Adaptation of the pressure sense is a matter of every- day experience. So long as we sit still, we are hardly aware of the pressure of our clothes; and the man who is 148 Cutaneous Senses looking for the spectacles that he carries on his forehead has become a stock figure in the comic papers. The posi- tive after-images of pressure ordinarily escape notice, since attention turns rather to the object arousing the sensation than to the sensation itself. They may, however, be in- tensive and of long duration;- the deformation of the skin persists awhile, after the removal of the stimulus, and this after-affect of stimulus shows itself in a continuance of sensation.
It seems, at first, hardly credible that the end- organs of pres- sure should not be differentiated for the reception of different kinds of stimuli. When we think of the great variety of our tactual experience, and when we remember further that the same stimulus has markedly different effects if applied to different parts of the skin, we are almost forced to believe in a number of qualitatively distinct sensations. Nevertheless, the verdict of experiment is de- cisive here, as it was decisive in the somewhat similar case of taste. And we must not forget the facts on the other side. First, the stimuli that normally affect the skin are areal stimuli, appealing to a group of diversely tuned pressure organs; and the texture of the skin itself, and the nature of the underlying tissues, vary from place to place. There is, then, every chance in ordinary experience for typical differences in the intensity and the temporal course of pres- sure sensations. Now the sensations which we have termed contact, pressure, and granular pressure, although they are evoked by differ- ent intensities of the same stimulus and are on that account usually considered as different intensities of the same quality, are at least as distinct as red and pink, or yellow and orange; and if we may not call them psychological qualities, we must at least say that they do the same service for touch that true qualitative differentiation does for other senses. Secondly, the greater number of normal stimuli affect other organs, cutaneous or subcutaneous, besides those of pressure. Hence most of our tactual experience does, in strictness, consist of more than one quality, because it derives from more than one sense. Thirdly, as has been said above, the atten- § 40. The Temperature Senses tion is generally concerned rather with the stimulating object than with the sensation which it excites. Here touch borrows from sight in much the same way as taste borrows from smell; visual characters of form, size, texture, etc., are so firmly associated to the feel of the stimulus that the skin gets the credit of a good deal of work done by the eye.
We shall have occasion, in § 50, to analyse some of the com- moner tactual complexes. In the meanwhile, this general state- ment of the various factors which enter into them may lessen the strangeness of the experimental results.
§ 40. The Temperature Senses. — If you draw the rounded point of a lead pencil slowly and lightly across the back of the hand, or, better, across the surface of the closed eye- lids, you will get, here and there, definite flashes of cold. There is a continuous sensation of pressure, due to the direct or indirect stimulation of pressure spots by deforma- tion of the skin; but this continuum is dotted by sensations from the cold spots.
For systematic work, it is best to use a hollow point of metal, which can be kept at a constant temperature by the passage of a stream of water. The average natural temperature of the healthy skin may be put at about 330 C.1 With the metal point held at I2°-I5° C, one obtains the char- acteristic sensation from the cold spots, and with the point at 37°-40° C, the sensation from the warm spots. Both are larger, more ex- tended than the sensation of pressure. The cold seems to lance down from above; it is set up at once in its full in- FlG. 21. Apparatus for the in- vestigation of the temperature senses. C, cold; W, warm water; P, metal point.
1 Degrees C. may be converted into degrees F. by the formula | C. + 32 = F.
150 Cutaneous Senses tensity; it might be described as a solid point of cold. The warm often seems to well up from beneath; it is thinner, more diffuse than the cold, and comes gradually to its full intensity.
