'Introduction 9 



The principal organic compounds found in the body are 

 usually divided into three great classes : 



1. Proteids, or albuminous substances. 



2. Carbohydrates (starches, sugars, and gums). 



3. Fats. 



The extent to which these three great classes of organic 

 materials of the body exist in the animal and vegetable 

 kingdoms, and are utilized for the food of man, will be dis- 

 cussed in the chapter on food (Chapter V). 



12. The Proteids. The proteids, because they contain the 

 element nitrogen while the others mentioned above do not, 

 are frequently called nitrogenous, and the other two are 

 known as non-nitrogenous substances. 



The proteids, the type of which is egg albumin, or the 

 white of egg, are found in muscle and nerve, in glands, in 

 blood, and in nearly all the fluids of the body. A human 

 body is estimated to contain on an average about 18 per 

 cent of albuminous substances. 



In succeeding chapters we shall have occasion to refer to 

 various proteids and substances allied to them as they exist 

 in muscle (myosin), coagulated blood (fibrin), and bones 

 (gelatin). 



13. The Carbohydrates. The carbohydrates are formed of 

 carbon, hydrogen, and oxygen, the last two in the propor- 

 tion to form water. Thus, we have animal starch, or glyco- 

 gen, stored up in the liver. Sugar, as grape sugar, is also 

 found in the liver. 



14. The Fats. The body of an average man contains 

 about 10 per cent of fats. These are formed of carbon, 

 hydrogen, and oxygen, in which the latter two are not in the 

 proportion to form water. The fat of the body consists of 

 a mixture which is liquid at the ordinary temperature. 



