SKIN-SENSATIONS
The senses, according to a time-honoured classification, are five
in number—smell, sight, taste, hearing, and common sensation, or
touch; but such a classification of our sensations and of the organs
which originate them is too crude for modern needs. Already we have
shown that, whereas the nose and the tongue afford the same kind of
information, the ear affords information of two, perhaps of three,
different kinds. Within the realm of common sensation we pick out three
special senses served by specialized sense-organs—touch, cold and
heat—and, possibly, a fourth, served by non-specialized nerves, to
which alone the epithet “common” properly applies.
The skin is supplied with nerves—naked fibrils—in the richest
abundance. They are most easily demonstrated in the layer which covers
the cornea, thanks to its transparency; in this, as shown in Fig. 41,
having branched on the front of the fibrous tissue of which the cornea
is composed, the nerves pass towards the surface, forming connections
with every one of its cells, or, at any rate, with every cell of the
more superficial of the three or four layers of which the epithelium
is made up. Ramified nerve-twigs of this type do not, under ordinary
conditions, convey any sensations to consciousness. So long as the
skin-cells with which they are connected are healthy, the nerve-twigs
establish for them connections with the central nervous system by which
their nutrition is regulated; but they carry no impulses to which we
can direct attention. The movement of blinking is accompanied by no
sensation until the edges of the eyelids come in contact. A pencil
pressed against the lid evokes touch-sensations from the skin, but
none from the cornea which underlies it. When a tiny beetle injures
the surface of the cornea by scratching the epithelial cells with
its horny wings and legs, the ruptured nerve-filaments convey to
consciousness impulses, or, as we prefer to express it, an influence
which is felt as pain. But even the pain caused by injury to the cornea
is trifling as compared with that which originates in the under-sides
of the lids, where not only is the epithelium supplied with branching
nerve-twigs, but specialized organs of touch are present to localize
the seat of injury. Everywhere the epithelium covering the surface
of the body is so abundantly supplied that a successful staining of
nerve-filaments induces one to think that every epithelial cell has
its nervous affiliation. These are the nerves of common sensation, if
we retain the term; but sensation so common, so obscure, so little
differentiated that we know no more about it than we know about the air
which envelops our hands and faces on a warm, windless day. Yet the
air, when it moves, gives rise to a dim, broad, generalized sensation,
which may be focussed into definiteness by a sensitive nerve.
[Illustration: FIG. 41.—VERTICAL SECTION OF THE EPITHELIUM WHICH
COVERS THE SURFACE OF THE CORNEA, AND OF A SMALL PORTION OF THE CORNEAL
SUBSTANCE, HIGHLY MAGNIFIED.
The black lines are naked nerve-fibres (stained with
chloride of gold), which are distributed amongst
the cells of the more superficial strata of the
epithelium in very great abundance. The corneal
substance is composed of sheets of transparent
fibres with intervening cells. As the fibres of the
several sheets cross one another at various angles,
they are cut, some transversely, others in the
direction of their length.]
An observer who has devoted himself for many years to the investigation
of skin-sensations, and especially of the “referred pains” which are
due to diseases of the viscera, recently caused the large cutaneous
nerve which supplies the thumb side of the forearm and hand to be cut
in his own arm, in order that he might study carefully the revival
of sensations. He found that he never lost his ability to recognize
displacements of the tissues beneath the skin. Pacinian bodies and
other end-organs of deep-lying nerves recorded pressure and tension
caused by pushing or rubbing with a blunt instrument. Seven weeks after
the injury he began to recognize stimuli that do harm—hot things,
cold things, pricking with a pin—although his power of localizing
the spot injured was extremely vague. In seven weeks, that is to say,
the protopathic nerves, which do not follow the same definite lines
as the nerves of the special senses, but form open networks with many
alternative paths, had re-established their skin connections. Only
gradually and very slowly did critical sensations return—the ability
to distinguish degrees of warmth, to recognize as separate two points
of a pair of compasses, to feel a touch with cotton-wool.
