72 THE FROG CHAP. 



Now the digestible part of the substance of animals 

 consists mainly of two classes of chemical compounds, 

 called proteids and/dtfs. The most familiar example of a 

 proteid is white of egg : other proteids, of varying com- 

 position, 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 follow- 

 ing proportions : 



Carbon - - from 51-5 to 54-5 per cent. 



Hydrogen - 6-9 7-3 ,, 



Oxygen - - 20-9 23-5 ,, 



Nitrogen - ,, 15-2 ,, 17-0 



Sulphur - ,, 0-3 ,, 2-0 



Fats differ from proteids in containing no nitrogen or 

 sulphur : they are formed of carbon, oxygen, and hydro- 

 gen, the number of atoms of hydrogen being typically 

 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, etc. The vegetable substances, such as corn and 

 grass, used as food by animals, 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. 



