like that of heat, is probably a blend of the three
elementary sensations of warmth, cold and pain.

[Illustration: Fig. 32.--Diagram of various sorts of sensory
end-organ found in the skin.

A is a hair end-organ; the sensory axons can be seen coiling around
the root of the hair; evidently a touch on the hair, outside, would
squeeze the coiled axon and stimulate it. The hair is a bit of
"accessory apparatus."

B is a touch corpuscle, consisting of a coiled axon-end surrounded
by a little cone of other tissue.

C is an end-bulb, presumably belonging to the temperature sense. It
has, again, a coiled axon-end surrounded by other tissue. The
"coils" are really much more finely branched than the diagram shows.

D is a free-branched nerve end, consisting simply of a branched
axon, with no accessory apparatus. It is the pain-sense organ.

E is a corpuscle of a type found in the subcutaneous tissue, as well
as in more interior parts of the body. It contains an axon-end
surrounded by a layered capsule.]

The stimulus that arouses the touch sensation is a bending of the
skin. That which arouses warmth or cold is of {200} course a
temperature stimulus, but, strange as it may seem, the exact nature of
the effective stimulus has not been agreed upon. Either it is a
warming or cooling of the skin, or it is the existence of a higher or
lower temperature in the skin than that to which the skin is at the
moment "adapted". This matter will become clearer when we later
discuss adaptation. The stimulus that arouses the pain sensation may
be mechanical (as a needle prick), or thermal (heat or cold), or
chemical (as the drop of acid), or electrical; but in any case it must
be strong enough to injure or nearly to injure the skin. In other
words, the pain sense organ is not highly sensitive, but requires a
fairly strong stimulus; and thus it is fitted to give warning of
stimuli that threaten injury.

Several kinds of sensory end-organ are found in the skin. There is the
"spherical end-bulb", into which a sensory axon penetrates; it is
believed to be the sense organ for cold. There is the rather similar
"cylindrical end-bulb" believed to be the sense organ for warmth.
There is the "touch corpuscle", found in the skin of the palms and
soles, and consisting, like the end-bulbs, of a mass of accessory
cells with a sensory axon ramifying inside it; this is an end-organ
for the sense of touch. There is