qualities the sense of smell is capable of revealing. The only definite
classification of smell qualities is that based on their pleasantness or the
opposite. We also borrow a few terms and speak of sweet or fragrant odors and
fresh or close smells. There is some evidence when we observe animals, or even
primitive men, that the human race has been evolving greater sensibility to
certain odors, while at the same time there has been a loss of keenness of what
we call scent.
Various Sensations from the Skin.—The skin, besides being a protective and
excretory organ, affords a lodging-place for the end-organs giving us our sense
of pressure, pain, cold, warmth, tickle, and itch. Pressure seems to have for its
end-organ the hair-bulbs of the skin; on hairless regions small bulbs called the
corpuscles of Meissner serve this purpose. Pain is thought to be mediated by free
nerve endings. Cold depends on end-organs called the bulbs of Krause; and
warmth on the Ruffinian corpuscles.
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.