The end-organs of temperature may be stimulated either from without or from within: from without, by the application to the skin of a cold or warm object, by radiant heat or the proximity of a cold body, by the action of substances like mustard, pepper, alcohol, menthol; from within, by the organic changes occurring in fever, in extreme fear, in an access of shame, etc. They respond to any decided change of the local level of temperature, the cold organs if the change is negative, and the warmth organs if it is positive. Like the pres- sure spots, they are tuned to different inten- sities of stimulus: some warm spots will give only a lukewarm sensation at 400 C, and Fig. 22. Cold and warm SQme cold ts on, a CQol sensatjon at I2° spots on an area (naturai size) of the back C- There are no qualitative differences under of the hand. The dots the general headings warm and cold.
represent the cold, the Cold spots may ^ found, without dimcircles the warm spots.,...,,,., — M Blix 1883 culty, by help of a lead pencil or a carpenter's spike. The warm spots are less easy of determination: partly because the warmed point quickly cools, and partly because the sensations themselves are duller and less insistent than those of cold. This difference of character makes it probable that the warmth organs are deeper seated than the cold organs. The latter may perhaps be identified with the terminal bulbs of Krause, and the former with the corpuscles of Ruffini.
Temperature spots are found, like pressure spots, over practi- cally the whole extent of the skin. The distribution of the three sets of organs differs in different regions. On the average, there are about 13 cold spots and 2 warm spots to the sq. cm.
It is a curious fact that the cold spots, which are not affected by the stimuli ordinarily used in the determination of warm spots, § 40- The Temperature Senses 151 give a clear sensation of cold if stimulated by temperatures above 450 C. This has been termed the paradoxical sensation of cold. Why the end-organs of cold should suddenly respond, at this particular temperature, is not known. Paradoxical sensations of warmth, — sensations aroused at the warm spots by a very cold stimulus, — have never been observed in the normal subject. It may be that they do not occur; or it may be that the end-organs are too deeply set in the skin to be reached by a punctiform cold stimulus. In pathological cases of anaesthesia to cold, the patients will occasionally declare that ice, applied to the skin, feels warm. It is, however, doubtful whether such statements do not rest upon a confusion.
If an areal stimulus of 450 C. or over is applied to a part of the skin which includes both cold and warm spots, we have the percep- tion of heat. In general, this appears as a simple and unana- lysable quality. It may, however, be analysed by a suitable ex- perimental procedure. Let the stimulus be set, say, at 400 C, and gradually increased. At first we get only the sensation of warmth. With a temperature of some 450, the paradoxical cold sensation also appears, and grows stronger and stronger as the rising temperature affects more and more of the cold spots. Under these conditions, it is possible, with practice, to distinguish the two component sensations, though each, as the attention turns to it, seems to be coloured by the other.
The temperature senses have a wide range of adapta- tion. In the winter we grow accustomed to cold, and in the summer to warmth, so that a warm winter's and a cool summer's day are judged by very different standards. Water that at the first plunge seems unpleasantly cold, and a room that strikes us on our entry as oppressively hot, soon become indifferent; we are even surprised at the comments of later arrivals.
Intensive stimuli, of brief duration, give a positive after- image. A long-continued and intensive cold stimulus is 152 Cutaneous Senses also followed by an after-sensation of cold. The removal of a continued warm stimulus, on the other hand, leaves a sensation of coolness.
It has been found possible, by experiment, to adapt the ringers to a temperature as low as n° C, so that a stimulus of 120 feels warm, and again to a temperature as high as 390, so that a slightly lower stimulus feels cold. If the skin is cooled by contact with an object which is kept constantly at io°, the paradoxical cold sensation will be aroused by a stimulus of 350, instead of the nor- mal 450. The following rough experiment gives striking proof of adaptation. Prepare three bowls of water, cold, lukewarm and warm. Hold the hands in the lukewarm water until this feels alike to both. Now place the one hand in the cold, the other in the warm water; let them remain for 1 min. Finally, dip both hands into the lukewarm water: it will seem decidedly cold to the warmed and decidedly warm to the cooled hand.
The after-image of cold following long-continued and intensive cold stimulation is a little paradoxical; we should rather expect an after-sensation of warmth. The cold may, in fact, be the para- doxical cold sensation, aroused in this case by the rush of warm blood to the cold-adapted organs.
