348 COORDINATION AND SENSATION 



surface. These cause the neurons to discharge impulses 

 which on reaching the cerebrum cause the sjensations. 

 Sensations are necessary for intelligent and purposeful 

 action and for acquiring all kinds of knowledge. To 

 enable the stimuli to act to the best advantage in starting 

 the impulses, special devices, called sense organs, are 

 employed. These receive the terminations of the neurons, 

 and by their special structure enable the most delicate 

 stimuli to start impulses. The simpler forms of sense 

 organs are those of touch, temperature, taste, and smell. 



Exercises. I. Compare-sensations and reflex actions with reference 

 to their nature and cause. Give steps in the production of each. 



2. Give examples of sensation stimuli. State the purpose of sense 

 organs. 



3. How do general sensations differ from special sensations? 



4. Of what value is pain in the protection of the body? 



5. Show that sensations lead to the higher forms of mental activity, 

 such as emotion and imagination. 



6. Of what value to the body is the " localizing of the sensation " ? 



7. What kinds of sense organs are found in the skin? State the 

 purpose of each. 



8. Through what sense avenues is one made aware of solids, of 

 liquids, and of gases ? 



9. Of what special protective value is the sense of smell ? 



PRACTICAL WORK 



To demonstrate the Pacinian Corpuscles. Spread out the mesen- 

 tery from the intestine of a cat and hold it between the eye and the 

 light. Pacinian corpuscles will appear as small translucent bodies hav- 

 ing the general form of grains of wheat. Secure a portion of the mesen- 

 tery over a circular opening in a thin piece of cork and examine it with 

 a microscope of low power. Follow the course of the nerve fiber to the 

 nerve from which it branches. 



To show Relative Sensitiveness of Different Parts of the Skin. 

 Holding a bristle between the fingers, bring the end in contact with the 

 skin, noting the amount of pressure necessary to cause a sensation of 



