INNERVATION OF THE HUMAN SKIN.
For at least seventy years the surface of the human skin has been
the subject of so much physiological observation and experiment that
Professor Sherrington considers the literature connected with it to
be probably greater than in any other branch of physiology. Most of
this study centres round the skin as a receptive field and problems of
the nervous system. It is easy to see why this should be in the case
of an organ so great as the skin, covering all the other structures
and organs and exposed through ages of evolution to the vicissitudes
of an inconceivable number of stimuli. And one outcome of this study
is to show that, metaphorically speaking, the skin is a mosaic, and
not the confused and blurred production of a child of four years old
who has been given a sheet of paper and a paint-box. There is order
in this field, and even without calling in final causes, plan and
purpose. Beside the protective function exercised by the skin it plays
a large part, through its nervous endowment, in the processes by which
the brain is made aware of the surrounding phenomena, thus conveying
intelligence to the centre of life only less important than that of the
special senses. It is maintained here that the result of the various
physical stimuli, of which pain, cold, warmth and touch are the chief,
is that certain functions and structures of the skin have arisen in
response to them.
This is, no doubt, to beg the question of origin, and if the balance of
evidence be seen to be against this view the order of events would need
to be stated differently. But the position is clear, whether correct
or not, and if it be shown to be erroneous it will at least have good
“lighthouse value.”
Observed Facts.
Briefly stated the facts of the innervation of the skin are of two
orders, anatomical and physiological; the former examined by the aid
of the microscope, the latter by physiological experiments of a varied
kind. The chief aspect in which these are viewed here is the mode of
distribution of these two groups of fact, and it is held that this
strongly suggests without proving it, the alleged mode of origin of
both.
TABLE I.--_Distribution of Touch Corpuscles_:--
In the deep connective tissue of the dermis there are:--
In the thumb about 70.
" " index finger 105.
" " middle finger 60.
" " whole hand 500.
They are numerous over finger joints and front of elbow joint.
In all 530 about the joints of the upper extremity.
317 about those of the lower extremity.
TABLE II.--(_From Schäfer’s Text Book of Physiology_):--
Average of Meissner’s corpuscles to each square millimetre, which is
approximately one five hundredth part of a square inch:--
Palmar surface of distal phalanx of index finger 21
Palmar surface of second phalanx of index finger 8
Palmar surface of first phalanx of index finger 4
Palmar surface of metacarpus of little finger 2
Plantar surface of distal phalanx of great toe 7
Middle of sole of foot 2
Flexor surface of forearm in each sq. mm. 1
Distal end of flexor surface of forearm in each sq. mm.:--
1 to each 6 sq. mm. approximately.
* Absent from the cornea, and conjunctiva of the upper eyelid and
from the glans penis.
TABLE III.--_Distribution of Touch Spots_:--
These must be distinguished, of course, from the touch _corpuscles_
of the preceding list and the subjective element in the study of them
must be borne in mind.
If an area, as of the calf of the leg, be prepared, by cutting short
the small hairs, and examined, it is found that there are about 15
touch spots in each square centimetre, which is about one-fifth of a
square inch.
In another area so treated the hairs are counted and the following
result is given:--
1. On the dorsal surface of the forearm 78 touch spots are found in
an area containing 15 hairs.
2. On the flexor surface of the forearm 147 touch spots are found in
an area containing 22 hairs.
3. On the scalp 66 touch spots are found in an area containing 38
hairs.
Schäfer says: “An area of the dorsum of the distal phalanx of a
finger contains about seven times as many touch spots as an equal
area between the shoulders. Regions poor in touch spots are the
flexor surface of the upper arm, the upper third of the thigh, the
leg above the inner malleolus, the neck, and in general the skin over
subcutaneous surfaces of bone.”[81]
[81] Schäfer’s _Text-Book of Physiology_.
TABLE IV.--_Distribution of Cold and Warmth Sensations._
The Scale includes twelve grades of sensation in cold, and eight in
warmth sensations, and commences with the regions which yield the
maximal intensity of sensation.
_Cold Sensations._
1. Tips of fingers and toes, malleoli, ankle.
2. Other regions of digits, tip of nose, olecranon.
3. Chin, palm, gums, glabella (a small central area just above bridge
of nose).