§ 41. The Pain Sense. — If you hold the shaft of a pin loosely between the forefinger and thumb of the right hand, and bring the point down sharply but lightly upon the back of the left hand, you will sense first the impact itself, and then, after a brief but noticeable interval, something finer, like a prick or a thrill. This second sensation is due to a moderate stimulation of the specific organ of pain.
For accurate results, the skin must be shaved, and softened with soap and water; and the surface must be explored by help of a very delicate hair-point. The sensa- tion obtained from the pain spots then occurs in three stages: first, as a bright, itchy sensation; secondly, as prick or wiry thrill; and thirdly, as punctiform pain. It is always delicate and lively, and has less body than the sensation of pressure.
The end-organs of pain may be stimulated, from without, by mechanical, thermal, electrical or chemical means. They re- spond most easily to chemical stimulation, as, for example, to acid FIG. 23. Pressure and pain spots on an area (here multiplied by 16) of the back of the hand. There are 2 pressure spots and 16 pain spots. The latter are marked as circles, the former as triangles; the hairs to which the pressure spots belong are indicated by the heavy lines and semicircles. — M. von Frey, 1896.
dropped upon the cutaneous surface or to intracutaneous injection of salt solution. They may also be stimulated from within, by the chemical action of substances produced by inflamed tissue. Like the other cutaneous organs, they are tuned to different intensities of stimulus.
Pain spots occur, apparently, over the whole extent of the skin proper. Their distribution does not coincide with that either of the pressure or of the temperature spots. On the average, there Cutaneous Senses seem to be ioo to 200 pain spots to the sq. cm. With the excep- tion of lips, teeth, and tip of tongue, the mouth cavity shows but little sensitivity to pain; a large area of each cheek is entirely free from pain spots. — The organs of pain are, perhaps, to be found in the free intraepithelial nerve-endings.
The sensation of pain is often blended with sensations of pres- sure and temperature. The following table gives the facts for the temperature senses: Warmth Organs Cold Organs Pain Organs Bitter cold.
+ + + The sensation of prick or pain resembles the sensation of warmth in its slow and gradual rise to full intensity. It resembles those of pressure and cold in its persistence after the removal of stimulus; the prick aroused by the impact of the pin on the back of the hand may last, as positive after-image, for 10 sec. or more.
The pain sense does not appear to show the phenomenon of adaptation. If a pain spot is repeatedly stimulated, the sensation recurs, and presently the surrounding area be- comes sore and irritable. It is true that, in everyday life, we learn to disregard pains of moderate intensity, such as muscular soreness or a continued slight rheuma- tism; but we ignore them, as we ignore distracting noises, because the attention is occupied with other topics, not because they fade out with time.
The dense distribution of the pain spots, the qualitative differ- entiation of their response to stimulus, and the long duration of § 42- Theory of the Cutaneous Senses 155 the after-image, account in general for the experiences of cutting, burning, scratching, abrading the skin. Whether they account in detail for all our experiences of cutaneous pain is still an open question. We said above that the granular sensation of pressure is often tinged with a faint ache. If, now, a fold of the skin — say, the fold between the fingers — is grasped firmly with a pair of forceps, the same dull ache appears. The pain in these cases is unlike any superficial pain, but like that of a severe acid burn. It may be, then, that there is another set of pain organs, deeper seated in the skin, whose characteristic sensation should be de- scribed as an ache. — The pains derived from subcutaneous tissues are discussed in § 56.
All pains at high intensities are extremely unpleasant or dis- agreeable. It is therefore only natural that, in ordinary speech, we refer to any very disagreeable experience as painful. But it must be distinctly understood that the sensations proceeding from the pain organs are not necessarily painful, in the sense that they are necessarily disturbing or unpleasant. The bright, itchy sensa- tion and the sensation of prick occur as often as not in indifferent or pleasant complexes; it is only in the third stage, that of puncti- form pain, that the sensation hurts. And even here, the pains aroused by minor injuries to the tissues seem in many cases to be insistent, interesting, rather than actually painful.
§ 42. Theory of the Cutaneous Senses. — We have seen that the end-organs of the pressure sense 1 are the hair- bulbs and the Meissner corpuscles. How are these organs affected by pressure stimuli? Under what precise condi- tions are they thrown into function?
