26 CHEMICAL COMPOSITION OF LIVING MATTER 



and is composed mainly of calcium carbonate and calcium 

 phosphate. 



Gases. — Oxygen and carbon dioxide occur in all living 

 matter. They may be free, as in the lungs, or absorbed 

 in the fluids of the body. These two are the most impor- 

 tant gases present, but others are present in smaller 

 quantities. 



Carbon compounds. — The compounds of this element 

 in living matter are so numerous and so important that 

 they have been given the name of organic compounds, 

 or life compounds. They may be classified under two 

 groups, nitrogenous and non-nitrogenous. 



Nitrogenous carbon compounds. — As the name indi- 

 cates, these compounds contain the element nitrogen in 

 addition to the elements carbon, hydrogen, and oxygen. 

 They are of many forms. Proteid, which contains sulphur, 

 in addition to the above mentioned elements, is the name 

 given to the form which is perhaps most abundant, and 

 which under the name of albumin and fibrin in the blood, 

 myosin in the muscles, and casein in milk, is very im- 

 portant in its relation to vital processes. When proteid 

 burns it loses part of its carbon and the result is another 

 nitrogen compound called urea, which is given off as a 

 waste by the kidneys. Still another form of nitrogen 

 compound is found in the gelatine of the bones and the 

 keratin of the hair and nails. 



Non-nitrogenous carbon compounds. — In this gi-oup 

 are placed all carbon compountls found in living matter, 

 which do not contain nitrogen. This group is divisible 

 into two sub-groups, carbohj'dratcs and fats. 



Carbohydrates. — Besides carbon these compounds con- 

 tain only hydrogen and oxygen, which are always in the 



