INTRODUCTION 5 



and so indicates the quantity of digestible protein which 

 the food must contain. 



The amides in food are much simpler bodies than 



proteins I e.g., urea itself is an amide: C0<] NH^/' ^^^ 



they can on oxidation give rise to a certain amount of 

 heat or energy, but in the horse have no power of re- 

 building or repairing waste in muscular tissue. Amides 

 chiefly occur in unripe, immature plants, and it is of 

 some importance that the percentage of the nitrogenous 

 matter existing in this form should be stated when the 

 chemical analysis of such a food is given. In ripe 

 foods, such as cereal grains and concentrated foods, 

 amides are present in small quantities only, and are 

 practically negligible. 



Of the other proximate principles in a food, the 

 carbo-hydrates, consisting of starches, sugars, and 

 soluble cellulose, are made up of carbon, hydrogen, 

 and oxygen. They supply heat and energy on being 

 broken down in the body, and, if given in excess of 

 working requirements, are deposited in the form of 

 fat. 



The fats, also made up of the same three elements, 

 carbon, hydrogen, and oxygen, require much more 

 oxidising to split them up into their ultimate products 

 — carbon dioxide and water — and, therefore, give rise 

 to a proportionately greater amount of heat and energy 

 than carbo-hydrates. 



Both fats and carbo-hydrates are excreted chiefly by 

 the lungs in the form of carbon dioxide (CO2) and water 

 vapour (H^O), and are completely oxidised, so that the 

 whole of their potential energy is given up to the 

 body. 