If a point is set down upon the skin, the pressure is greatest directly beneath the point, and rapidly decreases 1 The author knows of no skin-model that is satisfactory for psychological purposes. He uses the three Deyrolle models {Coupe de la peau de Vinie- rieur de la main, Coupe de la peau montrant V organisation d'un follicule pileux, and Coupe de Vextremite d'un doigl), upon which the end-organs men- tioned in the text have been painted.
156 Cutaneous Senses with increase of distance from it, — whether the distance be measured laterally, upon the surface of the skin, or ver- tically, into its substance. In other words, the point coin- cides with the maximum of what we may call a gradient of pressure. The gradient will be steep or gentle, according as the stimulus is strong or weak; it will take shape slowly or quickly, according as the point is applied gradually or with sudden impact.
Experiments show that the quick formation of such a pressure gradient is the adequate stimulus to the pressure organs. They show also that the gradient may be positive or negative; the pressure spots respond to pull as well as to pressure. It is not easy to say just what changes occur in the skin as the gradient is formed. There must, how- ever, be a redistribution of the liquid contained in the tissues, and there may be local concentration; probably, therefore, the mechanical effect of pressure upon the sur- face of the skin is translated into a chemical action upon the end-organs.
Reference to the pressure gradient enables us to explain two observations which have aroused much discussion among psychol- ogists. If the hand is dipped into water, or even into mercury, no pressure is sensed over the immersed area, but there is a distinct ring of pressure sensation at the place of emergence. The reason is, evidently, that here and here only does a noticeable pressure gradient occur. Again, if two objects of the same weight but of different size are laid successively upon the resting skin, the smaller appears the heavier. The reason is that the pressure gradient is steeper for the smaller, and more gentle for the larger object.
No theory of the temperature senses can as yet be offered; we have not even identified, with certainty, the terminal organs through which the sensations are aroused.
If, as seems probable, the organs of the pain sense are represented by the free nerve-endings of the epidermis, we have to account for the fact that the deeper-lying pressure spots are more readily stimulated by contact with material objects than the superficial pain spots. The reason lies in the nature of the cutaneous tissues. The epidermis is hard and inelastic, like a board; the cutis is soft and elastic, like sponge rubber. Hence, under ordinary circumstances, the stimulus is transmitted to the cutis, while the epidermis is not affected. When the epidermis is pierced by a fine point, or the dead cells of the outermost layer are cleared away and the tissue softened by soap and water, the superficial pain organs respond earlier than the organs of pressure.
§ 43. Tickle and Itch. — We described the sensation of contact as being somewhat ticklish, and the weak sensation of prick as being itchy. It would seem, then, that our everyday experiences of tickling and itching might be referred to a diffuse stimulation of the pressure spots and pain spots.
We get tickle, as a matter of fact, by brushing lightly, as with a feather, over a field of hairs. But we also get it, much more insistently, by brushing over a hairless surface, such as the red area of the lips, the palm of the hand, or the sole of the foot. The sense quality appears to be the same in the two cases. It is not quite easy to see how the pressure spots in the hairless regions can be stimulated by a contact which is too light to deform the skin: possibly, however, the application of the stimulus changes the pres- sure of blood in the superficial capillaries, and the nerve- endings are thus indirectly affected. The tickle that oc- curs when you are seized under the arms, or clutched on 158 Cutaneous Senses the knee, appears deeper-seated than the tickle produced by brushing the skin.
Itching may be referred with some confidence to the organs of cutaneous pain. It is the result of certain skin- diseases, of superficial wounds and burns, of the bite of insects, etc., — that is, of conditions by which the nerve- endings of the epidermis are directly affected.
