CHAPTER XXII 



THE NERVOUS 

 SYSTEM 



Mimosa, a sensitive plant. 



A leaf that catches flies. 



How are plants and animals adjusted to their environments ? What is 

 the structure of the brain f What are the functions of the brain? Is 

 phrenology a science? What scientific studies haw been made of the 

 nervous system ? 



Irritability. Plants and animals must adjust themselves to their 

 environment in order to survive. Different conditions in the 

 environment provoke responses in organisms. Certain of the re- 

 sponses of the amoeba were considered in the discussion on the 

 functions of the amoeba. If the response is very definite and with- 

 out exception for a given stimulus, that response is known as a 

 tropism. Such responses are characteristic of plants and of ani- 

 mals without a nervous system. In animals with a nervous sys- 

 tem, if the response is definite, mechanical, and without excep- 

 tion, the reaction may still be called a tropism. For example, 

 the swarming of the bees, the fluttering of moths around a light, 

 and the burrowing of worms into the earth are frequently called 

 tropisms. When an animal has a well-developed nervous sys- 

 tem, the responses are more varied and individual. The nervous 

 system governs and regulates the responses to stimuli. In this 

 latter case the response is called a nervous reaction instead of a 

 tropism. The response of the organism, whether it is a nervous 

 reaction or a tropism, is due to the activity of the protoplasm in the 



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