Encyclopædia Britannica, Ninth Edition/Touch
TOUCH may be defined as a sense of pressure, referred usually to the surface of the body. It is often understood as a sensation of contact as distinguished from pressure, but it is evident that, however gentle be the contact, a certain amount of pressure always exists between the sensitive surface and the body touched. Mere contact in such circumstances is gentle pressure; a greater amount of force causes a feeling of resistance or of pressure referred to the skin; a still greater amount causes a feeling of muscular resistance, as when a weight is supported on the palm of the hand; whilst, finally, the pressure may be so great as to cause a feeling of pain. The force may not be exerted vertically on the sensory surface, but in the opposite direction, as when a hair on a sensory surface is pulled or twisted. Touch is therefore the sense by which mechanical force is appreciated, and it presents a strong resemblance to hearing, in which the sensation is excited by intermittent pressures on the auditory organ. In addition to feelings of contact or pressure referred to the sensory surface, contact may give rise to a sensation of temperature, according as the thing touched feels hot or cold. These sensations of contact, pressure, or tempera ture are usually referred to the skin or integument cover ing the body, but they are experienced to a greater or less extent when any serous or mucous surface is touched. The skin being the chief sensory surface of touch, it is there that the sense is most highly developed, both as to delicacy in detecting minute pressures and as to the char acter of the surface touched. Tactile impressions, pro perly so called, are absent from internal mucous surfaces, as has been proved in men having gastric, intestinal, and urinary fistulae. In these cases, touching the mucous surface caused pain, and not a sensation of touch.
Organs of Touch.—Comparative Sketch.—The organs of touch present many varieties of form, from a simple filament of sensitive protoplasm to a highly complex end-organ connected with the commencement of a sensory nerve-fibre. The bodies of the lowest organisms are formed of contractile protoplasm, and mechanical con tact with any resisting substances causes a change of form. Here is the simplest kind of touch a response on the part of any portion of the surface of the body to a mechanical stimulus. The pseudopodia of the Rhizopoda are also organs of touch, and probably the cilia, the flagellfe, and the short rod-like bodies seen on many Infusoria belong to the same class of sensory organs. Among the Coslentera (hydroid polyps, tubularians, Hydromcdusae, Medusse, Anthozoa or sea-anemones) tentacles are found, usually arranged in circles around the mouth or on portions of the body engaged in locomo tion, as on the margins of the umbrella of Medusae. These have a large amount of sensibility, and serve as organs of touch. In some also there are stiff hairs on the tentacles and around the mouth, more differentiated tactile organs. The Vcrmes show organs of touch in the form of modified cells of the integument, connected with sensory nerves. These cells often assume the form of stiff rods projected from the surface (tactile setae). Such are often found over the whole body of Turbellaria and Ncmertina, on the tentacles of Bryozoa, on the head segment of Lumbricidse, and on the tentacles and antennae of Chsetopoda. In the latter group of animals tactile organs are also found in ring-like arrangements, called cirrhi, on the foot-stumps or parapodia. In some Hinidinca (leeches) compli cated tactile rods are embedded in cup-shaped organs scattered over the body. Large prominences of the cuticle, called tactile papillae, are also found in many of the Vermes near the oral and genital orifices. The Echiiwdcrmata have also special parts devoted to touch, and these show their highest differentiation in the tentacles of the Holothuroida. ArtJiropoda show tactile organs in the form usually of rod-like bodies projecting from the surface of the appendages and chiefly connected with nerves passing to ganglionic cells. In Crustacea such organs are found on the antennae and other appendages, and on the antennae in Myriapoda and Insecta. In the latter they are also found on the tarsal joints of the feet. The ap pearance of these rod-like bodies is seen in fig. 1.
Fig. 1.—Nerve-ending with tactile rods, from the proboscis of a fly (Musca). n, nerve; g, ganglionic swelling; s, tactile rods; c, fine hairs of cuticle (Leydig).
