SENSATIONS OF MOTION.
I treat of these in a separate chapter in order to give them the
emphasis which their importance deserves. They are of two orders:
1) Sensations of objects moving over our sensory surfaces; and
2) Sensations of our whole person's translation through space.
=1) The Sensation of Motion over Surfaces.=--This has generally been
assumed by physiologists to be impossible until the positions of
_terminus a quo_ and _terminus ad quem_ are severally cognized, and the
successive occupancies of these positions by the moving body are
perceived to be separated by a distinct interval of time. As a matter of
fact, however, we cognize only the very slowest motions in this way.
Seeing the hand of a clock at XII and afterwards at VI, I judge that it
has moved through the interval. Seeing the sun now in the east and again
in the west, I infer it to have passed over my head. But we can only
_infer_ that which we already generically know in some more direct
fashion, and it is experimentally certain that we have the feeling of
motion given us as a direct and simple _sensation_. Czermak long ago
pointed out the difference between _seeing the motion_ of the
second-hand of a watch, when we look directly at it, and noticing the
fact that it has _altered its position_, whilst our gaze is fixed upon
some other point of the dial-plate. In the first case we have a specific
quality of sensation which is absent in the second. If the reader will
find a portion of his skin--the arm, for example--where a pair of
compass-points an inch apart are felt as one impression, and if he will
then trace lines a tenth of an inch long on that spot with a
pencil-point, he will be distinctly aware of the point's motion and
vaguely aware of the direction of the motion. The perception of the
motion here is certainly not derived from a preëxisting knowledge that
its starting and ending points are separate positions in space, because
positions in space ten times wider apart fail to be discriminated as
such when excited by the compass-points. It is the same with the retina.
One's fingers when cast upon its peripheral portions cannot be
counted--that is to say, the five retinal tracts which they occupy are
not distinctly apprehended by the mind as five separate positions in
space--and yet the slightest _movement_ of the fingers is most vividly
perceived as movement and nothing else. It is thus certain that our
sense of movement, being so much more delicate than our sense of
position, cannot possibly be derived from it.
_Vierordt, at almost the same time, called attention to certain
persistent illusions, amongst which are these_: If another person gently
trace a line across our wrist or finger, the latter being stationary, it
will feel to us as if the member were moving in the opposite direction
to the tracing point. If, on the contrary, we move our limb across a
fixed point, it will seem as if the point were moving as well. If the
reader will touch his forehead with his forefinger kept motionless, and
then rotate the head so that the skin of the forehead passes beneath the
finger's tip, he will have an irresistible sensation of the latter being
itself in motion in the opposite direction to the head. So in abducting
the fingers from each other; some may move and the rest be still, but
the still ones will feel as if they were actively separating from the
rest. These illusions, according to Vierordt, are survivals of a
primitive form of perception, when motion was felt as such, but ascribed
to the whole 'content' of consciousness, and not yet distinguished as
belonging exclusively to one of its parts. When our perception is fully
developed we go beyond the mere relative motion of thing and ground, and
can ascribe absolute motion to one of these components of our total
object, and absolute rest to another. When, in vision for example, the
whole field of view seems to move together, we think it is ourselves or
our eyes which are moving; and any object in the foreground which may
seem to move relatively to the background is judged by us to be really
still. But primitively this discrimination is not perfectly made. The
sensation of the motion spreads over all that we see and infects it. Any
relative motion of object and retina both makes the object seem to move,
and makes us feel ourselves in motion. Even now when our whole field of
view really does move we get giddy, and feel as if we too were moving;
and we still see an apparent motion of the entire field of view whenever
we suddenly jerk our head and eyes or shake them quickly to and fro.
Pushing our eyeballs gives the same illusion. We _know_ in all these
cases what really happens, but the conditions are unusual, so our
primitive sensation persists unchecked. So it does when clouds float by
the moon. We _know_ the moon is still; but we _see_ it move faster than
the clouds. Even when we slowly move our eyes the primitive sensation
persists under the victorious conception. If we notice closely the
experience, we find that any object towards which we look appears moving
to meet our eye.
But the most valuable contribution to the subject is the paper of G. H.
Schneider,[27] who takes up the matter zoölogically, and shows by
examples from every branch of the animal kingdom that movement is the
quality by which animals most easily attract each other's attention. The
instinct of 'shamming death' is no shamming of death at all, but rather
a paralysis through fear, which saves the insect, crustacean, or other
creature from being _noticed at all_ by his enemy. It is paralleled in
the human race by the breath-holding stillness of the boy playing 'I
spy,' to whom the seeker is near; and its obverse side is shown in our
involuntary waving of arms, jumping up and down, and so forth, when we
wish to attract someone's attention at a distance. Creatures 'stalking'
their prey and creatures hiding from their pursuers alike show how
immobility diminishes conspicuity. In the woods, if we are quiet, the
squirrels and birds will actually touch us. Flies will light on stuffed
birds and stationary frogs. On the other hand, the tremendous shock of
feeling the thing we are sitting on begin to move, the exaggerated start
it gives us to have an insect unexpectedly pass over our skin, or a cat
noiselessly come and snuffle about our hand, the excessive reflex
effects of tickling, etc., show how exciting the sensation of motion is
_per se_. A kitten cannot help pursuing a moving ball. Impressions too
faint to be cognized at all are immediately felt if they move. A fly
sitting is unnoticed,--we feel it the moment it crawls. A shadow may be
too faint to be perceived. If we hold a finger between our closed eyelid
and the sunshine we do not notice its presence. The moment we move it to
and fro, however, we discern it. Such visual perception as this
reproduces the conditions of sight among the radiates.
