Chemical Changes in Animal Organism. 271 



chiefly in the form of urea and uric acid. (For the for- 

 mulae of these two substances see pp. 102 and 254.) The 

 former predominates in the case of mammals, whereas the 

 latter is present in the larger amount in the case of birds. 

 In addition, certain smaller amounts of nitrogen are elimi- 

 nated in the urine in the form of ammonia, creatinine, and 

 amino-acids, and small amounts of other substances. Sul- 

 phates and phosphates are also eliminated in the urine. 

 The combustion of the proteins is, therefore, never complete. 

 Finally, the faeces contain the undigested, or partially 

 digested, materials consisting of more or less complex 

 organic substances, such as vegetable fibres, proteins 

 (mucous), undigested fats, and other products which have 

 escaped absorption in the alimentary tract. The chief 

 factors to be dealt with in the consideration of the initial 

 and final products of metabolism may be summarized in 

 the following scheme : 



INITIAL PRODUCTS. FINAL PRODUCTS. 



The food (ingested through the 



mouth) 



Fats (with the elements O H 0). 

 Carbohydrates (with elements 



C-H-0). 

 Proteins (with elements 



C- H- O- N- Sand sometimes P). 

 Water. 



The oxygen (inspired by the lungs). 



expired by lungs. 



excreted by 

 the kidneys 

 in the urine. 



Carbon dioxide 

 Water 



Nitrogenous excreta ^ 

 Urea 

 Uric acid 

 Ammonia 

 Creatinine 

 Amino - acids and 

 other nitrogenous 

 substances 

 Water 

 Wholly or partially "\ 



digested material I excreted in 

 unabsorbed from j the faeces. 

 alimentary tract J 

 Water and almost neg- \ excreted by 

 ligible amounts of I the skin in 

 nitrogenous and j the form of 

 other matter J perspiration. 



* Sulphates, phosphates, sodium chloride, and other products are also 

 excreted in the urine. 



This scheme deals with the utilization of foodstuffs, taking 



