194 HUMAN BIOLOGY 



an engine has a mechanism for the intake of materials to be 

 used in it. Gland cells discharge from their opposite extrem- 

 ities the products which they make (see Fig. 2). Engines 

 have attachments for the utiHzation of the power generated. 

 This polar organization of the cell, providing for intake and 

 output and other attributes which it is unnecessary to 

 mention here, is referred to as polarity. 



The motive power for the cell and the engine is derived 

 from the combustion, or burning, of material coming from 

 without. Both of them are transformers, for energy is not 

 "created" anywhere in the known universe, it is merely 

 changed from one form to another. Sunhght acting upon 

 the green coloring matter of plants causes in some mysterious 

 way the hberation of oxygen and the formation of compounds 

 high in carbon and hydrogen. These substances are the 

 fuel. They are present in abundance in food and wood, 

 coal and oil. Combustion is brought about by the addition 

 of oxygen from the air. The energy developed through this 

 process of oxidation is much more economically used in the 

 cell than in the engine, for in the latter a large part of it is 

 lost by heat radiation. An impulse passing along a nerve 

 fiber generates heat, but only to about 1/1,000,000 of a 

 degree. 



Waste is discharged from the cells into the surrounding 

 body fluids (see Chap, xi) and is finally eliminated through 

 the lungs, kidneys, digestive tract and skin. In the engine 

 it is carried away in smoke and disposed of as ashes. If 

 such products accumulate instead of being removed in. an 

 orderly way both machines become clogged and cease to 

 function. 



Electrical forces are harnessed in the cell and by the 

 engine. Without them life would be impossible. In cells 

 they are usually barely detectable, but in rare instances 

 these vital units are grouped together to form organs which 

 are highly charged and are capable of giving a dangerous 

 electrical shock, or when appropriately connected up, to 

 ring a bell vigorously. This can be done by the electrical 

 organs of some fishes. Though most cells are rhythmical 

 or periodic in their action they are, as we have intimated, 

 never electrically at rest. By rest is here meant complete 