4. Occiput, patella, wrist.
5. Clavicle, neck, forehead, tongue.
6. Buttock, upper eyelid.
7. Lower eyelid, popliteal space, sole, cheek.
8. Inner aspect of thigh, arm above elbow.
9. Intercostal spaces along region of axillary line.
10. Areola of mamma.
11. Nipple, flank.
12. Certain areas of loins and abdomen.
_Warmth Sensations._
Absent from lower gums, mucosa of cheek at second lower molar and
cornea.
1. Tips of fingers and toes, cavity of mouth, conjunctiva, patella.
2. Remaining surfaces of digits, middle of forehead, olecranon.
3. Glabella, chin, clavicle.
4. Palm, buttock, popliteal space.
5. Neck.
6. Back.
7. Lower eyelid, cheek.
8. Nipple, loin.
TABLE V.--_Distribution of Cold and Warmth Spots._
By stimulation of cold or warmth spots there is shown, not only
the quality and quantity of the stimulus, but the locality. When
punctiform stimuli are applied to pairs of cold spots and pairs of
warmth spots marked “local sign” is found. This Goldscheider showed
to be higher for cold than warmth spots.
Cold Spots. Warmth Spots
Palm .8 mm. Do. 2 mm.
Cheek, Chin and
forehead 0.8 mm. Do. 5.0 mm.
Upper arm 2 mm. Do. 3 mm.
Back of hand, leg,
thigh 3 mm. Do. 4 mm.
Forearm 3 mm. Do. 3 mm.
Back, chest, abdomen 2 mm. Do. 5 mm.
Thus on the palm of the hand two pairs of cold spots .8 mm. apart
are distinguished by this punctiform stimulation, whereas on this
surface two pairs of warmth spots are only distinguished when they
are 2 mm. apart on the cheek, chin or forehead and cold spots are
distinguished when .8 mm. apart on the same surfaces warmth spots
when 5 mm. apart.
TABLE VI.--Average lowest distances in millimetres on different areas
of skin where two points are felt as two or minimal distances from
which double sensation is obtained.
Skin Region. Adult Man. Boy aged Twelve.
Tip of tongue 1.1 1.1
Palmar surface of tip of
finger (index) 2.3 1.7
Red surface of lip 4.5 3.9
Palmar surface of 2nd
phalanx of finger 4.5 3.9
Dorsal surface of 3rd phalanx
of finger 6.8 4.5
Side of tongue 9.0 6.8
Tongue 27 mm. from tip 9.0 6.8
Plantar surface of distal
phalanx of great toe 11.3 6.8
Surface of palm of hand 11.3 9.0
Dorsal surface 2nd phalanx
of finger 11.3 9.0
Forehead 22.6 18.0
Back of ankle 22.6 20.3
Back of hand 31.6 22.6
Forearm and leg 40.6 36.1
Dorsal surface of foot 40.6 36.1
Surface on outer border of
sternum 45.1 38.8
Back of neck 54.1 36.1
Middle of back 67.1 31.6 to 40.6
Upper arm and thigh 67.1 31.6 to 40.6
TABLE VII.--(_According to Weber’s Law._) Average differences in
different regions of skin of sensation of pressure.
Forehead }
Lips }
Dorsum of tongue } 1/30 to 1/40
Cheeks }
Temple }
Finger nail }
Dorsal surface of forearm, leg, }
and thigh }
Dorsal surface of hand }
Dorsal surface of 1st and 2nd } 1/10 to 1/20
phalanges of fingers }
Palmar surface of finger }
Palmar surface of hand }
Flexor surface of forearm }
Dorsum of foot }
Dorsal surface of toes }
Plantar surface of toes } More than 1/10
Sole of foot }
Surface of leg and thigh }
Thus on the forehead differences of pressure are distinguished when
they are increased by 1/30, whereas on the dorsum of the foot they
have to be increased by 1/10 to be distinguished. This is carried out
by impact of little balls of a light substances such as pith.