Ciliated tentacular processes exist in the larva of Brachiopoda which are probably touch organs, but there are no definite organs of this kind in the adult form. The Mollusca have the sense of touch widely diffused. All the soft parts of the body are capable of feeling when touched, and in various situations there are fine hair-like prolongations Fio. i. Nerve-ending from cells. These are supplied with with tactil e rods, from nerves, and are touch organs. Such (MIKCU) ^n^nerve g are found on the edge of the mantle ganglionic swelling; s, tactile rods; in Lamellibranchiata, where they c, fine hairs of cuticle (Ley dig), may be in rows; they also exist on the siphons, and "they serve to watch over the particles that get into the mantle cavity with the water" (Gegenbaur). Processes of a tactile kind are also found on the epipodium, the edge of the mantle, and the cephalic tentacles in many Gasteropoda, and on the dorsal tufts of the Nudibranchiata. Here and there also there are enlargements of the integument covered with cilia and supplied by a nerve which have been regarded as touch organs, but are by some supposed to be connected with smell (see SMELL). The Tunicata have cells with long filamentous processes in the integument, which are probably tactile in function. In the great majority of fishes touch is limited to the lips, to parts of the fins, and to special organs called barbels. In the Cyprinoids there is a fold of skin bordering the mouth which is highly tactile. The lip of the sturgeon is covered with numerous papillae; the sucking lip of the lamprey is papillose and highly sensitive. The fins are in many fishes modified to serve as organs of touch. Thus the gurnards (Triglidx) have three soft flexible rays detached from the fin, and "the filiform radial appendages of the Polynemidas, the prolonged ventral fins of Osphromenus, Trichogaster, and other Labyrintliibranchs, and of the Ophidiidss," are examples of this class of organs (Owen). The barbels are long slender processes of skin, either single or in pairs, found in the Siluridse, loaches, barbels, cods, sturgeons, and in the parasitic Myxinidas. The nerves for the barbels come from the fifth pair of cranial nerves. "A cod, blind by absence or destruction of both eyeballs, has been captured in good condition, and it may be sup posed to have found its food by exploring with the symphysial barbule, as well as by the sense of smell" (Owen). Bodies some what similar to the Pacinian corpuscles (to be afterwards described) were discovered by Savi in 1844 in the torpedo; they are arranged in linear series on the anterior part of the mouth and nostrils, and over the fore part of the electrical organs. Each is composed of two capsules, one connected with the other, and containing a granular substance in which the nerve end is embedded. Peculiar mucous glands are also found outside the electrical organs of the torpedo which are believed to minister to touch. Similar organs exist in sharks, and John Hunter dissected the snout of the spotted dog-fish (Scyllium) "to show the manner of the nerves ramifying, as also their apparent termination in this part, each ultimate nerve appearing to terminate in the bottom of a tube or duct, the sides of which secrete and convey a thick mucus to the skin." These "nervo-mucous" organs are found in the sides and under part of the head and on the fore part of the trunk.
The Amphibia and Reptilia do not show any special organs of touch. The lips of tadpoles have tactile papillae. Some snakes have a pair of tentacles on the snout, but the tongue is probably the chief organ of touch in most serpents and lizards. All reptiles possessing climbing powers have the sense of touch highly developed in the feet.
Birds have epithelial papillae on the soles of the toes that are no doubt tactile. These are of great length in the capercailzie (Tetrax urogallus), "enabling it to grasp with more security the frosted branches of the Norwegian pine trees " (Owen). It has been suggested that the delicate "papillose" digits of the smaller birds assist them in nest-building by having the sense of touch highly developed. Around the root of the bill in many birds there are special tactile organs, assisting the bird to use it as a kind of sensi tive probe for the detection in soft ground of the worms, grubs, and slugs that constitute its food. Special bodies of this kind have been detected in the beak and tongue of the duck and goose, called the tactile corpuscles of Merkel, or the corpuscles of Grandry (fig. 2).
Fig. 2.—Tactile Corpuscles from duck's tongue. n, nerve.
