CHAPTER XV

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.

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.

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.

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.”