134 THE BIOLOGY OF THE FROG chap. 



CHAPTER VII 



THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 



One of the characteristics of all forms of life is the need 

 of food. The matter which composes the bodies of living 

 organisms is being continually broken down and eliminated 

 as waste products. New matter is consequently required 

 to make good the loss if the vital process be kept going. 

 In the frog a part of the material is taken from the oxygen 

 of the air and from the water absorbed through the skin ; 

 but neither of these sources supplies the carbon, nitrogen, 

 and other elements which form essential parts of all living- 

 substance. Life phenomena are associated especially with 

 certain compounds called proteids. These are complex 

 substances containing carbon, oxygen, hydrogen, and nitro- 

 gen, and, in many cases, also sulphur, phosphorus, calcium, 

 potassium, sodium, iron, and occasionally other elements. 

 Living substance, or protoplasm, is of proteid nature, but 

 it is probable that it is a group of compounds rather 

 than a particular compound which we might express by a 

 definite chemical formula. This living matter is the sub- 

 ject of chemical changes which are spoken of under the 

 general term metabolism. The synthetic or building-up 

 processes by which this substance is formed from simpler 

 compounds are called anabolism ; the opposite, or tearing- 

 down processes by which it is resolved into simpler sub- 

 stances are known as katabolism. If an organism grows, it is 

 evident that the anabolic side of the process must predomi- 



