138 THE PRINCIPLES OF AGRICULTURE. 



The blood consists of a colorless fluid, containing an 

 immense number of little flattened disks, called corpus- 

 cles. Most of these are red, and give the blood its color. 



The purpose of these disks seems to be to absorb oxy- 

 gen from the air in the lungs, and carry it through the 

 body, until it is needed to oxidize the elements of food in 

 the blood, or the tissues of the body. 



As the disks return from the lungs they have a bright 

 red color, which is supposed to be due to the presence of 

 oxygen that they have absorbed. On returning to the 

 heart, after passing through the body, they assume a 

 purple shade. 



The elements of food in the blood are burned, or ox- 

 idized, by uniting with the oxygen of the corpuscles, pro- 

 ducing animal heat or force. Particles of the body are 

 also oxidized and replaced by new particles. 



Excretion. The blood not only brings together food 

 and oxygen to produce this oxidation, but carries away 

 the waste products. 



When the carbo-hydrates and fats of food are burned 

 in the system, the result is carbonic acid and water. 

 When nitrogenous substances are burned, not only car- 

 bonic acid and water are produced, but also certain 

 salts, the most important of which is called urea. 



Carbonic acid escapes from the blood partly through 

 the skin, but mostly through the lungs ; urea and other 

 salts, through the kidneys ; and water, through the skin, 

 lungs, and kidneys. 



The Nature of Animals. An animal upon the farm may 

 be regarded as a kind of machine, capable of performing 

 a certain amount of work. 



This work consists in converting food into flesh, milk, 

 wool, etc., or into mechanical force, for the service of 



