FOOD REQUIREMENTS. 91 



really nutritious food-stuffs are composed of materials which are 

 chemically like the tissues, although, as we shall see, we have no 

 grounds for believing that the different chemical groups of nutri- 

 tive stuffs are exclusively destined to replace corresponding sub- 

 stances in the body. On the contrary, we have good reason to 

 think that within the body the conversion of one group into 

 another is very common. 



In Chapter III. the tissues of the animal body were shown to 

 consist of chemical compounds, which have been classified into 

 certain groups. And it has also been stated that the tissues are 

 constantly undergoing chemical changes inseparable from their 

 life, and that for these changes a supply of nutritive material is 

 necessary. 



The nutriment required for an animal is, then, made up of 

 substances which may be divided into the same chemical groups 

 as the tissues of the body : viz., proteids, fats, carbohydrates, salts, 

 and water. So that each of the various substances which we 

 make use of as food, contains in varying proportions several of 

 the different kinds of nutrient material, either naturally or arti- 

 ficially mixed so as to form a complex mass, the important item 

 water being the only one which is commonly used by itself. These 

 substances may be considered to be the chemical bases of the food, 

 as they are also the chemical bases of the animal body. 



The following classification shows the relationships between 

 the chief items of nutritious matters, from a chemical point of 

 view, and their distribution in the various foods we commonly eat. 



I. ORGANIC. 



1. Nitrogenous 



A. Albuminous abundant in eggs, milk, meat, 



peas, wheaten flour, etc. 



B. Albuminoid in soups, jellies, etc. 



2. Non-Nitrogenous 



A. Carbohydrates (sugar, starch) abundant in 



all kinds of vegetable food, and in milk, and 

 present in small quantity in meat, fish, etc. 



B. Fats in milk, butter, cheese, fat tissues of 



meat, many vegetables, oils, etc. 



