the various
terminal organs. Thus, when the skin is pinched, the sensation is referred
to the skin, although the perception is in the brain. We may think it is
the eyes that see objects; in reality, it is only the brain that takes
note of them.

This is largely the result of education and habit. From a blow
on the head one sees flashes of light as vividly as if torches actually
dance before the eyes. Impressions have reached the seeing-center in the
brain from irritation of the optic nerve, producing the same effect as
real lights would cause. In this case, however, knowing the cause of the
colors, the person is able to correct the erroneous conclusion.

As a result of a depraved condition of blood, the seeing-center itself may
be unduly stimulated, and a person may see objects which appear real. Thus
in an attack of delirium tremens, the victim of alcoholic poisoning sees
horrible and fantastic creatures. The diseased brain refers them as usual
to the external world; hence they appear real. As the sufferer's judgment
is warped by the alcoholic liquor, he cannot correct the impressions, and
is therefore deceived by them.

313. Organs of Special Sense. The organs of special sense, the means
by which we are brought into relation with surrounding objects, are
usually classed as five in number. They are sometimes fancifully called
"the five gateways of knowledge"--the skin, the organ of touch; the
tongue, of taste; the nose, of smell; the eye, of sight;
and the ear, of hearing.

[Illustration: Fig. 124.--Magnified View of a Papilla of the Skin, with a
Touch Corpuscle.]

314. The Organ of Touch. The organ of touch, or tactile sensibility,
is the most widely extended of all the special senses, and perhaps the
simplest. It is certainly the most precise and certain in its results. It
is this sense to which we instinctively appeal to escape from the
illusions into which the other senses may mislead us. It has its seat in
the skin all over the body, and in the mucous membrane of the nostrils.
All parts of the body, however, do not have this sense in an equal degree.

In Chapter IX. we learned that the superficial layers of the skin covers
and dips in between the papillae. We also learned that these papillae are
richly provided with blood-vessels and sensory nerve fibers (sec. 234).
Now these nerve fibers terminate in a peculiar way in those parts of the
body which are endowed with a very delicate sense of touch. In every