734 



PHYSIOLOGY OF DIGESTION AND SECRETION. 



Their chemical composition is unknown. Proteins, fats, and car- 

 bohydrates are complex organic substances whose chemical struc- 

 ture is known in whole or in part. When eaten and digested they 

 enter the body liquids, and are employed either in the synthesis 

 of the more complex living matter or they undergo various chem- 

 ical changes, spoken of in general as metabohsm, which result 

 finally in the breaking up of their complex molecules with a libera- 

 tion of some of their internal energy. The chemical changes of 

 metabolism or nutrition are, in the long run, mainly exothermic — 

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

 chemical or internal energy that held the complex molecules 

 together assumes the form of heat when these molecules are 

 broken down by oxidative changes to simpler, more stable struc- 

 tures, 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 chem- 

 ical energy are either useless or 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 analzye it to determine its com- 

 position as regards water, salts, vitamins, proteins, fats, and 

 carbohydrates. The following table compiled by Munk from the 