It may be remarked of these tabulated results that on the one hand
they are the results of work extending over some seventy years and
numerous observers, and on the other that, broadly looked at, _they all
tell the same story_ of stimuli in their incidence on the skin--those
of pain, cold, warmth and touch. There is also one thread of origin
running through all, and that is that the regions most exposed to the
four stimuli show the highest development of specialised function and
structure.
Some Aspects of the Nervous System.
It has been said with some truth that the telephone has struck a
mortal blow at such serenity of life as the Juggernaut Car of modern
progress has left us. But if it has done nothing else it has furnished
the physiologist with a good illustration when he sets out to expound
the functions and arrangement of the elements of the central nervous
system and its peripheral expansion. In addition to this general
light upon a great matter the vivid experience of many an Englishman
during the recent years of war adds point to a subordinate phase of
the general story of the telephone, for it represents my contention
as to the origin or initiative of the sensorial areas of the mosaic
under consideration. Modern persons may be divided into two classes,
those who want and those who do not want the telephone, and the former
may be sub-divided into A, those who can, and B those who cannot
get it (or could not). A and B from the present point of view may
be termed Receptors, though to call the B people by that name is to
speak Hibernically. With this war-time experience in our minds, we
may picture a vast period of time during which the stimuli of pain,
cold, warmth and touch were hammering on the skin both before it
began to lose its chief hairy covering, and after that process had
left man still a hairy animal, but with much-diminished amount of his
ancient heritage. These stimuli fell upon the skin very much as the
class A, among telephone receptors, spent numerous fruitless stimuli
on Postmasters-General, Ministers in Parliament and in “short” bitter
letters to our bright little _Daily Pope_, and who yet found themselves
not “connected up,” as the saying goes. There is no knowing how long
it was before they had enough effect on the delicate nerve fibrils
struggling up into the epidermis and produced receptors or were
“connected up” to the exchange or central nervous system. I am inclined
to liken the pain stimuli to the short letters referred to, the cold
and warmth stimuli to those addressed to the Postmasters-General and
the touch stimuli to those which fell upon Ministers at question time.
Another comparison of the peripheral portion of the nervous system to
common things has at times forced itself upon my mind when reflecting
on the stimuli which are continually assaulting the skin, as I have
watched on the Needles’ Downs a flock of sheep on a summer evening
returning to their fold. As the sun begins to set they are scattered
over the western end of the Downs, still cropping the short grass
clothing those chalk and flint slopes which from immemorial time has
alone flourished there. They wander singly or in small groups on such
parts of the slope as the intrusive golfer still allows, and gradually
fall into larger groups which follow somewhat indefinite paths. As
they move further and further towards home they are seen to follow
one another in single file on some score or more of clearer paths,
and finally converge into one well-beaten and broad path until they
descend the northern slope and pass out by a single roadway into which
a gate opens, and so reach the haven where they would be. Here one has
a simple picture of the common stimuli of the skin, at first indefinite
and ineffectual, by their cumulative action producing an individual
receptor and its nerve connection with the central system.
Professor Leonard Hill[82] also gives a view of the general action of
the nervous system and compares it to control of the police force. He
supposes a murder to have been committed in a village, and that the
local policeman telegraphs to the local town ordering the roads to
be searched. The policeman is the tactile sense-organ, the telegraph
wire is the sensory nerve, the telegraph office in the local town
is the spinal cord, from this office a message is sent to the town
police-station by another wire and the police are set in motion. The
police are the muscles, the wire that sets them in motion in the motor
nerve. The message is also sent to neighbouring towns and to London,
that is to say, other local offices (parts of the spinal cord) and the
head office (the brain) are informed of the crime or sensory impulse.
The central office in London directs the operations controlling the
local police office. The whole order of events need not be here
described because it goes beyond my immediate purpose, but it is enough
to say that attached to the head office are the cleverest detectives
(higher sense-organs) and in these are kept records of past crimes,
lines of action of the police, and success or non-success of their
investigations.
[82] _Manual of Human Physiology._ Leonard Hill, p. 369.
Following on this picture he speaks of the way in which conscious
actions become automatic and makes a statement to the effect that
“_There is evidence to show that the axons_ (or processes of the
nerve-cells which extend unbroken from nerve-cell to its termination)
_become covered with a adulated coat as each new tract is formed. Thus
the structure, like the habit, becomes fixed_”--and--“_It would appear
as if, by repeated experiences, tracts and pathways must be beaten
through the nervous system_”[83] (Italics not in original).