Similar bodies have been found in the epidermis of man and mammals, in the outer root-sheath of tactile hairs or feelers. They consist of small bodies composed of a capsule enclosing two or more flattened nucleated cells, piled in a row. Each corpuscle is separated from the others by a transparent protoplasmic disk. Nerve fibres terminate either in the cells (Merkel) or in the protoplasmic intercellular matter (Ranvier, Hesse, Izquierdo). Another form of end-organ has been described by Herbst as existing in the mucous membrane of the duck's tongue. These corpuscles of Herbst are like small Paciuian corpuscles with thin and very close lamellae. Developments of integument devoid of feathers, such as the "wattles" of the cock, the "caruncles" of the vulture and turkey, are not tactile in their function.
In the great majority of Mammalia the general surface of the skin shows sensitiveness, and this is developed to a high degree on certain parts, such as the lips, the end of a teat, and the generative organs.
Fig. 3.—Tactile Corpuscle from the hand.
Where touch is highly developed, the skin, more especially the epidermis, is thin and devoid of hair. In the Monkeys tactile papillae are found in the skin of the fingers and palms, and in the skin of the prehensile tails of various species (Ateles). Such papillæ also abound in the naked skin of the nose or snout, as in the shrew, mole, pig, tapir, and elephant. In the Ornithorhynclius the skin covering the mandibles is tactile (Owen). In many animals certain hairs acquire great size, length, and stiffness. These constitute the vibrissæ, or whiskers. Each large hair grows from a firm capsule sunk deep in the true skin, and the hair bulb is supplied with sensory nerve filaments. In the walrus the capsule is cartilaginous in texture. The marine Carnivora have strong vibrissæ which "act as a staff, in a way analogous to that held and applied by the hand of a blind man" (Owen).
Fig. 4.—Tactile Corpuscles from clitoris of rabbit. n, nerve.
Each species has hairs of this kind developed on the eyebrows, lips, or cheeks, to suit a particular mode of existence, as, for example, the long fine whiskers of the night-prowling felines, and in the aye-aye, a monkey having nocturnal habits. In the Ungulata the hoofs need no delicacy of touch as regards the discrimination of minute points. Such animals, however, have broad, massive sensations of touch, enabling them to appreciate the firmness of the soil on which they tread, and under the hoof we find highly vascular and sensitive lamellle or papillte, contributing no doubt, not only to the growth of the hoof, but also to its sensitiveness. The Cetacca have mimerous papillae in the skin, regarding which John Hunter remarks: "These villi are soft and pliable; they float in water; and each is longer or shorter according to the size of the animal. In the spermaceti whale they are about a quarter of an inch long; in the grampus, bottlenose, much shorter; in all they are extremely vascular; they are sheathed in corresponding hollows of the epiderm." In some whales the skin is thrown into numerous longitudinal plaits on the under and fore part of the body (Balænoptera). Prof. Owen remarks regarding these: "It is peculiar to the swifter swimming whales that pursue mackerel and herring, and may serve to warn them of shoals, by appreciation of an impulse of the water rebounding therefrom, and so conveying a sense of the propinquity of sunken rocks or sand-banks. Sensitiveness to the movements of the ambient ocean is indicated by certain observed phenomena. The whale-fishers aver that when a straggler is attacked its fellows will bear down from some miles distance, as if to its assistance; and it may be that they are attracted by perception of the vibration of the water caused by the struggles of the harpooned whale or cachalot" (Owen's Comparative Anatomy, vol. iii. p. 189). Bats have the sense of touch strongly developed in the wings and external ears, and in some species in the flaps of skin found near the nose. These "nose-leaves" and expanded ears frequently show vibratile movements, like the antennae of insects, enabling the animal to detect slight atmospheric impulses. In the vampires (Desmodi) and fruit-eating bats (Pteropi) the auricular and nasal appendages are small; such sensitive tactile guides or warners in flight are only needed in the bats of active food, which must follow in swift evolutions, like the swallows, but in gloom, the volatile insects that people the summer air at dawn or dusk" (Owen). There is little doubt that many special forms of tactile organs will be found in animals using the nose or feet for burrowing. A peculiar end-organ has been found in the nose of the mole, while there are "end-capsules" in the tongue of the elephant and "nerve rings" in the ears of the mouse.
End-Organs of Touch in Man.—In man three special forms of tactile end-organs have been described, and can be readily demonstrated.
