Sight and Hearing.—The sense organs of sight and hearing are highly complicated structures, and will be considered in the chapters following.
Summary.—Sensations are certain activities of the mind that result from excitations within the body or at its[pg 348] surface. These cause the neurons to discharge impulses which on reaching the cerebrum cause the sensations. 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.—1. 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?