136 THE BIOLOGY OF THE FROG chap. 



amount. If now we could measure the energy expended by 

 the organism by radiating heat and performing work during 

 the time this material is consumed, we should probably find 

 it to be equal to the difference between the potential energy 

 of the food and that of the eliminated products. All of our 

 experience goes to prove that the great law of conservation 

 of energy applies as strictly to organisms as to the phe- 

 nomena of the inorganic world. Living beings are not 

 sources of energy in themselves, but are dependent upon 

 their environment for energy as much as they are for the 

 material composing their bodies. 



In order that food material may be assimilated or built 

 up into the tissues of the bodies, it must be rendered solu- 

 ble, so that it can pass through the lining of the alimen- 

 tary canal into the blood and lymph, and from these fluids 

 through the walls of the cells in the different parts of the 

 body. This process of converting food into a soluble state 

 ready for absorption is called digestion. There are certain 

 mechanical processes involved in digestion, such as (in higher 

 animals) chewing the food, moving it about by the contrac- 

 tions of the walls of the stomach, and passing it along the 

 intestine by the peristaltic contraction of the walls. The 

 frog, however, like most lower vertebrates, does not chew the 

 food taken into the mouth, but swallows it whole down the 

 very distensible esophagus into the stomach, where it is acted 

 upon by the gastric juice. The principal part of the process 

 of digestion consists in the chemical changes produced in food 

 by the action of the various digestive fluids. These changes 

 are mainly of the nature of fermentations caused by sub- 

 stances called enzymes, or ferments. What the chemical 

 nature of enzymes is still remains very much in the dark, 

 since they cannot be completely freed from their associa- 

 tion with other substances, but it is probable that they are 



