TOUCH, THE TEMPERATURE SENSE, THE MUSCULAR SENSE, AND PAIN.
=Nerve-endings in the Skin.=--"Many of the afferent skin-nerves end in
connection with hair-bulbs; the fine hairs over most of the cutaneous
surface, projecting from the skin, transmit any movement impressed on
them, with increased force, to the nerve-fibres at their fixed ends.
Fine branches of axis-cylinders have also been described as penetrating
between epidermic cells and ending there without terminal organs. In or
immediately beneath the skin several peculiar forms of nerve end-organs
have also been described; they are known as (1) _Touch-cells_; (2)
_Pacinian corpuscles_; (3) _Tactile corpuscles_; (4) _End-bulbs_."[23]
[Illustration:
FIG. 24.--End-bulbs from the conjunctiva of the human eye,
magnified.
]
These bodies all consist essentially of granules formed of connective
tissue, in which or round about which one or more sensory nerve-fibres
terminate. They probably magnify impressions just as a grain of sand
does in a shoe, or a crumb does in a finger of a glove.
=Touch, or the Pressure Sense.=--"Through the skin we get several kinds of
sensation; touch proper, heat and cold, and pain; and we can with more
or less accuracy localize them on the surface of the body. The interior
of the mouth possesses also three sensibilities. Through touch proper we
recognize pressure or traction exerted on the skin, and the force of the
pressure; the softness or hardness, roughness or smoothness, of the body
producing it; and the form of this when not too large to be felt all
over. When to learn the form of an object we move the hand over it,
muscular sensations are combined with proper tactile, and such a
combination of the two sensations is frequent; moreover, we rarely touch
anything without at the same time getting temperature sensations;
therefore pure tactile feelings are rare. From an evolution point of
view, touch is probably the first distinctly differentiated sensation,
and this primary position it still largely holds in our mental
life."[24]
Objects are most important to us when in direct contact. The chief
function of our eyes and ears is to enable us to prepare ourselves for
contact with approaching bodies, or to ward such contact off. They have
accordingly been characterized as organs of anticipatory touch.
"The delicacy of the tactile sense varies on different parts of the
skin; it is greatest on the forehead, temples, and back of the forearm,
where a weight of 2 milligr. pressing on an area of 9 sq. millim. can be
felt.
"In order that the sense of touch may be excited neighboring skin-areas
must be differently pressed. When the hand is immersed in a liquid, as
mercury, which fits into all its inequalities and presses with
practically the same weight on all neighboring immersed areas, the sense
of pressure is only felt at a line along the surface, where the immersed
and non-immersed parts of the skin meet.
=The Localizing Power of the Skin.=--"When the eyes are closed and a point
of the skin is touched we can with some accuracy indicate the region
stimulated; although tactile feelings are in general characters alike,
they differ in something besides intensity by which we can distinguish
them; some sub-sensation quality not rising definitely into prominence
in consciousness must be present, comparable to the upper partials
determining the timbre of a tone. The accuracy of the localizing power
varies widely in different skin regions and is measured by observing
the least distance which must separate two objects (as the blunted
points of a pair of compasses) in order that they may be felt as two.
The following table illustrates some of the differences observed:
Tongue-tip 1.1 mm. (.04 inch)
Palm side of last phalanx of finger 2.2 mm. (.08 inch)
Red part of lips 4.4 mm. (.16 inch)
Tip of nose 6.6 mm. (.24 inch)
Back of second phalanx of finger 11.0 mm. (.44 inch)
Heel 22.0 mm. (.88 inch)
Back of hand 30.8 mm. (1.23 inches)
Forearm 39.6 mm. (1.58 inches)
Sternum 44.0 mm. (1.76 inches)
Back of neck 52.8 mm. (2.11 inches)
Middle of back 66.0 mm. (2.64 inches)
The localizing power is a little more acute across the long axis of a
limb than in it; and is better when the pressure is only strong enough
to just cause a distinct tactile sensation than when it is more
powerful; it is also very readily and rapidly improvable by practice."
It seems to be naturally delicate in proportion as the skin which
possesses it covers a more movable part of the body.
[Illustration: FIG. 25.]
"It might be thought that this localizing power depended directly on
nerve-distribution; that each touch-nerve had connection with a special
brain-centre at one end (the excitation of which caused a sensation with
a characteristic local sign), and at the other end was distributed over
a certain skin-area, and that the larger this area the farther apart
might two points be and still give rise to only one sensation. If this
were so, however, the peripheral tactile areas (each being determined by
the anatomical distribution of a nerve-fibre) must have definite
unchangeable limits, which experiment shows that they do not possess.
Suppose the small areas in Fig. 25 to each represent a peripheral area
of nerve-distribution. If any two points in _c_ were touched we should
according to the theory get but a single sensation; but if, while the
compass-points remained the same distance apart, or were even
approximated, one were placed in _c_ and the other on a contiguous area,
two fibres would be stimulated and we ought to get two sensations; but
such is not the case; on the same skin-region the points must be always
the same distance apart, no matter how they be shifted, in order to give
rise to two just distinguishable sensations.
