CH. xxviil.] FOOD. 455 



CHAPTER XXVIII. 



FOOD. 

 THE chief chemical compounds or proximate principles in food 



are :- 



jids . . ... .. . .\ 



ohydrates . . . . . j-i 



1. Proteids 



2. Carbohydrates [organic. 



3. Fats 



4. Water. 



5. Salts j- morgamc. 



In milk and in eggs, which form the exclusive foods of 

 young animals, all varieties of these proximate principles are 

 present in suitable proportions. Hence they are spoken of as 

 perfect foods. Eggs, though a perfect food for the developing 

 bird, contain too little carbohydrate for a mammal. In most 

 vegetable foods carbohydrates are in excess, while in animal 

 food, like meat, the proteids are predominant. In a suitable 

 diet these should be mixed in proper proportions, which must 

 vary for herbivorous and carnivorous animals. 



A healthy and suitable diet must possess the following cha- 

 racters : 



1. It must contain the proper amount and proportion of the 

 various proximate principles. 



2. It must be adapted to the climate; to the age of the 

 individual and to the amount of work done by him. 



3. The food must contain not only the necessary amount of 

 proximate principles, but these must be present in a digestible 

 form. As an instance of this, many vegetables (peas, beans, 

 lentils) contain even more proteid than beef or mutton, but are 

 not so nutritious, as they are less digestible, much passing oft' 

 in the faeces unused. 



The nutritive value of a diet depends chiefly on the amount 

 of carbon and nitrogen it contains. A man doing a moderate 

 amount of work will eliminate, chiefly from the lungs, in the 

 form of carbonic acid, from 250 to 280 grammes of carbon per 

 diem. During the same time he will eliminate, chiefly in the 

 form of urea in the urine, about 15 to 1 8 grammes of nitrogen. 

 These substances are derived from the metabolism of the tissues, 

 and various forms of energy, mechanical motion and heat being 



