362 BOARD OF AGRICULTURE. 



All species of animals, birds, fishes, reptiles, insects,, 

 worms and infusorial earths, together with the seeds of the 

 grasses and many varieties of vegetables, fruits, roots and 

 nuts, are used by mankind as food ; and some tribes are 

 cannibals, as were our English ancestors only a few hundred 

 years ago. 



Climatic influences modify food requirements. Hundreds 

 of millions of people living in tropical countries subsist 

 chiefly on rice and tropical fruits ; those living in arctic 

 regions consume enormous quantities of fats and fat meats, 

 without fruits or starch foods. The diet of one region would 

 be fatal to people living in the other. The food required in 

 infancy, middle life, old age and disease, varies very widely, 

 and its selection is influenced by many considerations. 



The constituent elements of foods are the same as those of 

 the tissues they are to nourish, — carbon, hydrogen, oxygen, 

 nitrogen, sulphur, phosphorus, calcium, magnesium and 

 iron are the essentials. We do not use these elements di- 

 rectly as foods, but in selecting our aliment we deal with 

 their combinations as found in living organisms, plants and 

 animals, or in organic products produced by the agency of 

 life, having chemical combinations of inorganic materials in 

 their substance, such as water, and the salts of lime, potash 

 and iron. The organic structure of many foods contains 

 compounds of which nitrogen is an important part, and such 

 arc called nitrogenous ; others, composed of carbon, hydrogen 

 and oxygen, variously combined, arc non-nitrogenous. The 

 nitrogenous foods are chiefly used in tissue building, and the 

 non-nitroo:enous in the 2i;encration of heat, — both are used 

 in tissue building and the production of heat and force, but 

 to an unequal degree. Physiologically considered, ali- 

 mentary substances may be classed under four divisions of 

 alimentary principles : — 



I. Nitrogenized principles. Albumen, fibrine, casein 

 and gluten. The proteids. 



II. Fats and oils. 



III. Carbo-hydrates. Starch and sugar. 



IV. Inorganic materials. Minci-al salts and water. 

 Nitrogen is an essential clement in the structure of animal 



tissues, so that without it animal life in any form would be 



