the hair end-organ, consisting of a
sensory axon coiled about the root of the hair; this, also, is a touch
receptor. Finally, there is the "free-branched nerve end", consisting
simply of the branching of a sensory axon, with no accessory apparatus
whatever; and this is the pain receptor. Perhaps the pain receptor
requires no accessory apparatus because it does not need to be
extremely sensitive.

Now since we find, in the skin, "spots" responsive to four quite
different stimuli, giving four quite different sensations, and
apparently provided with different types of end-organs, it has become
customary to speak of four skin senses in place of the traditional
"sense of touch". We {201} speak of the pain sense, the warmth sense,
the cold sense, and the pressure sense, which last is the sense of
touch proper.

The Sense of Taste

Analysis has been as successful in the sense of taste as in cutaneous
sensation. Ordinarily we speak of an unlimited number of tastes, every
article of food having its own characteristic taste. Now the interior
of the mouth possesses the four skin senses in addition to taste, and
many tastes are in part composed of touch, warmth, cold or pain. A
"biting taste" is a compound of pain with taste proper, and a "smooth
taste" is partly touch. The consistency of the food, soft, tough,
brittle, gummy, also contributes, by way of the muscle sense, to the
total "taste". But in addition to all these sensations from the mouth,
the flavor of the food consists largely of odor. Food in the mouth
stimulates the sense of smell along with that of taste, the odor of
the food reaching the olfactory organ by way of the throat and the
rear passage to the nose. If the nose is held tightly so as to prevent
all circulation of air through it, most of the "tastes" of foods
vanish; coffee and quinine then taste alike, the only _taste_ of each
being bitter, and apple juice cannot be distinguished from onion
juice.

But when the nose is excluded, and when cutaneous and muscular
sensations are deducted, there still remain a few genuine tastes.
These are sweet, sour, bitter and salty--and apparently no more. These
four are the elementary taste sensations, all others being compounds.
The papillae of the tongue, with their little "pits" already spoken
of, correspond to the "spots" of the skin, with this difference,
however, that the papillae do not each give a single sensation. Some
of them give only two, some only three o