(1) The End-Bulbs of Krause.—These are oval or rounded bodies, from 1360 to 1170 of an inch long. Each consists of a delicate capsule, composed of nucleated connective tissue enclosing numerous minute cells. On tracing the nerve fibre, it is found that the nerve sheath is continuous with the capsule, whilst the axis cylinder of the nerve divides into branches which lose themselves among the cells. Waldeyer and Longworth state that the nerve fibrils terminate in the cells, thus making these bodies similar to the cells described by Merkel (ut supra). See fig. 5.
Fig. 5.—End-Bulb from human conjunctiva. a, nucleated capsule; b, core; c, entering nerve-fibre terminating in the core at d.
These bodies are found in the deeper layers of the conjunctiva, margins of the lips, nasal mucous membrane, epiglottis, fungiform and circumvallate papillae of the tongue, glans penis and clitoris, mucous membrane of the rectum of man, and they have also been found on the under surface of the "toes of the guinea-pig, ear and body of the mouse, and in the wing of the bat" (Landois and Stirling). In the genital organs aggregations of end-bulbs occur, known as the "genital corpuscles of Krause" (fig. 4). In the synovial membrane of the joints of the fingers there are larger end-bulbs, each connected with three or four nerve-filaments.
(2) The Touch Corpuscles of Wagner and Meissner.—These are oval bodies, about 1300 of an inch long by 1500 of an inch in breadth. Each consists of a series of layers of connective tissue arranged transversely, and containing in the centre granular matter with nuclei (fig. 7).
Fig. 6.—End-Bulb from conjunctiva of calf. n, nerve.
One, two, or three nerve fibres pass to the lower end of the corpuscle, wind transversely around it, lose the 480 TOUCH IG. 7. Vertical Section of the Skin of the palm of the hand, a, blood-vessel ; 6, papilla of the cutis vera; c, capillary; d, nerve-fibre passing to a touch-cor- pascle ; e, Wagner s touch-corpuscle ; /, nerve- fibre, divided transversely; g, cells of the Malpighian layer of the skin. (From Landois and Stirling, after Biesiadecki.) white substance of Scliwann, penetrate into the corpuscle, where the axis cylinders, dividing, end in some way un- known Thecor puscles do not con tain any soft core, but are apparently built up of irregu lar septse of con nective tissue, in the meshes of which the nerve fibrils end in ex pansions similar to Merkel s cells. Dr Thin describes simple and com pound corpuscles according to the number of nerve fibres entering them. These bodies are found abun dantly in the palm of the hand and sole of the foot, where there may be as many as 21 to every square millimetre (1 mm. = ^ inch). They are not so numerous on the back of the hand or foot, mamma, lips, and tip of the tongue, and they are rare in the genital organs. " Kollmann describes three special tactile areas in the hand : (1) the tips of the fingers, with 24 touch corpuscles in a length of 10 mm. ; (2) the three eminences lying on the palm behind the slits between the fingers, with 5*4-2*7 touch -corpuscles in the same length ; and (3) the ball of the thumb and little finger, with 3*1-3 5 touch corpuscles. The first two areas also contain many of the corpuscles of Vater or Pacini, whilst in the latter these corpuscles are fewer and scattered. In the other parts of the hand the nervous end- organs are much less developed" (Landois and Stirling). (3) The Corpuscles of Vater or Pacini. These, first described by Vater so long ago as 1741, are small oval bodies, quite visible to the naked eye, from -j- 1 ^ to -^ of an inch long and %-% to ^5- of an inch in breadth, attached to the nerves of the hands and feet. They can be readily demonstrated in the mesentery of the cat (fig. 8). Each corpuscle consists of 40 to 50 lamellae or coats, like the folds of an onion, thinner and closer to gether on approaching the centre. Each lamella is formed of an elastic material mixed with delicate con nective tissue fibres, and the inner surface of each is lined by a single continuous layer of endothelial cells. A double-contoured nerve fibre passes to each. The white substance of Schwann becomes continuous with the lamellae, whilst FlG 8 ._ Vatei , s or Padni . g Cor . the axis Cylinder passes into the puscle. a, stalk ; t>, nerve-fibre body, and ends in a small knob or t^sue^nveiopej e, aSs cynn- in a pleXUS. Sometimes a blood- der, with its end divided at/. vessel also penetrates the Pacinian body, entering along with the nerve. Such bodies are found in the subcutaneous tissue on the nerves of the fingers and toes, near joints, attached to the nerves of the