In ourselves, the main function of the peripheral parts of the retina is
that of sentinels, which, when beams of light move over them, cry 'Who
goes there?' and call the fovea to the spot. Most parts of the skin do
but perform the same office for the finger-tips. Of course _movement of
surface under object is (for purposes of stimulation) equivalent to
movement of object over surface_. In exploring the shapes and sizes of
things by either eye or skin the movements of these organs are incessant
and unrestrainable. Every such movement draws the points and lines of
the object across the surface, imprints them a hundred times more
sharply, and drives them home to the attention. The immense part thus
played by movements in our perceptive activity is held by many
psychologists to prove that the muscles are themselves the
space-perceiving organ. Not surface-sensibility, but 'the muscular
sense,' is for these writers the original and only revealer of objective
extension. But they have all failed to notice with what peculiar
intensity muscular movements call surface-sensibilities into play, and
how largely the mere discernment of impressions depends on the mobility
of the surfaces upon which they fall.
Our _articular surfaces are tactile organs_ which become intensely
painful when inflamed. Besides pressure, _the only stimulus they receive
is their motion upon each other_. To the sensation of this motion more
than anything else seems due the perception of the position which our
limbs may have assumed. Patients cutaneously and muscularly anæsthetic
in one leg can often prove that their articular sensibility remains, by
showing (by movements of their well leg) the positions in which the
surgeon may place their insensible one. Goldscheider in Berlin caused
fingers, arms, and legs to be passively rotated upon their various
joints in a mechanical apparatus which registered both the velocity of
movement impressed and the amount of angular rotation. The minimal felt
amounts of rotation were much less than a single angular degree in all
the joints except those of the fingers. Such displacements as these,
Goldscheider says, can hardly be detected by the eye. Anæsthesia of the
skin produced by induction-currents had no disturbing effect on the
perception, nor did the various degrees of pressure of the moving force
upon the skin affect it. It became, in fact, all the more distinct in
proportion as the concomitant pressure-feelings were eliminated by
artificial anæsthesia. When the joints themselves, however, were made
artificially anæsthetic, the perception of the movement grew obtuse and
the angular rotations had to be much increased before they were
perceptible. All these facts prove, according to Herr Goldscheider, that
_the joint-surfaces and these alone are the seat of the impressions by
which the movements of our members are immediately perceived_.
=2) Sensations of Movement through Space.=--These may be divided, into
feelings of rotation and feelings of translation. As was stated at the
end of the chapter on the ear, the labyrinth (semicircular canals,
utricle and saccule) seems to have nothing to do with hearing. It is
conclusively established to-day that the semicircular canals are the
organs of a sixth special sense, that namely of rotation. When
subjectively excited, this sensation is known as _dizziness_ or
_vertigo_, and rapidly engenders the farther feeling of nausea.
Irritative disease of the inner ear causes intense vertigo (Ménière's
disease). Traumatic irritation of the canals in birds and mammals makes
the animals tumble and throw themselves about in a way best explained by
supposing them to suffer from false sensations of falling, etc., which
they compensate by reflex muscular acts that throw them the other way.
Galvanic irritation of the membranous canals in pigeons cause just the
same compensatory movements of head and eye which actual rotations
impressed on the creatures produce. Deaf and dumb persons (amongst whom
many must have had their auditory nerves or labyrinths destroyed by the
same disease which took away their hearing) are in a very large
percentage of cases found quite insusceptible of being made dizzy by
rotation. Purkinje and Mach have shown that, whatever the organ of the
sense of rotation may be, it must have its seat in the head. The body is
excluded by Mach's elaborate experiments.
The semicircular canals, being, as it were, six little spirit-levels in
three rectangular planes, seem admirably adapted to be organs of a sense
of rotation. We need only suppose that when the head turns in the plane
of any one of them, the relative inertia of the endolymph momentarily
increases its pressure on the nerve-termini in the appropriate ampulla,
which pressure starts a current towards the central organ for feeling
vertigo. This organ seems to be the cerebellum, and the teleology of
the whole business would appear to be the maintenance of the upright
position. If a man stand with shut eyes and attend to his body, he will
find that he is hardly for a moment in equilibrium. Incipient fallings
towards every side in succession are incessantly repaired by muscular
contractions which restore the balance; and although impressions on the
tendons, ligaments, foot-soles, joints, etc., doubtless are among the
causes of the compensatory contractions, yet the strongest and most
special reflex arc would seem to be that which has the sensation of
incipient vertigo for its afferent member. This is experimentally proved
to be much more easily excited than the other sensations referred to.
When the cerebellum is disorganized the reflex response fails to occur
properly and loss of equilibrium is the result. Irritation of the
cerebellum produces vertigo, loss of balance, and nausea; and galvanic
currents through the head produce various forms of vertigo correlated
with their direction. It seems probable that direct excitement of the
cerebellar centre is responsible for these feelings. In addition to
these corporeal reflexes the sense of rotation causes compensatory
rollings of the eyeballs in the opposite direction, to which some of the
subjective phenomena of _optical vertigo_ are due. Steady rotation gives
no sensation; it is only starting or stopping, or, more generally
speaking, acceleration (positive or negative), which impresses the
end-organs in the ampullæ. The sensation always has a little duration,
however; and the feeling of reversed movement after whirling violently
may last for nearly a minute, slowly fading out.
The cause of the _sense of translation_ (movement forwards or backwards)
is more open to dispute. The seat of this sensation has been assigned to
the semicircular canals when compounding their currents to the brain;
and also to the utricle. The latest experimenter, M. Delage, considers
that it cannot possibly be in the head, and assigns it rather to the
entire body, so far as its parts (blood-vessels, viscera, etc.) are
movable against each other and suffer friction or pressure from their
relative inertia when a movement of translation begins. M. Delage's
exclusion of the labyrinth from this form of sensibility cannot,
however, yet be considered definitively established, so the matter may
rest with this mention.