foods. 159 



absorption is possible. This is the sole end of digestion. The aliments 

 are in the digestive tract split up into their nutritive elements, which are 

 prepared for their absorption, while the non -nutritive portions are 

 expelled. Thus, the blood of animals is continually receiving additions 

 from the food, which it carries to different organs and tissues. Certain 

 of these are fixed or assimilated, replacing analogous substances rendered 

 unfit for carrying on the vital functions ; the others are modified and 

 destroyed. There is thus an incessant movement in the animal economy, 

 a continuous interchange of material, and a double current of entering 

 and expelled materials. This double current is marked by two series of 

 chemical phenomena,— the one terminating in the fixation of the nutri- 

 tive principles in the economy, in their assimilation ; the other in their 

 decomposition, their retrogressive metamorphosis, or their disassimila- 

 tion. The ensemble of these two chemical phenomena constitutes 

 nutrition. 



Foods of animals are destined to supply the waste of tissues. They 

 must, therefore, to be complete, embrace all the tissue-ingredients which 

 are liable to waste. The statement, therefore, of these tissue-constituents 

 will be also a statement of the essential food-stuffs. The chief constitu- 

 ents of the blood, flesh, and other tissues, as we have seen, may be classi- 

 fied as follow : — 



Okganic. 



Nitrogenous. Non-nitrogenous. 



Albuminous Bodies and Carbo-hydrates and 



their Derivatives. Hydro-carbons. 



Inorganic. 



Water, Sodium Chloride, 



Alkaline Phosphates, Sodium Sulphate, 



Phosphatic Earths '(Calcium, Magnesium), Potassium Sulphate, 



Magnesium and Calcium Carbonates, Iron, 



Potassium Chloride, Silicon. 



It is rarely the case, however, that these simple nutritive substances 

 are taken separately as food. Ordinarily the alimentary substances are 

 formed of mixtures, in various proportions, of the simple nutritive 

 substances. Thus, water that we drink contains mineral salts in solution. 

 Meat contains water, albuminous bodies, salts, and fats, while milk 

 contains all the alimentaiy principles. AYe must therefore distinguish 

 between simple nutritive substances and foods which contain several of 

 these bodies. 



In addition to the simple food-stuffs, there are other substances not 

 belonging to an y of the above classes which have certain nutritive values, 

 such as alcohol, organic acids, tea, coffee, and essential oils. These are 



