CHARACTERISTICS OF ANIMALS 19 



going on in it. Analyses of dead tissues show that the compounds 

 are very complex, yet not more than twelve separate elements can 

 be demonstrated. This is a relatively small number when com- 

 pared to the eighty or more chemical elements known to science. 

 Of these twelve elements only the following six are the most im- 

 portant—carbon, hydrogen, oxygen, nitrogen, phosphorus, and 

 sulphur. These materials of construction of the body are linked 

 together in different ways to form molecules of enormous size. 



Carbon enters the animal body as a constituent of organic 

 compounds, and leaves it either as carbonic acid gas or in simple 

 organic compounds, such as urea. This element makes up a 

 large part of the animal body. 



Hydrogen occurs in nature in such combinations as water and 

 ammonia. It is in these forms that it is taken up by plants 

 and converted into a constituent of organic compounds. Hy- 

 drogen is given off by the body in the same forms as acquired, 

 that is, as water and in combination with nitrogen to form 

 ammonia, and as a constituent of organic compounds. 



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

 It is essential to the animal, for lif e 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. 



Phosphorus, although present in the smallest quantity of 

 any of the six principal elements, is one of the most important 

 that enters into the formation of the body. It never occurs 

 in the free state but is present in a greater diversity of compounds 

 and plays a more vital function than any other element. It enters 

 the body as a constituent of vegetable and animal foods, and is 

 excreted largely in the urine, although traces are found in the 

 feces. Forbes of the Ohio Experiment Station in speaking of 

 phosphorus said: "Structurally, it is important as a constituent 

 of every cell nucleus and so of all cellular structures; it is also 

 prominent in the skeleton, in milk, in sexual elements, glandular 

 tissue, and the nervous system. Functionally, it is involved 

 in all cell multiplication, in the activation and control of enzyme 

 actions, in the maintenance of neutrality in the organism, in 

 the conduct of nerve stimuli, and through its relation to osmotic 

 pressure, surface tension, and imbibition of water by colloids it 



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