HOW IS THE HUMAN MACHINE PROTECTED? 359 



skeleton nonliving heavy steam 



protective tissues organs food 



oxygen nitrogen carbon hydrogen 



self protection build energy 



blood cells tear down oxidize 



repair wastes living dead 



The auto and the human body are alike because they both 



release (1) when they (2) fuels. They both have (3) 



coverings, both have a framework or (4) , both use (5) 



to release energy, both have to get rid of their (6) But 



they differ greatly because the human machine can (7) and 



(8) its structure out of (9) , and it is (10) directed. 



It is made of (11) material the units of which are called 



(12) , while the auto is made from many (13) materials. 



STORY TEST 



SALLIE TELLS ABOUT CELLS 

 Read carefully and critically. List all the errors and suggest corrections. 



My teacher has asked me to tell the class what cells are and 

 what they do. Your body and mine are made of cells. They 

 are tiny units of living stuff so small that single cells cannot be 

 seen except with a microscope. Like bricks, they are all exactly 

 alike in size and shape, the only difference being that big animals 

 and plants have big cells while small ones have little cells. As a 

 body grows larger, the cells grow in size, so this is another reason 

 why larger things have larger cells. Every cell has a nucleus and 

 grows by dividing, so that one cell may form two, these two, four, 

 and so on. This is all I know about cells. 



PROBLEM II. HOW IS THE HUMAN MACHINE 

 PROTECTED ? 



The Skin, Its Uses and Its Care. We all know that 

 the skin is a protective covering and that if we cut or 

 break it, we feel a sensation of pain. The figure on page 

 361 shows us that the skin is made up of cells, the outer 

 layer of which is largely dead, while the inner layer is very 

 much alive. It is this part that bleeds when we scratch 

 or cut ourselves, for it is really supplied with tiny blood 

 vessels. If you look at the surface of the skin on your 



