CHAPTER 13 • HOW DO ORGANISMS RESIST INJURY? 



1 How can a sick person get well ? 



2 What makes an organism sick? 



3 Are all kinds of diseases caused by bacteria ? 



4 Are blood sicknesses inherited? 



5 Does vaccinating prevent all diseases? 



6 What is antitoxin? 



7 Are there antitoxins for all diseases ? 



8 Can any medicine be suitable for all kinds of sickness ? 



9 How does one become immune to certain diseases? 



10 What is the difference between a serum and a vaccine? 



Living things are always exposed to mechanical injuries. A fish snaps at 

 another fish and gets away with only part of the prey, A wind blows a 

 bough off a tree or tears off a piece of bark. A bird catches a lizard by the 

 leg, and the lizard slinks off on his remaining three. A parasite gets inside 

 an animal and destroys part of the tissues, or it excretes substances that are 

 poisonous to the host. Such dangers are parts of the risks of living. 



If an injury is too extensive, or if too much of an organism is removed 

 or destroyed, death is likely to result. But how much is too much? What 

 happens when the injuries are chemical, or result from poison? How does 

 a sick organism recover? How much punishment can an organism take 

 and still continue to live? 



How Much Damage Can an Organism Endure? 



Healing and Regeneration^ Plants and animals of nearly all classes 

 repair mechanical injury by growing new tissue that closes the wound. In 

 organisms like ourselves the gap in the tissues at first fills with fluid from 

 the surrounding cells and spaces — lymph. Then the surrounding cells 

 multiply rapidly, forming new cells. (This rapid formation of new cells 

 is called proliferation.) Such healing is almost universal. But it is not 

 equally effective among all species, nor among all the tissues of a given 

 plant or animal. 



At one extreme, planarians will regenerate, or regrow, complete indi- 

 viduals from rather small fractions of worms (see illustration opposite). The 

 earthworm can regrow the missing part if its hind end is cut off. Oystermen 

 who formerly tried to slaughter the destructive starfish by chopping them 

 with hoes and shovels discovered that the enemy could regenerate the parts 



iSeeNo. 1, p. 246. 

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