According to a theory set forth in this book (p. 312), pain is not a
set of sensations, but a condition of the central nervous system which
renders it unduly excitable, or excitable in a particular manner, to
impulses which have the same local origin as the nerve-current which
sets up the condition of pain. When a nerve of the skin has been cut,
the epithelial ramifications are renewed before any specialized tactile
or other sense-organs have regained their nervous connections. When
the area which has regained its surface ramifications, but has not
regained its sense-organs, is injured, no localization of pain results.
Indeed, the obscure sensations which are then experienced if the skin
be injured can hardly be described as painful. The ramified nerves
pour their agitation into the grey matter of the spinal cord; but it
is not the agitation _per se_ which causes pain. It is the passage of
impulses through the agitated area that gives to them, when they reach
consciousness, not only a topographical meaning, but also a distressful
feeling. Until the specialized organs of the skin have been restored to
working order, there are no impulses to pass through the agitated grey
matter, and therefore no feelings of pain. According to this view there
are two systems of afferent nerves, the protopathic and the specialized
or critical. The former is very widely and very abundantly distributed
to the surface of the body, the lungs, the alimentary canal, and other
viscera. It has no end-organs, no defined tracts in the central nervous
system, no definite connections with the cortex of the great brain.
The currents which it conducts, if they originate in the visceral part
of this system, have no direct effect in consciousness; but if they
originate on the surface of the body, or in the alimentary canal at
the lower end of the œsophagus, or in certain other situations, they
co-operate with stimuli of heat, cold, or traction. The critical system
works in a more definite way. Its impulses originate in sense-organs.
Starting with a certain potential, they are transmitted by the
discharge of a succession of linked neurones. When they reach the
cortex their potential is sufficiently high to evoke consciousness.
Their distribution in the cortex is as definite as their origin.
Specialized sense-organs are necessary for the origin of all
sensations. Within the epithelium are certain cells which look as
if they were specialized for sensory purposes. The deeper sheet, or
derma, of the skin is abundantly provided with structures in which
nerves end in the most elaborate and complicated ways (Fig. 42). They
are found especially in the papillæ of connective tissue, which, set
in rows, form the ridges that one can see at the finger-tips and in
various other situations. All of these organs are made up of groups
of epithelial cells which, displaced from the epidermis, have sunk
into the derma, with the nerves connected with them. In their further
development the nervous part of the apparatus is complicated by
branching, the branches being thickened and usually flattened into
ribbons, which lie on the external surfaces of the cells or between
them. A more or less marked capsule is provided for the organ by
condensation of connective tissue.
Anyone can convince himself that the skin is not uniformly sensitive.
He may test it first for the minimal stimulus which excites a sensation
of touch. With a hair of the head—it must not be a very fine one—cut
across with scissors, and held between finger and thumb at the right
distance from the cut end, the skin of the palm of the hand is prodded.
Every here and there a spot is found which is insensitive to so slight
a pressure. These spots are neither large nor very close together.
If the hairless skin of the arm between the elbow and the armpit
be investigated in the same way, much larger blank areas are met
with—oval patches more than ¼ inch in diameter. When a hairy surface
is tested, it is found that contact with a hair can always be felt; and
when the hairs are shaved, the touch-spots are found to extend around
or from the points at which hairs pierce the epidermis. Touchless areas
lie between them. Hair-follicles receive tufts of nerve-filaments, and
it appears that they are the chief organs of touch. “Touch-corpuscles,”
which are found in great numbers in the papillæ of the skin of the
fingers and elsewhere, may probably be regarded as, genetically,
hair-follicles which have not developed hairs.
[Illustration: FIG. 42.—SENSE-ORGANS SUSCEPTIBLE TO PRESSURE.
All are formed on essentially the same plan; a
fibrous capsule invests a group of epithelial
cells amongst which a nerve ramifies. The simplest
form is known as a Grandry’s corpuscle-a nerve
ending in one or two plates between two or three
epithelial cells. These organs are found in great
numbers in the bills of aquatic birds. If a duck
is watched whilst it is gobbling mud at the margin
of a pond, it will be seen to have a remarkable
capacity for discriminating between the shells of
small snails, which it can crush, and stones, which
it needs to drop from its bill. Its bill is also
provided with small Pacinian corpuscles (Fig. 43).
