PHYSIOLOGICAL CHEMISTEY 271 



All the food-stuff digested by the animal is not decomposed, a 

 certain amount of it being used in order to build up the tissues 

 themselves (e.g. muscle, glands, etc.), and a certain amount being 

 laid aside as storage material (e.g. fat, glycogen), which the organism 

 can use as food in times of need. 



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 some cases, 

 nitrogen. The organic substances are divided into two groups de- 

 pending on whether or not they contain nitrogen. The nitrogenous 

 food-stuffs include protein, which is the most important constituent 

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

 possible. The non-nitrogenous food-stuffs include the fats and carbo- 

 hydrates, both of which may be regarded as combustion materials; 

 fat, moreover, is the principal storage substance for surplus food-stuff 

 assimilated. 



The chemical composition of fats and carbohydrates is fairly accu- 

 rately known, but at the present date we are only beginning to 

 understand the structure of the apparently much more complex 

 protein molecule. Much less do we know of the chemical constitu- 

 tion of living protoplasm of which protein is the chief constituent, 

 for living matter cannot be analysed since it is killed by the process 

 of analysis, and the results obtained show only the decomposition 

 products of dead matter. 



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

 elementary constituents of the organism, and 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 investi- 

 gate the nature of the chemical interchanges in the organism, and, 

 in order to do this, we shall require to study the chemical compo- 

 sition of the various excretory bodies given off in the urine and other 

 excreta. 