abdominal plexuses of the FIG. 9. jEsthesiometer of Sieveking. sympathetic, on the coccygeal gland, on the dorsum of the penis and clitoris, in the meso-colon, in the course of the intercostal and periosteal nerves, and in the capsules of lymphatic glands (William Stirling). Physiology of Touch in Man. Such are the special end- organs of touch. It has also been ascertained that many sensory nerves end in a plexus or network, the ultimate fibrils being connected with the cells of the particular tissue in which they are found. Thus they exist in the cornea of the eye, and at the junctions of tendons with muscles. In the latter situation " flattened end-flakes or plates " and " elongated oval end-bulbs " have also been found (Sachs, Rollett, Golgi). A consideration of these various types of structure show that they facilitate inter mittent pressure being made on the nerve endings. They are all, as it were, elastic cushions into which the nerve endings penetrate, so that the slight variation of pressure will be transmitted to the nerve. Probably also they serve to break the force of a sudden shock on the nerve endings. Sensitiveness and Sense of Locality. The degree of sensitiveness of the skin is determined by finding the smallest distance at which the two points of a pair of compasses can be felt. This method, first followed by Weber, is em ployed by phy sicians in the diagnosis of nervous affections involving the sensitiveness of the skin. The following table shows the sensitiveness in millimetres for an adult, whilst the corresponding numbers for a boy 12 years of age are given within brackets (Landois and Stirling, after Weber) : Millimetres. Tip of tongue I l Third phalanx of finger, volar surface 2-2 "3 Red part of the lip 4 5 Second phalanx of finger, volar surface First phalanx of finger, volar surface 5-5 5 Third phalanx of finger, dorsal surface 6 8 Tip of nose 6 8 [4 5] Head of metacarpal bone, volar 5-6*8 [4*5] Ball of thumb 6 5-7 Ball of little finger 5*5-6 Centre of palm 8-9 Dorsum and side of tongue ; white of the lips ; metacarpal part of the thumb 9 Third phalanx of the great toe, plantar surface, [6 8] Second phalanx of the fingers, dorsal surface... 11 3 [9] Back.... 11-3 [9] Eyelid 11-3 [9] Centre of hard palate 13 5 Lower third of the fore-arm, volar surface 15 In front of the zygoma [H 3] Plantar surface of the great toe 15 8 Inner surface of the lip 20 - 3 Behind the zygoma 22 6 [15 8] Forehead...... . 22 6 [18] Occiput 27-1 [22-6] Back of the hand 31 6 [22-61 Under the chin 33 8 [22 -6] Vertex 33 8 [22*6] Knee 36*1 Sacrum (gluteal region) 44*6 Fore-arm and leg 45 1 Neck 54*1 [36-1] Back of the fifth dorsal vertebra ; lower dorsal and lumbar region 54*] Middle of the neck 677 Upiier arm; thigh; centre of the back 67*7 [31 "6- 40 0] These investigations show not only that the skin is sensitive, but that one is able with great precision to distinguish the part touched. This latter power is usually called the sense of locality, and it is influenced by various conditions. _ The greater the number of sensory nerves in a given area of skin the greater is the degree of accuracy in distinguishing different points. Contrast in this way the tip of the finger and the back of the hand. Sensi tiveness increases from the joints towards the extremities, and, as pointed out by Vierordt, sensitiveness is great in parts of the body that are actively moved. The sensibility of the limbs is finer in TOUCH 481 the transverse axis than in the long axis of the limb, to the extent of | on the flexor surface of the upper limb and on the extensor surface (Landois). It is doubtful if exercise improves sensitive ness, as Francis Galton found that the performances of blind boys were not superior to those of other boys, and he says that " the guidance of the blind depends mainly on the multitude of col lateral indications, to which they give much heed, and not their superiority to any one of them." When the skin is moistened with indifferent fluids sensibility is increased. Suslowa made the curious discovery that, if the area between two points distinctly felt be tickled or be stimulated by a weak electric current, the impressions are fused. Stretching the skin, and baths in water containing carbonic acid or common salt, increase the power of localizing tactile impressions. In experimenting with the com passes, it will be found that a smaller distance can be distinguished if one proceeds from greater to smaller distances than in the re verse direction. A smaller distance can also be detected when the points of the compasses are placed one after the other on the skin than when they are placed