270 Biological Chemistry. 



lowest part of the alimentary tract in the form of the faeces. 

 A small part of the waste products is eliminated in the 

 perspiration through the pores of the skin, but this is 

 almost negligible. It is not proposed to deal in this place 

 with the mechanisms of the various parts of the body, 

 such as with the circulation, the mode of action of the 

 heart pump, and the lungs ; for the essential details 

 reference must be made to a manual of physiology. From 

 the more purely chemical standpoint the following factors 

 are of chief interest: I. The intake of the organism, 

 which includes the food ingested and the oxygen inspired. 

 II. The output of the organism, which includes the carbon 

 dioxide expired by the lungs, the nitrogenous and other 

 substances excreted by the kidneys in the urine, and the 

 materials unabsorbed from (or excreted into) the alimentary 

 tract, which are eliminated in the form of the faeces. In 

 the investigations of the total metabolism of the organ the 

 intake is balanced against the output, and the initial and 

 end products only are investigated. 



Now the chief organic constituents of the food are the 

 fats, the carbohydrates, and the proteins. The final pro- 

 ducts of the two first-named groups are carbon dioxide and , 

 water when combustion is complete ; these are also the 

 final products of combustion of these substances in the 

 animal body. Proteins, on the other hand, contain nitrogen, 

 sulphur, and in certain cases (the nucleo-proteins and phos- j 

 pho-proteins) phosphorus. The end products which would 

 be produced by the complete combustion of proteins in < 

 excess of oxygen (as in a combustion furnace), are carbon 

 dioxide, water, nitrogen (or oxides of nitrogen), sulphuric 

 acid, and phosphoric acid. These are actually end pro- 

 ducts produced by the combustion of protein in the 

 animal body in all cases except that of nitrogen. This 

 latter element is eliminated from the body in the urine 



