[006 EXPERIMENT STATION RECORD. 



drates :: 1 : 2.5 : 1; that is (having the requisite amount of nitrogen, 

 used probably for the repair of tissue or for some vital process less 

 understood, or for both), it is theoretically and within certain limits 

 unimportant which of the nutrients supplies the necessary energy. An 

 abundance of carbohydrates and fat will, within limits, make up for a 

 deficiency of protein; that is to say, the body will get on with a small 

 amount of protein provided a correspondingly large amount of nitrogen- 

 free material is supplied. This is often spoken of as the power of 

 carbohydrates and fat to protect protein. 



1 10) The nutrients of the food combine in the body with the oxygen 

 of the air and undergo combustion, thus liberating energy for the body. 



METABOLISM OF NITROGEN. 



Metabolism experiments of the kind here referred to may be divided 

 into four classes: (1) Those which show the amount of nitrogen (with 

 or without ash constituents) consumed in the food and excreted in the 

 urine and feces; (2) those which also account for the carbon (with or 

 without oxygen and hydrogen), known as respiration experiments; (3) 

 those which in addition to the above factors take account of the energy 

 of the food and excretory products as well as the energy of internal 

 and external muscular work and the heat radiated from the body; and 

 (4) those in which the respiratory quotient is determined, i. e., the 

 ratio of carbon dioxid excreted in the breath to oxygen consumed from 

 the air. 



The first line of experimenting is much the simplest, as to both appa- 

 ratus required and manipulation, and it also affords data which are of 

 much importance not only in judging of the metabolism of protein, but 

 also as to the general condition of the animal. In experiments on the 

 metabolism of nitrogen as ordinarily conducted, the nitrogen in the food, 

 urine, and feces is determined. The urine contains nearly all the metab- 

 olized nitrogen excreted from the body. Under ordinary conditions the 

 amount in the feces is small. Apparatus has been devised for the collec- 

 tion of the urine. The collection of the feces is a comparatively simple 

 matter. The nitrogen in the urine and feces may be readily deter- 

 mined by the ordinary methods. The sum of the nitrogen in the urine 

 and feces (representing the outgo) subtracted algebraically from the 

 nitrogen in the food (income) shows whether a gain or loss of nitrogen 

 in the body is taking place, i. e., whether or not nitrogen is being 

 retained in the form of increased tissue, reserve material, etc. If the 

 income is greater than the outgo there is a gain of nitrogen in the body; 

 if it is less than the outgo there is a loss ; if it is just equal to the outgo, 

 nitrogen equilibrium has been reached. Comparing the income and 

 outgo is often spoken of as determining the nitrogen balance. 



Nitrogen metabolism in its relation to feeding experiments. — The feeding 

 experiment as ordinarily made depends upon considerations like those 

 mentioned above (p. 1004). Yet it is doubtless true that few consider 