simultaneously. If the points of the compasses are unequally heated, the sensation of two contacts becomes confused. An anaemic condition, or a state of venous con gestion, or the application of cold, or violent stretching of the skin, or the use of such substances as atropine, daturin, morphia, strychnine, alcohol, bromide of potassium, cannabin, and hydrate of chloral blunt sensibility. The only active substance said to increase it is caffein. Absolute sensitiveness, as indicated by a sense of pressure, has been determined by various methods. Two different weights are placed on the part, and the smallest difference in weight that can be perceived is noted. "Weber placed small weights directly on the skin ; Aubert and Kammler loaded small plates ; Dohrn made use of a balance, having a blunt point at one end of the beam, rest ing on the skin, whilst weights were placed on the other end of the beam to equalize the pressure ; Eulenberg invented an instrument like a spiral spring paper-clip or balance (the barfesthesiometer), having an index showing the pressure in grammes ; Goltz employed an india-rubber tube filled with water, and this, " to ensure a con stant surface of contact, bent at one spot over a piece of cork, is touched at that spot by the cutaneous part to be examined, and, by rhythmically exerted pressure, waves analogous to those of the arterial pulse are produced in the tube" (Hermann); and Landois invented a mercurial balance, enabling him to make rapid variations in the weight without giving rise to any shock (figured in Landois and Stirling s Physiology, p. 1155). These methods have given the following general results. (1) The greatest acuteuess is on the forehead, temples, and back of the hand and forearm, which detect a pressure of 002 gramme; fingers detect 005 to 015 gramme; the chin, abdomen, and nose 04 to 05 gramme. (2) Goltz s method gives the same general results as Weber s experiment with the compasses, with the exception that the tip of the tongue has its sensation of pressure much lower in the scale than its sensation of touch. (3) Eulenberg found the following gradations in the fineness of the pressure sense : the forehead, lips, back of the cheeks, and temples appreciate differences of -$ to -fa (200 : 205 to 300 : 310 grammes). The back of the last phalanx of the fingers, the forearm, hand, 1st and 2d phalanges, the palmar surface of the hand, forearm, and upper arm distinguish differences of -5*5 to ^ (200 : 220 to 200 : 210 grammes). The front of the leg and thigh is similar to the forearm. Then follow the back of the foot and toes, the sole of the foot, and the back of the leg and thigh. Dohrn placed a weight of 1 gramme on the skin, and then determined the least additional weight that could be detected, with this result .- 3d phalanx of finger, 499 gramme ; back of the foot, 5 gramme ; 2d phalanx, 771 gramme ; 1st phalanx, 82 gramme ; leg, 1 gramme; back of hand, 1 156 grammes; palm, 1 lOtJ grammes; patella, 1 5 grammes; forearm, 1 99 grammes; umbilicus, 3 "5 grammes ; and back, 3 8 grammes (Landois and Stirling). (4) In passing from light to heavier weights, the acuteness increases at once, a maximum is reached, and then with heavy weights the power of distinguishing the differences diminishes (Hering, Bieder- mann). (5) A sensation of pressure after the weights have been removed may be noticed (after-pressure sensation), especially if the weight be considerable. (6) Valentine noticed that, if the finger were held against a blunt- toothed wheel, and the wheel were rotated with a certain rapidity, he felt a smooth margin. This was ex perienced when the intervals of time between the contacts of suc cessive teeth were less than from ^^ to ^f T f a second. The same experiment can be readily made by holding the finger over the holes in one of the outermost circles of a large syren rotating quickly : the sensations of individual holes become fused, so as to give rise to a feeling of touching a slit. (7) Vibrations of strings are de tected even when the number is about 1500 per second ; above this the sensation of vibration ceases. By attaching bristles to the prongs of tuning forks, and bringing these into contact with the lip or tongue, sensations of a very acute character are experienced, which are most intense when the forks vibrate from 600 to 1500 per second. Information from Tactile Impressions. These enable us to come to the following conclusions. (1) We note the existence of some thing touching the sensory surface. (2) From the intensity of the sensation we determine the weight, tension, or intensity of the pres sure. This sensation