

 from the venous and poisoned blood by these membranes 

 alone, it is enabled to act upon the blood and abstract from 

 it the carbon, and by this means purifies it, and the arterial 

 blood is fitted for the purposes of life. This purification 

 being performed, the chest contracts, and the lungs are 

 compressed into smaller compass, and a portion of air, 

 holding in it a quantity of carbon, and rendered poisonous 

 in its turn, is squeezed out. Immediately afterwards the 

 chest expands again, and the lungs expand with it, and pure 

 atmospheric air is drawn into them, which is immediately 

 thrown out again by the compression of the lungs, which, 

 like the preceding expiration, is poisoned by the carbon of 

 the blood. These alternate contractions of the chest and 

 lungs constitute what is termed breathing. 



When the horse is subjected to powerful exertion, it is 



