34 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



C. Bread contains the same constituents as flour, except that some of the 

 starch has been converted into dextrin and dextrose during baking (most 

 flours, however, contain a small quantity of sugar). Extract breadcrust 

 w^ith cold M^ater, and test the extract for dextrin (iodine test) and for dextrose 

 (Trommer's test). If hot water is used, starch also passes into solution. 



D. Meat. — This is our main source of proteid food. Cut up some lean 

 meat into fine shreds and grind these up with salt solution. Filter and test 

 for proteids. 



THE PRINCIPAL FOOD-STUFFS 



We can now proceed to apply the knovv^ledge we have obtained of 

 the proteids, carbohydrates, and fats to the investigation of some im- 

 portant foods. We do not actually nse as food the various organic 

 proximate principles in the pure condition ; it is necessary that in a 

 suitable diet these should be mixed in certain proportions, and in 

 nature we find them already mixed for us. The chief proximate 

 principles in food are : — 



1. Proteids 



2. Carbohydrates |" organic. 



3. Fats j 



4. Water ) 



5. Salts I inorganic. 



In milk and in eggs, which form the exclusive foods of young 

 animals, all varieties of these proximate principles are present in suit- 

 able proportions. Hence they are spoken of as perfect foods. Eggs, 

 though a perfect food for the developing bird, contain too little carbo- 

 hydrate for a mammal. In most vegetable foods carbohydrates are 

 in excess, while in animal food, like meat, the proteids are predomi- 

 nant. In a suitable diet these should be mixed in proper proportions, 

 which must vary for herbivorous and carnivorous animals. We 

 must, however, limit ourselves to the omnivorous animal, man. 



A healthy and suitable diet must possess the following characters : — 



1. It must contain the proper amoimt and proportion of the 

 various proximate principles. 



2. It must be adapted to the climate, to the age of the individual, 

 and to the amount of work done by him. 



3. The food must contain not only the necessary amount of proxi- 

 mate principles, but these must be present in a digestible form. As 

 an instance of this many vegetables (peas, beans, lentils) contain even 

 more proteid than beef and mutton, but are not so nutritious, as they 

 are less digestible, much passing off in the faeces unused. 



The nutritive value of a diet depends chiefly on the amount of 

 carbon and nitrogen it contains in a readily digestible form. A man 

 doing a moderate amount of work will eliminate, chiefly from the 



