vii THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 135 



nate over the katabolic. If katabolism predominates, or, in 

 other words, if waste exceeds repair, the organism must 

 diminish in size. 



Now the function of food is not merely to compensate for 

 the material which is broken down and eliminated, but to 

 afford the energy necessary to carry on the various activi- 

 ties of the organism. Food is to the body what fuel is to 

 a steam engine. The body is continually expending energy 

 in the form of heat. The amount of energy lost in this way 

 depends upon circumstances, and it may be comparatively 

 small when the temperature of the animal is only slightly 

 above freezing. But so long as life lasts there is some heat 

 produced, and this heat results from the breaking down of 

 some of the constituents of the body. Every movement 

 which the frog performs involves the expenditure of energy, 

 which must come ultimately from its food supply. An 

 organism has often been compared to a vortex which main- 

 tains its form, while the material of which it is composed is 

 subject to continual change. The matter composing the 

 tissues of an animal is not the same during successive years, 

 nor quite the same during successive days. It is being con- 

 tinually drawn through the vortex, where it gives up a part 

 of its energy for the maintenance of the vital processes. 

 The substances eliminated by an animal possess, therefore, 

 less energy than the food material taken in. The amount 

 of energy obtainable from a gram of any particular com- 

 pound, such as cane sugar, when it undergoes decomposi- 

 tion, may be measured with considerable accuracy. If we 

 measure the energy resulting from the splitting up of a cer- 

 tain amount of food substance and compare it with the 

 energy obtainable from an equal amount of the same kind 

 of food material after it has been eliminated from the 

 organism, we should find the latter to be much less in 



