282 Why Living Things Behave As They Do unit v 



with the auditory (hearing) nerve. what you learn may prove to be useful. 

 Sound waves pass through the outer ear The inner ear which picks up sound 



and canal and hit the eardrum at the stimuli is important to us in still another 



entrance to the middle ear. The vibra- way. Part of the inner ear opens into a 



tions are passed along the very small bony structure consisting of three arches 



bones known as the havmier, anvil, and which lie almost at right angles to one 



stirnip which stretch across the middle another. They, too, are filled with liquid 



ear. Then the vibrations reach the tiny and lined with a membrane containing 



membrane at the entrance to the snail sensitive cells attached to nerve endings, 



shell. These vibrations set up waves in As the head moves and turns, first some, 



the liquid which fills the spiral passage then others of these cells are stimulated 



(cochlea). The waves wash over sensi- by the liquid. Thus man's brain is kept 



tive receptors, stimulating them. Thus informed of the position of these arches 



sound stimuli are received in the inner {semicircular cajials), making it possible 



ear and sent by the auditory nerve to the for a human being to know the position 



cortex of the cerebrum. Do Exercise 12; of his body and keep his balance. 



Questions 



1. Name two properties of protoplasm which enable an animal to per- 

 form a reflex act. 



2. The two parts of the nervous system work together closelv. What 

 are they? What does each do? 



3. Name the nerve centers. How are the nerves related to the nerve 

 centers? 



4. Name in order, starting on the outside, all the structures that protect 

 the brain. Why is the spinal column strong and flexible? 



5. Name the three main regions of the brain. What is the arrangement 

 of gray and white matter in the cerebrum? In the cerebellum? In the 

 medulla? In the cord? 



6. In what two ways have we learned about the activities of each region 

 of the brain? Name the chief activities of the cerebrum. 



7. What are the activities of the cerebellum and medulla? Define co- 

 ordination. 



8. When you examine the brain of fish, frog, reptile, bird, dog, and man 

 what difl^erences do you note in the cerebrum? 



9. What is the unit of structure of the nervous system? Name and de- 

 scribe the parts of a neuron. Which property of protoplasm is highly 

 developed in a neuron? 



10. Where do the cell bodies of most neurons lie? Where else are they 

 found? 



