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PHYSIOLOGY OF DIGESTION AND SECRETION. 



molecules into simpler compounds. The chemical changes of metab- 

 olism or nutrition are, in the long run, mainly exothermic, that 

 is, they are attended by the production of heat. Some of the chem- 

 ical or internal energy that held the complex molecules together 

 assumes the form of heat, or perhaps muscular work, after these 

 molecules are broken down by oxidative changes to simpler, more 

 stable structures, such as water, carbon dioxid, and urea. Proteins, 

 fats, and carbohydrates form materials that the tissue cells are 

 adjusted to act upon after they have undergone certain changes 

 during digestion. Other complex organic compounds containing 

 chemical energy are either injurious to the tissues, or they have a 

 structure such that the tissues cannot act upon them. Such 

 substances cannot be considered as foods in the scientific sense. 

 When, therefore, we desire to know the food value of any animal 

 or vegetable product, we analyze it to determine its composition as 

 regards water, salts, proteins, fats, and carbohydrates. The 

 following table compiled by Munk from the analyses given by 

 Konig * may be taken as an indication of the average composition 

 of the most commonly used foods: 



COMPOSITION OF FOODS. 



An examination of this table shows that the animal foods, par- 

 ticularly the meats, are characterized by their small percentage in 

 carbohydrate and by a relatively large amount of protein or of 

 protein and fat. With regard to the last two foodstuffs, meats differ 



*See Konig, " Die menschlichen Nahrungs und Genussmittel"; and 

 Atwater and Bryant, "The Chemical Composition of American Food Mate- 

 rials," Bulletin 28, United States Department of Agriculture, 1899. 



