2, A M>t i, f<<l q/ Veterinary Physiology. 



carbon, returning the oxygen to the air, and thus supplying 

 to the atmosphere what animals are momentarily depriving 

 it of. Even the solid carbon in the form of coal must at 

 one time have been carbonic acid, for the reason just men- 

 tioned that this is the only form in which carbon can be 

 taken up by plants. Carbon enters the animal system with 

 the carbon of the food, and leaves it either as carbonic acid 

 or in compounds, such as urea, which readily yield it ; us 

 carbonic acid, therefore, it is again taken up by the plant. 



Hydrogen does not occur in the free state in nature, but 

 principally as water, and a very small quantity as ammonia ; 

 in these forms the hydrogen is taken up into the plant, by 

 which it is introduced into the animal, which gives it off' 

 again as water and ammonia, or a substance which readily 

 yields these. 



Oxygen is the most widely distributed of the elements, 

 forming one quarter by weight of the atmosphere, and eight- 

 ninths the weight of water ; it also forms, by means of its 

 compounds, one-half the weight of the earth's crust. It is 

 the only element which enters the animal or vegetable body 

 in a free state, and then only to a limited extent. We have 

 mentioned the combination of carbon with oxygen, and it is 

 this which principally supplies the plant with the oxygen 

 required ; at the same time the plant returning the oxygen 

 to the air, maintains the needful balance which should exist 

 between the carbonic acid and oxygen. 



Nitrogen exists largely in a free state, no less than four- 

 fifths of it being in this form in the atmosphere ; it has but 

 little affinity for other elements. In the form of ammonia, 

 nitrous and nitric acids, it enters the plant, free nitrogen 

 the plant cannot assimilate. In the form of vegetable 

 proteid it enters the animal, leaving it as urea, etc., which 

 by rapid decomposition yields ammonia. The animal cannot 

 utilize free nitrogen any more than the plant, though the 

 gas is found dissolved to a slight extent in some of the 

 fluids of the body. 



Sulphur exists largely in nature in combination as 

 sulphates of alkalies and alkaline earths ; in this form it 



