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:—

They are numerous over finger joints and front of elbow joint.

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:—

* 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:—

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

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.

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.

Thus on the palm of the hand two pairs of cold spots .8mm. apart are distinguished by this punctiform stimula­tion, whereas on this surface two pairs of warmth spots are only distinguished when they are 2mm. apart on the cheek, chin or forehead and cold spots are distinguished when .8mm. apart on the same surfaces warmth spots when 5mm. 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.

Table VII.—(According to Weber’s Law.) Average differences in different regions of skin of sensation of pressure.

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 Hill82 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.

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).

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.)

A passage from Professor McDougall’s Physiological Psychology may also be referred to at more length than it was in Chapter III., page25. 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

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.

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.

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 2mm. 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.