"It is probable that the nerve-areas are much smaller than the tactile;
and that several unstimulated must intervene between the excited, in
order to produce sensations which shall be distinct. If we suppose
twelve unexcited nerve-areas must intervene, then, in Fig. 25, _a_ and
_b_ will be just on the limits of a single tactile area; and no matter
how the points are moved, so long as eleven, or fewer, unexcited areas
come between, we would get a single tactile sensation; in this way we
can explain the fact that tactile areas have no fixed boundaries in the
skin, although the nerve-distribution in any part must be constant. We
also see why the back of a knife laid on the surface causes a continuous
linear sensation, although it touches many distinct nerve-areas. If we
could discriminate the excitations of each of these from that of its
immediate neighbors we should get the sensation of a series of points
touching us, one for each nerve-region excited; but in the absence of
intervening unexcited nerve-areas the sensations are fused together.
=The Temperature-sense. Its Terminal Organs.=--"By this we mean our
faculty of perceiving cold and warmth; and, with the help of these
sensations, of perceiving temperature differences in external objects.
Its organ is the whole skin, the mucous membrane of mouth and fauces,
pharynx and gullet, and the entry of the nares. Direct heating or
cooling of a sensory nerve may stimulate it and cause pain, but not a
true temperature-sensation; hence we assume the presence of temperature
end-organs. [These have not yet been ascertained anatomically.
Physiologically, however, the demonstration of special spots in the skin
for feeling heat and cold is one of the most interesting discoveries of
recent years. If one draw a pencil-point over the palm or cheek one will
notice certain spots of sudden coolness. These are the cold-spots; the
heat-spots are less easy to single out. Goldscheider, Blix, and
Donaldson have made minute exploration of determinate tracts of skin and
found the heat-and cold-spots thick-set and permanently distinct.
Between them no temperature-sensation is excited by contact with a
pointed cold or hot object. Mechanical and faradic irritation also
excites in these points their specific feelings respectively.]
[Illustration:
FIG. 26.--The figure marked C P shows the cold-spots, that marked H
P the heat-spots, and the middle one the hairs on a certain patch
of skin on one of Goldscheider's fingers.
]
=The feeling of temperature is relative to the state of the skin.= "In a
comfortable room we feel at no part of the body either heat or cold,
although different parts of its surface are at different temperatures;
the fingers and nose being cooler than the trunk which is covered by
clothes, and this, in turn, cooler than the interior of the mouth. The
temperature which a given region of the temperature-organ has (as
measured by a thermometer) when it feels neither heat nor cold, is its
_temperature-sensation zero_, and is not associated with any one
objective temperature; for not only, as we have just seen, does it vary
in different parts of the organ, but also on the same part from time to
time. Whenever a skin-region has a temperature above its sensation-zero
we feel warmth; and _vice versa_: the sensation is more marked the
greater the difference, and the more suddenly it is produced; touching a
metallic body, which conducts heat rapidly to or from the skin, causes
a more marked hot or cold sensation than touching a worse conductor, as
a piece of wood, of the same temperature.
"The change of temperature in the organ may be brought about by changes
in the circulatory apparatus (more blood flowing through the skin warms
it and less leads to its cooling), or by temperature-changes in gases,
liquids, or solids in contact with it. Sometimes we fail to distinguish
clearly whether the cause is external or internal; a person coming in
from a windy walk often feels a room uncomfortably warm which is not
really so; the exercise has accelerated his circulation and tended to
warm his skin, but the moving outer air has rapidly conducted off the
extra heat; on entering the house the stationary air there does this
less quickly, the skin gets hot, and the cause is supposed to be
oppressive heat of the room. Hence, frequently, opening windows and
sitting in a draught, with its concomitant risks; whereas keeping quiet
for five or ten minutes, until the circulation has returned to its
normal rate, would attain the same end without danger.
"The acuteness of the temperature-sense is greatest at temperatures
within a few degrees of 30° C. (86° F.); at these differences of less
than 0.1° C. can be discriminated. As a means of measuring absolute
temperatures, however, the skin is very unreliable, on account
of the changeability of its sensation-zero. We can localize
temperature-sensations much as tactile, but not so accurately."[25]
=Muscular Sensation.=--The sensation in the muscle itself cannot well be
distinguished from that in the tendon or in its insertion. In muscular
fatigue the insertions are the places most painfully felt. In muscular
rheumatism, however, the whole muscle grows painful; and violent
contraction such as that caused by the faradic current, or known as
cramp, produces a severe and peculiar pain felt in the whole mass of
muscle affected. Sachs also thought that he had demonstrated, both
experimentally and anatomically, the existence of special sensory
nerve-fibres, distinct from the motor fibres, in the frog's muscle. The
latter end in the 'terminal plates,' the former in a network.
