642 PHYSIOLOGICAL CHEMISTRY. 



other elements, such as aluminum, manganese, copper, etc., are some- 

 times found in the animal system, but they cannot be looked upon 

 as normal or necessary constituents. 



, The various kinds of animal food are derived chiefly from three 

 groups of organic substances, viz., carbohydrates (sugars, starch, etc.), 

 fats, and proteins or nitrogenous substances. The inorganic sub- 

 stances, such as phosphates, chlorides, etc., required by the animal in 

 the construction of bones, for the liberation of hydrochloric acid in 

 the gastric juice, etc., are generally found as constituents of various 

 kinds of food or are derived from drinking water. Milk contains all 

 the necessary organic or inorganic constituents ; bread is rich in phos- 

 phates, which latter are also found in smaller or larger quantities in 

 nearly all kinds of vegetable and animal food. 



Through the food are supplied those compounds which supply the 

 constituents that replace the exhausted material of the living cells, 

 and by chemical changes their inherent potential energy is converted 

 into the heat of the body and into the kinetic energy used in work- 

 ing the living mechanism. While the nitrogenous substances have 

 primarily the task of continuously replacing the wear and tear of the 

 nitrogenous tissues, they also serve, together with non-nitrogenous 

 food, to yield the animal heat, as also muscular and other power for 

 the work which the body performs. To a certain extent the different 

 nutrients can do the work of one another. Thus, the body can burn 

 protein in place of fats or carbohydrates, but neither of the latter can 

 replace the protein in building or repairing tissue. On the other 

 hand, the fats and carbohydrates, while being consumed, protect the 

 proteins. 



To some extent, the animal body may be regarded as a complicated machine, 

 in which the potential energy, supplied by the food, is converted into actual 

 energy of heat and mechanical labor. The main difference is that in our 

 machines the fuel serves as the source of energy only, while in the body the 

 food is mainly changed first into tissue (thus building up and renewing the 

 body constantly), serving as fuel afterward. While in the best steam-engine 

 only one-tenth of the fuel is utilized as mechanical work, one-fifth of the energy 

 of the food is realized in the human body. 



Heat and muscular power are forms of energy developed by the 

 consumption of food in the body. The amount of energy developed 

 is the measure of the food value of any nutrient, and the unit of 

 value is the calorie. 



While each individual substance generates a definite number of 

 calories during combustion, for practical purposes it is sufficiently 

 accurate to estimate the average amount of heat and energy in 1 



