160 PEACTICAL PHYSIOLOGY 



The chemical substances which exist in the food-stuffs and tissues 

 may be divided into inorganic and organic, the former include water 

 and the mineral salts, and the latter consist of organic compounds 

 containing the elements carbon, oxygen, hydrogen, and, in a certain 

 class of them, nitrogen. The organic substances are divided into two 

 groups depending on whether they contain nitrogen or not. The 

 nitrogenous food stuffs include proteid, which is the most important con- 

 stituent of the tissues, and without which, as a- food-stuff, animal life is 

 impossible. The non-nitrogenous include the fats and carbohydrates, 

 the latter being pre-eminently the combustion material, and the former 

 the storage material of the animal body. 



The chemical composition of fats and carbohydrates is accurately 

 known, but with regard to the structure of the proteid molecule we 

 know next to nothing. Much less, therefore, do we know of the 

 chemical constitution of living protoplasm of which proteid is the chief 

 constituent. Living matter cannot be analysed, for the mere process 

 of analysis necessarily kills it, and the results obtained show only the 

 decomposition products of dead matter. 



These bodies, fats, proteids, and carbohydrates, really represent the 

 elementary constituents of the organism, so that they are frequently 

 called the ' proximate principles.' 



We shall first of all study the chemical nature of the proximate 

 principles, then the variety and amount of these contained in the 

 various tissues and foods. 



We shall then be in a position to investigate the nature of the 

 chemical interchanges in the organism, and in order to do this we 

 shall require to study the chemical composition of various excretory 

 bodies given off in the urine and other excreta. 