Touch-corpuscles, more elaborate in form than the
one figured, are found in the papillæ of the skin
of the fingers and elsewhere. They appear to be
modified hair-follicles. End-bulbs occur in the
conjunctiva and elsewhere, and especially in the
peritoneum. Together with Pacinian corpuscles, they
are accountable for sensations connected with the
distension of the stomach and intestines.]
If sensitiveness to pain is investigated by tapping very gently with a
needle—or, better, by using a stiff horsehair fixed in a cleft stick,
from which it projects about ¼ inch—it will be found that every here
and there are spots which are exceedingly sensitive, whilst adjoining
them are areas which are moderately sensitive, and between these
areas small spots or stretches of skin which do not give the smarting
sensation even though the horsehair be pushed until it doubles up.
[Illustration: FIG. 43.—PACINIAN CORPUSCLE.
These organs are especially numerous in the
neighbourhood of tendons and ligaments. They
are also present beneath the skin of the hands
and feet. Their capsules are formed of a great
number of concentric lamellæ of connective tissue,
enclosing lymph-spaces. Within the capsule is a
core of finely granular substance, which also shows
a tendency to a lamellar disposition. The structure
of these relatively large sense-organs is highly
suggestive of sensitiveness to pressure, traction,
or rubbing.]
Testing now for sensitiveness to cold with a cold blunt metal point,
“cold-spots” can be mapped on the skin. If the metal is warmed to about
50° C., “heat-spots” are found. The different kinds of spot are very
irregularly distributed. They may coincide, or overlap, or leave blank
spaces. Their relative abundance varies. In some regions touch-spots,
in others cold-spots, in others heat-spots, are more closely grouped.
The tongue and the hand, and especially the tips of the fingers, are
most sensitive to touch; but whereas the tongue is also exceedingly
sensitive to warmth, the hands are relatively insensitive. Yet,
speaking generally, parts especially sensitive to touch are little
sensitive to temperature, and _vice versa_. Sensitiveness to cold is
much more widespread than sensitiveness to heat. It is concentrated in
the skin covering the abdominal viscera. A cold douche directed between
the shoulders is doubtfully felt as cold. There is no doubt whatever
about it when it strikes the skin over the stomach.
From these observations it appears that the skin contains three
sets of organs sensitive respectively to touch, cold, and heat.
Certain investigators hold that it also contains specific organs, or
nerve-endings, sensitive to painful stimulants; but in this case there
is the obvious difficulty of distinguishing between pain and touch. At
no spot can pure pain be evoked free from any consciousness of touch.
To a certain extent the combinations of epithelial cells and
nerve-endings in the skin fulfil the negative requirement of
sense-organs; each kind, whilst specially sensitive to its own specific
stimulant, is insensitive to stimulants of other kinds. But mutual
exclusion is not absolute in the case of cold and warmth. If a warmed
metal point be applied to a cold spot, it produces a sensation of cold.
Our feelings of warmth and cold are to a large degree comparative.
Luke-warm water feels cold to hands just taken out of hot water;
moderately cold water appears luke-warm to hands that have been in
contact with ice. The sensory apparatus for cold and heat soon adapts
itself, or, in physiological language, it is soon fatigued. If after a
prolonged bath at the body temperature a foot be plunged into very hot
water and withdrawn quickly, the feeling which first ensues is one of
cold. It is indistinguishable from the feeling provoked by dipping the
foot into cold water. The sensation of cold subsequently gives place
to one of painful warmth. This does not indicate that the heat-spots
have been waked out of their lethargy by excessive stimulation. On the
contrary, it is the cold-spots which, when they were first stimulated
by the very hot water, answered “Cold,” that now cry out “Hot”; for
both cold-spots and heat-spots, when strongly stimulated, yield the
same sensation. Indeed, it appears that the mind relies upon the
simultaneous stimulation of adjacent heat-spots and cold-spots for the
assurance that the thing with which the skin is in contact is really
hot. If two metal points, one kept warm and the other cold, are applied
simultaneously to two closely adjacent spots of skin, the resulting
sensation is “hot.” When the cold point is withdrawn, or replaced by a
second warm point, the sensation sinks to “warm.”