[83] _Op. cit._, p. 371.
Beside this I place a statement from Professor Graham Kerr as to his
view of the development of peripheral nerve-trunks. He is reviewing
the “outgrowth” theory of His, the “chain cell” theory of Balfour, and
the “Primitive Continuity” theory of Hensen, and expresses himself as
follows: “_It is suggested that the development of the actual nerve
fibril is simply the coming into view of a pathway produced by the
repeated passage of nerve impulses over a given route._”[84] (Italics
not in original.)
[84] _Text Book of Embryology._ Vertebrata with the exception of
Mammalia. Vol. II., 1919, p. 106.
A passage from Professor McDougall’s _Physiological Psychology_ may
also be referred to at more length than it was in Chapter III.,
page 25. Speaking of the automatization of voluntarily acquired actions
which have been explained by the view that purely reflex actions
carried out by mechanisms of the spinal level were also originally
acquired by our original ancestors as voluntary actions, he says, “This
view is usually associated with the name of Wundt, who has forcibly
advocated it. It implies, of course, the assumption that acquired
characters are in some degree transmitted from one generation to
another, a proposition which most biologists at the present time are
inclined to deny because they cannot conceive how such transmissions
can be effected. Nevertheless, the rejection of this view leaves
us with insuperable difficulties when we attempt to account for
the evolution of the nervous system, and there are no established
facts with which it is incompatible. If, therefore, we accept this
view we shall regard the congenital neural dispositions, both those
that determine pure reflexes and those that determine instinctive
actions, as having been acquired and consolidated under the guidance
of individual experience, with the co-operation, to a degree which we
cannot determine, of natural selection.”[85]
[85] _Physiological Psychology._ W. McDougall, p. 156 (1911).
These three statements from a physiologist, a zoologist, and a
psychologist, all of great eminence, though they differ in particular
problems studied, tell very strongly in favour of the position here put
forward as to initiative in the production of specialised innervation
of the skin.
Origin of Cold, Warm, Pain and Touch Spots.
The hair-clad skin of primitive man provided ample raw material for
the eventual differentiation of both end-organs and sensorial areas
which is found to-day. Not only did he possess what is called Common
Sensation in his skin but in the individual hairs lay a delicate
tactile structure, which, though probably inferior in delicacy, serves
a similar purpose to that of the vibrissæ on the muzzle of Felidæ. Each
hair, being deeply inserted into the skin and supplied with fine nerve
fibrils, when it is bent, acts as a lever communicating an impulse
to an afferent nerve trunk In an animal covered with thick hair the
sensory impulse conveyed might be exceedingly delicate, but, from the
nature of the case, of much more limited range than in one like man in
whom the hair is so greatly diminished in length and thickness.
It would be fruitless to speculate as to which of these four forms of
stimuli was the earliest to become effective in developing man.
Cold and Pain.
Two of them, cold and pain, may be termed _nocuous_; one, that of
touch, _useful_, and one, that of warmth, _indifferent_. If it be true,
as Professor Scott Elliott states,[86] that man’s earliest home had a
climate which “lies between the regular tropical, with wet, steaming,
impassable jungles, and the colder temperate zone, so affording chance
of acclimatisation in both directions,” the stimuli of cold would even
then not be wanting, however much they increased in severity when he
passed through glacial periods; but wherever, whenever, and at whatever
time he first became man he had to tread the Via Dolorosa in the course
of his hard and eventful life, and must have been well accustomed in
all regions of his skin to the stimuli of pain, working, as he did, for
his living, and fighting for it and his mate, with varied and powerful
enemies. Though it is correct to call both these fundamental stimuli
“nocuous,” this is all a matter of degree, and both the stimulus of
moderate cold, raising blood-pressure and activating metabolism, and
that of minor pains, would do little else than good in his education
for the higher terrestrial life to which he had descended. If he was
to learn effectually to take care of himself the discipline of both
moderate cold and pain would be as valuable to him then as in its
measure it is to his descendant to-day. The triumphs of medicine and
surgery could never have appeared if it were not for the beneficent
warning voice of pain that so generally accompanies disease.
