of tactual sensibility, so that a very slight stimulus, such
as the contact of the bed-clothes, becomes greatly exaggerated.

In diseased states of the nervous system these variations of sensibility
become much more striking. The patient who has hyperaesthesia fears to
touch a perfectly smooth surface, or he takes a knock at the door to be
a clap of thunder. The hypochondriac may, through an increase of
organic sensibility, translate organic sensations as the effect of some
living creature gnawing at his vitals. Again, states of anaesthesia lead
to odd illusions among the insane. The common supposition that the body
is dead, or made of wood or of glass, is clearly referable in part to
lowered sensibility of the organism.[32]

It is worth adding, perhaps, that these variations in sensibility give
rise not only to sensory but also to motor illusions. To take a homely
instance, the last miles of a long walk seem much longer than the first,
not only because the sense of fatigue leading us to dwell on the
transition of time tends to magnify the apparent duration, but because
the fatigued muscles and connected nerves yield a new set of sensations
which constitute an exaggerated standard of measurement. A number of
optical illusions illustrate the same thing. Our visual sense of
direction is determined in part by the feelings accompanying the action
of the ocular muscles, and so is closely connected with the perception
of movement, which has already been touched on. If an ocular muscle is
partially paralyzed it takes a much greater "effort" to effect a given
extent of movement than when the muscle is sound. Hence any movement
performed by the eye seems exaggerated. Hence, too, in this condition
objects are seen in a wrong direction; for the patient reasons that they
are where they would seem to be if he had executed a wider movement than
he really has. This may easily be proved by asking him to try to seize
the object with, his hand. The effect is exaggerated when complete
paralysis sets in, and no actual movement occurs in obedience to the
impulse from within.[33]

Variations in the condition of the nerve affect not only the degree, but
also the quality of the sensation, and this fact gives rise to a new
kind of illusion. The curious phenomena of colour-contrast illustrate
momentary alterations of sensibility. When, after looking at a green
colour for a time, I turn my eye to a grey surface and see this of the
complementar