n6 TEXT-BOOK OF PHYSIOLOGY. 



body which becomes manifest as heat and mechanic motion in the 

 transformations of the material used in the nutritive processes. 



It has been stated in a previous chapter that the animal body 

 may be regarded as a machine capable of performing each day a certain 

 amount of work by the expenditure of a definite amount of energy. In 

 the performance of its work, whether it be the raising of weights against 

 gravity, or the overcoming of friction, cohesion, or elasticity, the machine 

 suffers disintegration and metabolizes a portion of the food materials and 

 loses a portion of its available energy. Unlike other machines, however, 

 it possesses the power, within limits, of self-renewal, when supplied with 

 foods in proper quantity and quality. 



QUANTITIES OF FOOD PRINCIPLES REQUIRED DAILY. 



In order that the body may continue in the performance of its work and 

 yet retain a given weight, it is essential that the loss to the body daily shall 

 be exactly compensated by the introduction and assimilation of a corre- 

 sponding amount of food principles. If this condition is realized, the 

 body neither gains nor loses in weight, but remains in a condition of nutritive 

 equilibrium. The determination of the extent of the metabolism is made 

 from an examination of the quantity and composition of the daily excretions. 

 If therefore these are collected and analyzed, it will become possible to de- 

 termine from their chief constituents the extent and character of the tissue 

 and food metabolized. Thus the urea and other nitrogen-holding com- 

 pounds contained in the urine represent the proteins metabolized; the carbon 

 dioxid and water represent the fat and carbohydrates metabolized. There- 

 fore it becomes possible to determine from the amounts of the urea and 

 carbon dioxid eliminated, the different amounts of the food principles re- 

 quired to restore the nutritive equilibrium under any given condition. 

 As the activity of the nutritive changes varies in accordance with age, weight, 

 climatic conditions, work done, etc., and as the excreted products vary in the 

 same ratio, it is obvious that the required amounts of food will vary in 

 accordance with these varying conditions, if equilibrium is to be maintained. 



An experiment designed to collect the excretions for purposes of analysis 

 is termed a metabolism experiment; its object is to deduce from the amounts 

 of urea and other nitrogen-holding compounds, of carbon dioxid and water 

 discharged, the amount of the tissue and food metabolized, and hence from 

 them to calculate the amounts of the food principles and their ratio one to 

 another that must be returned to the body if nutritive equilibrium is to be 

 restored. This is accomplished by one of the many forms of respiration 

 appliances, which have been devised for animals and for man. The best 

 form of apparatus for determining the metabolism of man is that designed 

 by Benedict. 



Many metabolism experiments have been performed by different inves- 

 tigators under a great variety of conditions. The results, though differing 

 in some respects, have nevertheless a general average value. The following 

 table shows the results of a series of experiments made by Vierordt. On 

 the right under the term outcome, are arranged the amounts of the sub- 

 stances eliminated; on the left, under the term income, the amounts of the 