[86] _Prehistoric Man and His Story_, p. 92.
Through long ages of exposure to the stimuli of cold and pain came
response in the form of cold and pain spots, after minute struggles
between the static conservative tissues of the skin and the dynamic
force of repeated assaults upon them. In due time then receptors
appeared and each became connected with the central organs, by which
means better adapted motor reactions against “nocuous” cold and pain
became possible. In 1900 Professor Sherrington summed up the evidence
in Schafer’s work on Physiology against the existence “of separate
afferent fibres with their specific end-organs entrusted specifically
with carrying painful impressions to a pain centre,” but Professor
Starling in his later work on Human Physiology speaks of “a distinct
sense of pain,” probably subserved by a distinct set of nerve fibres,
but for the present purpose it is not necessary that agreement on such
a problem should be reached, for it is alone with pain _spots_ that we
are concerned. He also points out that on the one hand the cornea is
sensitive to only one of the four stimuli in question, that is, pain,
and on the other that the surface of the glans penis is sensitive to
cold and pain, but tactile sensation and warmth sensations are almost
entirely absent.
_Touch._--This form of stimulus and its response can only be reckoned
as useful to the organism, except that it may be, and often must be
indifferent. The great number of the touch spots can be understood
when it is declared by Professor Sherrington that almost invariably
there are one or more touch spots close to the emergence of each
hair,[87] and that they are very numerous also on the palmar and
plantar surfaces of the hand and foot. Of the four forms of cutaneous
stimuli those of touch are the only kind that have so far been proved
to have specialised corpuscles, the other three having developed the
physiological equivalent of cold, pain and warmth _spots_.
[87] _Schafer_, p. 922.
_Warmth spots_ are decidedly the least numerous of the four, those of
pain being, as stated by Professor Sherrington, the most numerous. It
is obvious that unless thermal stimuli become somewhat excessive they
hardly can be described as “stimuli,” being more or less neutral in
their action on a warm-blooded animal. This cannot be entirely so,
because it has been shown quite conclusively that warmth spots _do_
exist, though much less numerous than others. There is a significant
fact as to thermal reaction and that is that there are no pure
_heat_ spots like those of cold, for the stimuli of about 49° C
are so associated with those of pain that warmth spots alone are
distinguished, and among primitive man no stimuli of heat could impinge
on his skin, until he had learned the use of fire, more powerful than
those of solar heat.
Such stimuli of heat as the rays of the sun would occasionally
discharge on the skin would resolve themselves into the general
stimulus of pain, and in this direction a far shorter initiation
occurred than with any of the four normal cutaneous stimuli. The fact,
at any rate, of there being no _heat_ spots is to be noted.
It remains now, having quoted three writers eminent in physiology,
psychology and zoology in support of the modest thesis here put
forward for me to appeal to the authority of the facts contained in the
tables for such evidence as they can give, and to give a summary of
this.
Summary.
1. Table I. shows that the structures known as touch corpuscles are
distributed on those parts of the skin where the stimuli of touch fall
most and in proportion to the degree in which those parts are employed
in tactile discrimination; thus, most of all on the index finger (with
the exception of the tip of the tongue) next on the thumb and less on
the middle finger. There are 530 of these corpuscles to the upper and
317 to the lower extremity.
2. Table II. bears out the same conclusion, the average number of
corpuscles to a square millimetre being twenty-one on the terminal,
eight on the second and four on the first phalanges of the index
finger, whereas on the foot there are seven on the great toe much
exposed to stimuli and only two on the middle of the sole of the
foot, which is little exposed. The absence of them from the cornea
and conjunctiva, protected by quick and powerful reflexes from such
stimuli, and from the (normally) covered glans penis is in accordance
with the other results.
3. Table III. dealing with touch spots, shows that these are nearly
twice as numerous on the flexor as the dorsal surface of the forearm;
and nearly five times as numerous as on the scalp, where tactile
stimuli are few, and that the distal phalanx of a finger contains about
seven times as many as an area between the shoulders. The regions poor
in touch spots are shown to be those where relatively few tactile
stimuli can fall.
