296 THE ANIMALS AND MAN 



second, their oxidation in the cells results in the production 

 of energy, in the form of heat or motion; that is, they furnish 

 the body with building material and fuel. 



The proteid compounds are best fitted for this purpose, 

 though each food nutrient has its value. 



We may now consider each of the food nutrients sepa- 

 rately, and the place of each in our diet. 



Proteids or nitrogenous compounds (albumins, etc.).- 

 While the exact composition of proteid is unknown the 

 substances forming it are known to be carbon, oxygen, 

 hydrogen, nitrogen, sulphur and other elements. Such 

 foods as white of egg, lean meat, milk curd and the gluten of 

 wheat, contain large amounts of proteid. The proteid food 

 stuffs are the only food stuffs supplying nitrogen to the body. 

 This nitrogen is being constantly eliminated from the cells 

 and hence must be constantly restored to the cells. 



Proteids have been called flesh-producers or tissue-formers 

 because they possess all the elements for forming tissues and 

 cells, as muscles, nerves, etc. 



Plants manufacture proteid from sugars and certain min- 

 eral salts, the former supplying the carbon, hydrogen, and 

 oxygen, the latter supplying nitrogen, sulphur, and other 

 elements. Plants are therefore the original source of supply 

 for proteid food. While carnivorous animals obtain their 

 proteid by eating the flesh of other animals, these have 

 obtained it from plants. 



Tests For Proteid. 



a. Xanthoproteic Test. Boil the substance to be tested in strong 

 nitric acid (80%) ; a lemon yellow appears. Wash in water and add 

 enough ammonia to neutralize the acid. If the color changes to deep 

 orange proteid is present. 



b. Millon's Test. Pour Millon's reagent (solution of mercury in 

 nitric acid) over the substance to be tested and bring slowly to a 

 boiling point. If proteid is present the solution becomes rose red. 



Proteid burns with the odor of burning leather. 



Carbohydrates (starches, sugars, etc.). Carbohydrates con- 



