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 1 ? 