4. Table IV. gives cold and warmth sensations graded according to the
delicacy with which they are perceived in many regions of the skin.
The cold sensations are best distinguished on the parts normally most
exposed to cold, as the tips of fingers, malleoli, tip of nose, chin,
patella, wrist, and least on the protected areas, inner side of thigh,
flank, loins and abdomen. The warmth sensations are best distinguished
on the regions on which the stimuli of warmth has most frequently
fallen, tips of fingers and toes, cavity of mouth, palm of hand, less
so on the neck and loin. And the striking fact is noted that warmth
sensations are not felt in the lower gums, the inside of the cheek at
a certain level and the cornea, which again is protected from these
stimuli by its efficient reflex, whereas to the gums and inside of the
cheek most warmth stimuli have not been “stimuli” at all.
5. Table V. also gives results of the mode of distribution of cold
and warmth spots, examined with punctiform stimuli. The “local sign”
for cold is higher than that for warmth spots, and two of these
are distinguished as double when only 0.8 millimetres apart on the
palm, cheek, chin and forehead, whereas on the upper arm, back and
thigh, they are only distinguished as double when separated by two
millimetres, and this distance is the minimum at which warmth spots are
distinguished as two, that is 2 mm. on the palm, and five on cheek,
chin, forehead and back. This tells the same story as Table IV., of
past stimuli of cold and warmth.
6. Table VI. deals more elaborately than the others with double
sensation in different areas of the skin, the tip of the tongue being
the most accurate in this respect of all examined, and the tip of the
index finger next, which is to the great toe as 2.3 to 11.3, the palmar
surface of a finger half as accurate again as the dorsal surface, the
palm of the hand twice as accurate as the surfaces of the forehead and
back of ankle, nearly four times as much so as the dorsum of the foot
and six times as the skin of the middle of the back.
There is here a very close relation between the amount of exposure of
these various regions to tactile stimuli and their present equipment of
ability to discriminate between two small objects.
7. Table VII. deals with the sensation of pressure in certain groups of
areas, and shows that change of pressure is perceived about three or
four times as accurately on the forehead, lips and tongue, as on the
finger nail, back of forearm, hand, or fingers, and more than three or
four times on the back of the foot, and sole, and surface of leg and
thigh. In this group of observations also the rule is followed that
the greater and more frequent in man’s ancestral past have been the
exposure of his skin to variations of pressure, the greater is his
present power of accurate discrimination of them.
There are some scattered facts mentioned by Professor Sherrington
which are in keeping with the line here taken, that the formation of
receptors in the skin have their origin in accumulated stimuli. He
refers to the vain endeavours of Goltz to evoke the reflex croak of
the female frog by applying electrical stimuli to the skin, whereas
non-nocuous mechanical stimuli were the only stimuli that proved
effective.
He never was able to elicit the “extensor thrust” in the “spinal dog”
by any form of _electrical_ stimulation, but only by a particular kind
of mechanical stimulus. This peculiarity was also found in the pinna
reflex of the cat.
As to the scratch reflex in the dog it was only when it was _easily
elicitable_ that it could be evoked by electrical stimulation as well
as mechanical, and when it was not easily elicitable electrical stimuli
failed altogether while mechanical stimuli still evoked it.
He describes the receptor as a mechanism “_attuned_ to respond
specially to a certain one or ones of the agencies that act as stimuli
to the body,” and points to the fact that _electrical_ stimuli are
not of common occurrence in nature and no chance for adaptation to
evolve in the organism receptors appropriate for such stimuli has
been afforded. Such negative facts are at the least suggestive in
considering the question of the mode of origin of receptors and
end-organs, electrical stimuli being rare in nature.
The subject of the innervation of the skin and its receptors has been
treated here in a great measure by the aid of imagination, with some
evidence, and a good deal of reconstruction has been attempted, but
perhaps this will be pardoned by those who are prepared to carry out
a corresponding process with such as Pithecanthropus, Eoanthropus and
Saurian monsters from somewhat scanty osseous remains. Any biological
theory of the origin of these receptors than the one here put forward
is faced with some formidable difficulties, which are probably
insurmountable.