lly 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.

[Illustration: FIG. 17.--Diagram showing distribution of hot and cold
spots on the back of the hand. C, cold spots; H, hot spots.]

THE KINAESTHETIC 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, therefor