Cutaneous or skin sensation may arise from either mechanical stimulation, such as pressure, a blow, or tickling, from thermal stimulation from hot or cold objects, from electrical stimulation, or from the action of certain chemicals, such as acids and the like. Stimulated mechanically, the skin gives us but two sensation qualities, pressure and pain. Many of the qualities which we commonly ascribe to the skin sensations are really a complex of cutaneous and muscular sensations. Contact is light pressure. Hardness and softness depend on the intensity of the pressure. Roughness and smoothness arise from interrupted and continuous pressure, respectively, and require movement over the rough or smooth surface. Touch depends on pressure accompanied by the muscular sensations involved in the movements connected with the act. Pain is clearly a different sensation from pressure; but any of the cutaneous or muscular sensations may, by excessive stimulation, be made to pass over into pain. All parts of the skin are sensitive to pressure and pain; but certain parts, like the finger tips, and the tip of the tongue, are more highly sensitive than others. The skin varies also in its sensitivity to heat and cold. If we take a hot or a very cold pencil point and pass it rather lightly and slowly over the skin, it is easy to discover certain spots from which a sensation of warmth or of cold flashes out. In this way it is possible to locate the end-organs of temperature very accurately.

[Fig. 17.]—Diagram showing distribution of hot and cold spots on the back of the hand. C, cold spots; H, hot spots.

The Kinæsthetic Senses.—The muscles, tendons, and joints also give rise to perfectly definite sensations, but they have not been named as have the sensations from most of the other end-organs. Weight is the most clearly marked of these sensations. It is through the sensations connected with movements of muscles, tendons, and joints that we come to judge form, size, and distance.

The Organic Senses.—Finally, to the sensations mentioned so far must be added those which come from the internal organs of the body. From the alimentary canal we get the sensations of hunger, thirst, and nausea; from the heart, lungs, and organs of sex come numerous well-defined but unnamed sensations which play an important part in making up the feeling-tone of our daily lives.

Thus we see that the senses may be looked upon as the sentries of the body, standing at the outposts where nature and ourselves meet. They discover the qualities of the various objects with which we come in contact and hand them over to the mind in the form of sensations. And these sensations are the raw material out of which we begin to construct our material environment. Only as we are equipped with good organs of sense, especially good eyes and ears, therefore, are we able to enter fully into the wonderful world about us and receive the stimuli necessary to our thought and action.

4. PROBLEMS IN OBSERVATION AND INTROSPECTION

1. Observe a schoolroom of children at work with the aim of discovering any that show defects of vision or hearing. What are the symptoms? What is the effect of inability to hear or see well upon interest and attention?

2. Talk with your teacher about testing the eyes and ears of the children of some school. The simpler tests for vision and hearing are easily applied, and the expense for material almost nothing. What tests should be used? Does your school have the test card for vision?

3. Use a rotator or color tops for mixing discs of white and black to produce different shades of gray. Fix in mind the gray made of half white and half black; three-fourths white and one-fourth black; one-fourth-white and three-fourths black.