10 THE ANIMAL ORGANISM 



to which it is to be added. The flesh of a dead ox or sheep differs 

 considerably in composition from that of a hving man, and the 

 difference is increased by its digestion. In the course of incor- 

 poration the food has therefore to undergo chemical changes 

 by which it is converted into the substances which compose the 

 body, and these changes it undergoes by the activity of the living 

 matter itself. That is to say, the living substance has the power 

 of making, out of unlike materials, additional matter of its own 

 composition. The process by which this is done is known as 

 assimilation. Both absorption and assimilation are processes in 

 which work is done, and therefore involve the use of energy, 

 but their net result is to add to the amount of material composed 

 of complex molecules, and therefore to the amount of energy, 

 in the body. 



PROVISION OF ENERGY 



The mode in which the living body avails itself of energy 

 contained in its own substance depends upon the following facts. 

 When atoms unite to form molecules, energy is generally set 

 free, and the stabler the molecules formed, the greater, almost 

 invariably, is the amount of energy liberated at their formation. 

 The same amount of energy must be used to break up a molecule 

 as was set free when it was formed. The molecules that compose 

 the substances from which the body obtains its energy contain 

 carbon, hydrogen, oxygen, and sometimes nitrogen and other 

 elements, and are complex and relatively unstable and rich in 

 energy. The body breaks down these molecules so as to form 

 smaller and more stable molecules. The energy which is freed 

 in the formation of the stabler molecules is so much greater than 

 that which is required to break down those that are less stable 

 that a large balance of energy is set free, and becomes available 

 for the work of life. Usually the breaking-down process is con- 

 tinued until the carbon and hydrogen atoms are in the very 

 small and stable molecules of carbon dioxide and water — that 

 is to say, it is a complete oxidation. But it is not always so. The 

 substances which are broken down never contain enough oxygen 

 to combine with all the carbon and hydrogen in their molecules, 

 and therefore many animals and plants which live in surroundings 

 from which they cannot obtain additional oxygen are unable 

 to complete the process of disintegration. Thus the fungus known 

 as yeast, living in solutions which contain no dissolved oxygen, 



