CHAPTER 15 • WHAT DO THE NERVES DO? 



1 Are there any animals that have no nerves? 



2 How can some animals get along without special sense organs ? 



3 Do animals feel pain as we do? 



4 Are there any activities in the body that we cannot control? 



5 What is the use of pain? 



6 What is the "funny bone"? 



7 Would there be any harm in killing the nerves in the teeth ? 



8 Are people with larger heads smarter than those with smaller 



heads ? 



9 Is it true that if one of the senses is injured, the others become 



more keen to make up for it? 

 10 How can we tell whether other animals perceive the work! 

 through the senses just as we do? 



Among the lowest organisms, different stimuli may produce similar 

 effects. Thus an ameba contracts when touched, when suddenly illumi- 

 nated, when stimulated by some chemical substance or by a charge of elec- 

 tricity. In our own bodies the division of labor is so great that there are 

 several highly specialized organs — the eye, the ear, the tongue, and so on. 

 Each of these is sensitive to only a limited class of stimuli. Moreover, the 

 various organs respond in special ways. Sometimes there is a rather violent 

 reaction through sudden movement. Or a stimulus may bring about a 

 chemical change, as in the formation of an antitoxin or ih the increased 

 secretion from a gland. 



The most striking feature in the structure of higher animals is perhaps 

 the presence of the nerves. These specialize in receiving disturbances and 

 in transmitting them. How do the nerve cells really differ from other kinds 

 of cells? How do they influence the action of other kinds of cells? How 

 do they make the parts of the body work together? 



What Kinds of Cells Are Nerves? 



Special Functions and Special Cells In the ameba and other one-celled 

 species each cell carries on all the life functions — feeding and assimilating, 

 breathing and oxidation, moving, excreting, sensing, reproducing (see il- 

 lustration, p. 23). But the cells in larger plants and animals impress us 

 not with their similarities but rather with their differences — as between the 

 bone cell and the gland cell or between the skin cell and the muscle cell. 



In each special tissue we find an exaggeration of some special function 

 that is common to all protoplasm. Thus chemical processes of various kinds 



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