CHAPTER 10 • HOW DOES FOOD REACH 



THE DIFFERENT PARTS OF THE BODY? 



1 Is the sap of plants the same as the blood of animals ? 



2 Do all animals have blood ? 



3 How does the blood help to keep us alive? 



4 Of what is blood composed ? 



5 How does exercise speed up the heart? 



6 Do all animals have organs corresponding to hearts? 



7 How does blood clot ? 



8 How does the blood keep the body warm? 



9 What can the doctor tell from feeling the pulse ? or from listen- 



ing to the heart ? 



10 How can the blood of one person be made to work in the body 



of another? 



11 Can the blood of one animal be transfused into the body of 



another ? 



12 Why must blood be "typed" before a transfusion is made? 



In all except the very smallest plants and animals there is some way of 

 distributing materials among the different parts of the body. In the com- 

 mon plants one set of tubes carries water and dissolved salts from the roots, 

 by way of the stems, to the leaves; and another set of vessels carries organic 

 food from the leaves to other parts of the plant. The two currents are inde- 

 pendent of each other. They consist of different materials and are not con- 

 nected at any point. 



The red fluid that spurts out when the flesh is cut has always impressed 

 mankind as both important and mysterious. People have explained almost 

 everything they could observe or imagine about life by pointing to the blood. 

 It is truly a marvelous juice! The very color has itself been exciting and has 

 been widely used as a symbol. On flags and emblems it has represented the 

 blood that men have shed to ensure their rights and freedoms. It has also 

 represented the blood brotherhood of all humanity. 



Some of the ancient Greeks held the notion that the blood moves. That 

 the heart actually pumps blood and keeps it in circulation was first worked 

 out by the English physician William Harvey (1578-1657). Harvey's argu- 

 ment, from the facts then known, was perfect. There was in it, however, 

 one missing link: how does the blood get from the arteries to the veins? 

 Harvey could not tell. He was certain only that somehow it must. Nobody 

 then could know either the structure of the blood or the existence of 

 capillaries, for the microscope revealed its secrets only after Harvey died. 



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