It is often stated, as a paradox, that tickling makes us laugh, and is therefore pleasant, while it also makes us try to get away, and is therefore unpleasant Such a statement is, however, far too simple for the facts. Tickling may be either pleasant or un- pleasant, according to the region of the skin affected, the mode of application of stimulus, and the mood of the person tickled. Thus, it may be said in general that tickling on the sole of the foot or at the hairy orifices of nose and ear is distinctly unpleasant, while tickling on the palm of the hand or under the arm-pits is rather pleasant. Yet the flexing of the foot may change the un- pleasant into a pleasant experience, and tickle on the palm may be almost unbearably disagreeable. The arousal of laughter, again, is extremely capricious. We can tickle ourselves, and get pre- cisely the same sensations as when we are tickled by some one else; but we never make ourselves laugh. Nor can any rule be laid down as to the provocation of laughter by a certain form of stimulus, by stimulation of certain areas of the skin, etc. What holds of one person does not hold of another, or of the same per- son in a different mood. And the movements of escape are equally variable: a child may ask to be tickled to-day, and may beg not to be tickled to-morrow; a part of the body that is pleasurably ticklish in one child may be unpleasantly ticklish in an- other, and so on.
All this variety of detail means that the tickle-stimulus does not act simply upon the sense-organ, as a colour-stimulus acts upon the eye, but that it serves as the trigger, so to speak, to release certain inherited mechanisms of our nervous system. These mechanisms are, however, themselves liable to modification 01 § 43- Tickle and Itch 159 arrest from higher nervous centres, so that they are not touched off inevitably or with uniform result. The reason for such a physiological arrangement can only be guessed at. It is signifi- cant that the areas which are especially ticklish are highly vulner- able: in nearly all of them some important structure, such as a large artery, is close to the surface; and where this is not the case, as in the sole and the palm, an injury, even if not serious, is seriously disabling to the organism. It may be, then, that tic- kling represents a very old form of race-play, the play of combat. The movements of attack and of defence are playful forms of fighting, and the laughter shows that the whole affair is a friendly game. This interpretation is, of course, speculative. Neverthe- less, it accounts on the ground of utility for the persistence of the motor responses to tickling, while it also leaves room for their variety and uncertainty in our own case. On the whole, it prob- ably comes as near the truth as, in the absence of exact data, we can expect any theory to bring us.
References for Further Reading §§38-43. A. Goldscheider, Gesammelte Abhandlungen, i., 1898; C. S. Sherrington, Cutaneous Sensations, in E. A. Schafer's Text-book of Physiology, ii., 1900, 920 ff.; J. Sully, Ati Essay on Laughter, 1902; M. von Frey, Vorlesungen iiber Physiologie, 1904, 308 ff.; T. Thunberg, Drue/a-, Temperatur- und Schmerzempfindungen, in Nagel's Handbuch, iii., 1905, 647 ff.; H. Head, W. H. R. Rivers and J. Sherren, The Afferent Nervous System from a New Aspect, in Brain, xxviii., 1905, 99; L. Torek, Ueber das Wesen der Juckempftndung, in Zeitschnft f. Psychologie, xlvi., 1907, 23 ff; E. Murray, A Qualitative Analysis of Tickling: its Relation to Organic Sensation, in American Journal of Psychology, xix., 1908, 289 ff KINAESTHETIC SENSES § 44. The Kinaesthetic Senses. — In passing from the special senses to the group of organic sensations, we naturally turn first to the internal senses which are com- monly included in the sense of touch. These senses have their organs in the motor apparatus of the body; they are set in function by bodily movements; they enable us, without help from the eye, to judge of the position and movement of our limbs. Hence they have been termed, collectively, the kinaesthetic senses.1 The nature of the tissues which make up the motor apparatus, and the distribution of sense-organs within them, may be under- stood from the following illustration. Think of two long bones, which form a ball and socket joint, and of a single muscle, which passes across the joint and is attached by its tendons to the shafts of the bones. The opposed surfaces of the joint are covered with cartilage. This thins out, at its margin, into a layer of vascular connective tissue, the periosteum, which extends over the entire shaft. The joint is enclosed in a capsule of ligament; the inner surface of the capsule and the inner faces of the articular cartilages are lined with synovial or lubricating membrane. The muscle is composed of bundles of muscle fibres; it is divided into compartments by fascial or sheathing tissue, and is invested by a thicker sheath of the same material. The tendons are strong, fibrous cords, directly continuous at the one end with the fascia of the muscle and at the other with the periosteum of the bone.
