THE BALANCE OF NUTRITION 231 



the urine in the form of crystalline nitrogenous products, of 

 which urea is the most familiar and often the most abundant. 

 When these products are burned they yield a certain amount 

 of heat, thus showing that they still contain part of the gross 

 energy of the protein, and that, therefore, only a portion of the 

 latter has been transformed in the body. 



320. The urine. In the main, the urine is the vehicle for the 

 removal from the body of the incompletely oxidized products 

 of katabolism, although some unoxidized or partially oxidized 

 material also escapes from the body in the form of the excretory 

 products contained in the feces (317), and small amounts of 

 chemical energy are contained in the cutaneous excretory 

 products (198). 



The energy of the katabolic products contained in the urine 

 may be determined as in the case of the feces by burning the 

 dried residue in the calorimeter, a small correction being usually 

 necessary for unavoidable losses in drying. 



An approximate calculation of the chemical energy of the urine 

 may be based upon its nitrogen or better on its carbon content, using 

 the average ratio found in experiments on the same species, but these 

 ratios vary more or less in different cases, and in exact work direct 

 determinations are called for. 



321. Cutaneous excretion. The amount of chemical energy 

 removed in the perspiration is too small to be of any significance, 

 except possibly in experiments on severe work. 



In addition to the perspiration there is a continual small loss 

 of matter with its accompanying chemical energy in the form 

 of epidermal scales, hair, etc., sloughed off. These losses are 

 comparable to the excretory products in the feces, since they 

 consist essentially of incompletely katabolized body material. 

 Their amount is small but is sufficient to be taken account of 

 in exact experiments. 



Metabolizable energy 



322. General conception. It has been shown in the fore- 

 going paragraphs that more or less of the chemical energy of 

 the feed escapes unused from the body, the total thus rejected 

 being equal to the gross energy of the total excreta, solid, liquid 



