UTILISATION OF NUTRIENTS 45 



too small to be considered. It suffices, then, in 

 an estimation of the increase of flesh to determine 

 how much nitrogen is taken into the body in the 

 food, and how much leaves it in the urine and 

 faeces. If, then, the output of nitrogen is sub- 

 tracted from the income, it is easy to calculate the 

 amount of flesh that has been stored, for dry flesh, 

 free from fat and mineral matter, contains 16-67% 

 of nitrogen. Fresh lean meat contains, further, 

 77% of water. In a certain sense, then, the additions 

 of nitrogen and of flesh are the same. 



Although the food of an animal may contain 

 more or less nitrogen or even none at all, it is always 

 found that a certain quantity is present in the urine. 

 This comes from the decomposition of the constitu- 

 ents of the food or of the body tissue, and so the 

 nitrogen in the urine is a measure of the nitrogen 

 metabolism in the body. In investigations relative 

 to vital changes which take place in the organism, 

 particularly in relation to its food supply, the 

 quantity of nitrogen in the urine plays a very im- 

 portant part. Experiments on the formation of 

 body tissue are in general carried out in the same 

 way as those on the digestibility of food (p. 27), 

 only here, in addition to the faeces, the urine must 

 be collected without loss for a certain length of 

 time. The minimum time of collection should be 

 eight to ten days, and careful analyses of the urine 

 are necessary. During the drying of faeces small 



