CHAPTER IV 



THE MASTER TISSUES 



72. A complete animal (or plant) is called an organism. 

 The cells . constituting it and working harmoniously to- 

 gether were likened to a human community or town. 

 Such a comparison helps us to realize how complex and 

 mysterious is an animal body, and how wonderful are the 

 processes of life. But the relation between the cells is 

 much closer than that between the individuals of a com- 

 munity. A marked difference is that the human being can 

 move as a whole, can change place in space, can act with 

 all the organs in the organism working in unison. The 

 energy stored in the body enables it to do these things, 

 and the two tissues that chiefly expend the energy and 

 give us the ability to do things, to act, are the nerve tissue 

 and the muscular tissue. How is energy, or the ability to 

 do work, stored in the body ? 



73. Oxidation. We know that this energy arises from a 

 kind of combustion or slow burning, thus resembling in 

 its source the energy of the steam engine. That some- 

 thing besides wood or coal is necessary to a fire can be 

 shown by shutting off entirely the draught of the stove. 

 Fire and all other forms of combustion depend upon a 

 process called oxidation. This consists in the uniting of 

 oxygen, the active element of the air, with carbon, hydrogen, 

 and other elements in wood, coal, etc. Bread, meat, and 

 other foods contain these latter elements. That carbon is 

 in sugar, for instance, can be easily proved by charring 

 sugar upon the stove, the charcoal thus produced being a 



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