Great importance has been attached to the muscular sense as a factor in
our perceptions, not only of weight and pressure, but of the
space-relations between things generally. Our eyes and our hands, in
their explorations of space, move over it and through it. It is usually
supposed that without this sense of an intervening motion performed we
should not perceive two seen points or two touched points to be
separated by an extended interval. I am far from denying the immense
participation of experiences of motion in the construction of our
space-perceptions. But it is still an open question _how_ our muscles
help us in these experiences, whether by their own sensations, or by
awakening sensations of motion on our skin, retina, and articular
surfaces. The latter seems to me the more probable view, and the reader
may be of the same opinion after reading Chapter VI.
=Sensibility to Weight.=--When we wish to estimate accurately the weight
of an object we always, when possible, lift it, and so combine muscular
and articular with tactile sensations. By this means we can form much
better judgments.
Weber found that whereas ⅓ must be added to a weight resting on the hand
for the increase to be felt, the same hand actively 'hefting' the weight
could feel an addition of as little as 1/17. Merkel's recent and very
careful experiments, in which the finger pressed down the beam of a
balance counterweighted by from 25 to 8020 grams, showed that between
200 and 2000 grams a constant fractional increase of about 1/13 was felt
when there was no movement of the finger, and of about 1/19 when there
was movement. Above and below these limits the discriminative power grew
less.
[Illustration: FIG. 27 (after Wundt).]
=Pain.=--The physiology of pain is still an enigma. One might suppose
separate afferent fibres with their own end-organs to carry painful
impressions to a specific pain-centre. Or one might suppose such a
specific centre to be reached by currents of overflow from the other
sensory centres when the violence of their inner excitement should have
reached a certain pitch. Or again one might suppose a certain extreme
degree of inner excitement to produce the feeling of pain in all the
centres. It is certain that sensations of every order, which in moderate
degrees are rather pleasant than otherwise, become painful when their
intensity grows strong. The rate at which the agreeableness and
disagreeableness vary with the intensity of a sensation is roughly
represented by the dotted curve in Fig. 27. The horizontal line
represents the threshold both of sensational and of agreeable
sensibility. Below the line is the disagreeble. The continuous curve is
that of Weber's law which we learned to know in Fig. 2, p. 18. With the
minimal sensation the agreeableness is _nil_, as the dotted curve shows.
It rises at first more slowly than the sensational intensity, then
faster; and reaches its maximum before the sensation is near its acme.
After its maximum of agreeableness the dotted line rapidly sinks, and
soon tumbles below the horizontal into the realm of the disagreeable or
painful in which it declines. That all sensations are painful when too
strong is a piece of familiar knowledge. Light, sound, odors, the taste
of sweet even, cold, heat, and all the skin-sensations, must be moderate
to be enjoyed.
The quality of the sensation complicates the question, however, for in
some sensations, as bitter, sour, salt, and certain smells, the turning
point of the dotted curve must be drawn very near indeed to the
beginning of the scale. In the skin the painful quality soon becomes so
intense as entirely to overpower the specific quality of the sort of
stimulus. Heat, cold, and pressure are indistinguishable when
extreme--we only feel the pain. The hypothesis of separate end-organs in
the skin receives some corroboration from recent experiments, for both
Blix and Goldscheider have found, along with their special heat-and cold
spots, also special 'pain-spots' on the skin. Mixed in with these are
spots which are quite feelingless. However it may stand with the
terminal pain-spots, separate paths of _conduction_ to the brain, for
painful and for merely tactile stimulations of the skin, are made
probable by certain facts. In the condition termed _analgesia_, a touch
is felt, but the most violent pinch, burn, or electric spark destructive
of the tissue will awaken no sensation. This may occur in disease of the
cord, by suggestion in hypnotism, or in certain stages of ether and
chloroform intoxication. "In rabbits a similar state of things was
produced by Schiff, by dividing the gray matter of the cord, leaving the
posterior white columns intact. If, on the contrary, the latter were
divided and the gray substance left, there was increased sensitiveness
to pain, and possibly touch proper was lost. Such experiments make it
pretty certain that when afferent impulses reach the spinal cord at any
level and there enter its gray matter with the posterior root-fibres,
they travel on in different tracts to conscious centres; the tactile
ones coming soon out of the gray network and coursing on in a readily
conducting white fibre, while the painful ones travel on farther in the
gray substance. It is still uncertain if both impulses reach the cord in
the same fibres. The gray network conducts nerve-impulses, but not
easily; they tend soon to be blocked in it. A feeble (tactile) impulse
reaching it by an afferent fibre might only spread a short way and then
pass out into a single good conducting fibre in a white column, and
proceed to the brain; while a stronger (painful) impulse would radiate
farther in the gray matter, and perhaps break out of it by many fibres
leading to the brain through the white columns, and so give rise to an
incoördinate and ill-localized sensation. That pains are badly
localized, and worse the more intense they are, is a well-known fact,
which would thus receive an explanation."[26]
Pain also gives rise to ill-coördinated movements of defence. The
stronger the pain the more violent the start. Doubtless in low animals
pain is almost the only stimulus; and we have preserved the peculiarity
in so far that to-day it is the stimulus of our most energetic, though
not of our most discriminating, reactions.
* * * * *
=Taste, smell, as well as hunger, thirst, nausea, and other so-called
'common' sensations= need not be touched on in this book, as almost
nothing of psychological interest is known concerning them.