73 THE FROG chap. 



As we have seen, the frog is a carnivorous animal. Now 

 the digestible part of the substance of animals consists 

 mainly of two classes of chemical compounds, called proteids 

 a.ndfa(s. The most familiar example of a proteid is white 

 of egg : other proteids, of varying composition, are found in 

 muscle, in blood, and in other animal tissues. All are 

 composed of the five chemical elements — carbon, oxygen, 

 hydrogen, nitrogen, and sulphur — the five elements being 

 combined in the following proportions : — 



Fats differ frorri proteids in containing no nitrogen or 

 sulphur : they are formed of carbon, oxygen, and hydrogen, 

 the number of atoms of hydrogen being always more than 

 twice as great as the number of atoms of oxygen. 



It will be noticed that two important articles of diet are 

 absent from the above list, namely sugar and starch — the 

 latter the largest constituent of flour, oatmeal, rice, &c. 

 The vegetable substances used as food by animals, such as 

 corn and grass, contain these bodies in varying proportions 

 in addition to vegetable proteids, and there is no doubt 

 that the frog must eat a small quantity of such vegetable 

 food, if only in the stomachs of the herbivorous animals upon 

 which it preys. Now, starch and sugar belong to a group of 

 compounds called carbohydrates, composed of carbon, 

 oxygen, and hydrogen, but differing from fats in that the 

 number of atoms of hydrogen is always exactly double that 

 of the atoms of oxygen, as in water. Lastly, the food always 

 contains a certain quantity of saline or mineral matters, as 

 well as water. 



