Cold Orgcms. — The sense organs for cold prob-
ably have the form of end bulbs, lying nearer the
surface of the skin than do the warmth organs. The
spots sensitive to cold average between 10 and 15
per square centimeter of skin surface.
The adequate stimulus for cold is anything that
reduces within certain limits the temperature of the
sense organ. This may be done by the contact of the
skin with objects whose temperature is lower than
that of the sense organ at the time, by skin evapora-
tion, by heat radiation from the skin, and by the con-
striction of neighboring blood vessels. Certain sub-
stances, such as menthol, stimulate the cold organ
either by direct action or possibly by causing blood-
vessel constriction. A cold spot, curiously, may be
stimulated by a pointed hot rod of a temperature
45 to 50 degrees C. (The temperature of the blood
is 37 degrees C). When such a result is obtained,
we have what is called paradoxical cold.
Pam Orgcms. — Certain free nerve endings that
are found distributed over most of the body surface



20 GENERAL PSYCHOLOGY

are probably sense organs for pain. Pain is also
elicited by the stimulation of parts of the body other
than the skin surface, and possibly results from the
intense stimulation of sense opgans not primarily
concerned with pain.
The adequate stimuli to the pain organs are me-
chanical, thermal, and chemical, and must be of
much greater intensity than those that are neces-
sary to arouse the organs of touch and temperature.
The prolongation of these intense stimuli is usually
injurious to the animal.