1 The term was suggested by H. C. Bastian. See The Brain as an Organ § 44- The Kinaesthetic Senses 161 The muscles and tendons contain peculiar end- organs, known respectively as the muscle spindles and the spindles of Golgi. Pacinian corpuscles, or similar structures, have been found in fascial tissue, in ligament, in the synovial membrane, and in the periosteum of certain bones. Sensory nerve-endings occur also in the substance of the bony tissue. The sensitivity of the bone extends, apparently, up to the margin of the articular cartilages; whether it extends farther, so that the surface of the joint be- neath the cartilage is sensitive, we do not know.
In everyday life, the sensations of the kinaesthetic senses occur only as factors in what we may call touch- blends. In all such experiences as lifting, holding, grasp- ing, pushing, pulling, moving, handling, writing, playing a musical instrument, tying a knot, they are fused or blended with sensations from the skin. It is not sur- prising, then, that the skin should itself come to be looked upon as an active or motor organ, and should be credited with sense-qualities which are really derived from the deeper-lying tissues. The very fact that the motor ap- paratus is covered by the skin, that under normal con- ditions it cannot be separately stimulated, favours this confusion. Besides, the kinaesthetic sensations are, in general, very like the cutaneous; in one case, indeed, they seem to be indistinguishable from cutaneous pressure. For this reason, some psychologists still describe them as sensations of internal touch.
Nevertheless, the distinction between the sensitivity of the skin and that of the underlying tissues was drawn very early in psychology. Aristotle seems to have had an inkling of the difference, and it was clearly recognised in the sixteenth century.1 Modern writers have usually 1 Aristotle appears in general to bracket cutaneous and kinaesthetic sen- sations together, under the heading of touch. However, he says, in the His- 1 62 Kinaesthetic Senses ascribed the deeper-seated sensations to the muscles, and the muscular sense has accordingly been added, as a sixth sense, to the list of sight, hearing, taste, smell and touch. We have learned, however, that this list is itself incom- plete; and recent experiments, coupled with the study of pathological cases of partial anaesthesia, show that the muscles are but one, and that by no means the most important, of a number of sensitive tissues.
§ 45. The Muscular Sense. — To bring out the -special quality of muscular sensation, we must find a method of stimulating the muscle alone, that is, independently of skin, tendon and joint. The best way is to lay out the arm on a support; to render the skin and the subcutaneous connective tissue anaesthetic by cocaine injection or ether spray; and then to press down upon the body of the muscle. The result is, first, a sensation which is described as dull, dead, diffuse; it is simple in nature, cannot be named, but suggests areal pressure upon the skin. With increasing intensity of stimulus, the sensation takes on a dragging character: sometimes there seems to be a hard, dead lump in the muscle, sometimes the muscle fibres seem to be ground or rolled against one another. The general impression is that of a tired, overworked limb. Finally, the dragging sensation becomes sore, achy,- and the whole experience passes into dull pain.
The same series of sensations appears if a muscle is thrown into forced contraction by the electric current. Indeed, after a short tory of Animals: "The sense of touch resides in the simple parts, as in the flesh;...the capacity of action resides in the compound parts, as...the power of locomotion in the feet or wings." For an historical discussion of the subject, see T. Reid's Works, ed. by W. Hamilton, ii., 1872, 867, note II.
§ 46. The Tendinous Sense 163 practice in the laboratory, it is easy to identify the different stages, by introspection, in everyday life.
As the possible seat of muscular sensations, we have the fascial corpuscles and the muscle spindles. It is probable, from what we know of the joints (§ 47), that the corpuscles mediate a sort of pressure sensation. We may, then, provisionally, ascribe to them the dull, diffuse sensation, and to the spindles the dragging, sore, tired sensation that ultimately becomes pain.