20 THE HORSE IN HEALTH AND DISEASE 



Oxygen is the only element entering the body in a free state. 

 It is essential to the animal, for life cannot exist for any length 

 of time without it. 



Nitrogen enters the body in large amounts in protein combina- 

 tions, and leaves it as urea and other products which easily yield 

 ammonia. 



Sulphur is present in the smallest amount of any of the five 

 principal constituents of the body. It is taken into the body as a 

 part of the protein molecule, and is largely excreted in the urine as 

 sulphates. 



It is customary to divide the constituents of the animal body 

 into: 



(I) Inorganic compounds, which include the mineral elements 

 found in the bones and teeth, also the water and salts of the body. 

 They are recovered from the ash after burning the body. They 

 comprise various salts, of which calcium phosphate predominates. 

 None of them contains carbon. Approximately 80 per cent, of the 

 total ash compounds of fat animals are found in the bones. 



(II) Organic compounds, which make up by far the largest 

 portion cf the animal body. They are characterized by containing 

 carbon and are subdivided for study into: (1) nitrogenous sub- 

 stances; (2) non-nitrogenous substances. 



(1) The nitrogenous substances include all the organic com- 

 pounds having nitrogen in their molecules. They have various 

 characteristics and properties. For a complete discussion of these 

 substances references should be made to text-books on organic 

 chemistry. Collectively this group of highly complex compounds 

 is known as proteins. 



Proteins comprise a large part of the sohd matter of the muscles, 

 tendons, blood, skin, etc. They are mostly colloidal in nature, 

 only a very few having been obtained in crystalline form. Chemists 

 have not been able to synthetize any but the very simple proteins. 

 One characteristic of proteins is their inability to diffuse through 

 an animal membrane. The decomposition products of proteins 

 are varied and numerous. Carbonic acid, water, urea, and uric 

 acid are the final end-products, but many intermediate sub- 

 stances, such as glycin and leucin, which are really cleavage pro- 

 ducts of proteins, occur in the course of their destruction. 



Albuminoids are closely aUied to the proteins, but differ from 

 them by being insoluble in all neutral solvents. The best-known 



