*Summary.*—Sensations are certain activities of the mind that result from excitations within the body or at its 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? 9. Of what special protective value is the sense of smell? PRACTICAL WORK *To demonstrate the Pacinian Corpuscles.*—Spread out the mesentery from the intestine of a cat and hold it between the eye and the light: Pacinian corpuscles will appear as small translucent bodies having the general form of grains of wheat. Secure a portion of the mesentery 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 touch. Test the lips, tongue, tips of fingers, and palm and back of hand, trying different sizes of bristles. Has the degree of sensitiveness any relation to the thickness of the cuticle? *To show Perceptive Differences of Different Portions of the Skin.*—Place the points of a pair of dividers on the back of the hand of one who looks in the opposite direction. Is one point felt or two? Repeat several times, changing the distance between the points until it is fully determined how near the two points must be placed in order to be felt as one. In like manner test other parts of the body, as the tips of the fingers and the back of the neck. Compare results obtained at different places.