is in the first instance referred to the skin, but after the pressure has reached a certain amount muscular sensa tions are also experienced the so-called muscular sense. (3) The locality of the part touched is at once determined, and from this the probable position of the touching body. Like the visual field, to which all retinal impressions are referred, point for point, there is a tactile field, to which all points on the skin surface may be referred. (4) By touching a body at various points, from the difference of pressure and from a comparison of the positions of various points in the tactile field we judge of the configuration of the body. A number of " tactile pictures " are obtained by pass ing the skin over the touched body, and the shape of the body is further determined by a knowledge of the muscular movements necessary to bring the cutaneous surface into contact with different portions of it. If there is abnormal displacement of position, a false conception may arise as to the shape of the body. Thus, if a small marble or a pea be placed between the index and middle finger so as to touch (with the palm downwards) the outer side of the index finger and the inner side of the middle finger, a sensation of touching one round body is experienced ; but if the fingers be crossed, so that the marble touches the inner side of the index finger and the outer side of the middle finger, there will be a feeling of two round bodies, because in these circumstances there is added to the feelings of contact a feeling of distortion (or of muscular action) like what would take place if the fingers, for pur poses of touch, were placed in that abnormal position. Again, as showing that our knowledge of the tactile field is precise, there is the well-known fact that when a piece of skin is transplanted from the forehead to the nose, in the operation for removing a deformity of the nose arising from lupus or other ulcerative disease, the patient feels the new nasal part as if it were his fore head, and he may have the curious sensation of a nasal instead of a frontal headache. (5) From the number of points touched we judge as to the smoothness or roughness of a body. A body having a uniformly level surface, like a billiard ball, is smooth ; a body having points irregular in size and number in a given area is rough ; and if the points are very close together it gives rise to a sensation, like that of the pile of velvet, almost intolerable to some indivi duals. Again, if the pressure is so uniform as not to be felt, as when the body is immersed in water (paradoxical as this may seem, it is the case that the sensation of contact is felt only at the limit of the fluid), we experience the sensation of being in contact with a fluid. (6) Lastly, it would^ appear that touch is always the result of varia tion of pressure. No portion of the body when touching anything can be regarded as absolutely motionless, and the slight oscillations of the sensory surface, and in many cases of the body touched, produce those variations of pressure on which touch depends. Theories as to Touch. To explain the phenomenon of the tactile field, and more especially the remarkable variations of tactile sensibility above described, various theories have been advanced. (1) The one most generally known is that of E. H. Weber, as modified or restated by Lotze, Meissner, Czermak, and others. It assumes that, whilst we refer every tactile sensation to a certain position in the tactile field, we do not refer it merely to a point, but to a circular or oval area on the skin, called a circle of sensibility. Further, it is assumed that if two such circles touch or overlap they cannot be individually perceived, and that they can only be 30- individually perceived when one or more circles of sensibility inter vene, or, in other . fi words, when there is a non -irri tated sensory ele ment" between the two points touched (figs. 10 and 11). Each circle of sensibility may be supposed to be innervated by a distinct fibre. Thus, suppose the a. c f C a e ef c Fig. 10. Fig. 11. ~IGS. 10 and 11. Diagrams of Tactile Innervation. (From of the skin to be diagrammatically represented as in figs. 10 and 11, each square would be a " circle of sensibility." In more sensitive regions the squares would be smaller and the number of nerve terminations greater than in less sensitive regions. In fig. 10 the area contains nine "circles" and has nine nerve terminations, whilst in fig. 11, although the total area is the same, there are thirty -six "circles " and thirty-six nerve filaments. If the points of the compasses be placed at a and c in fig. 10 the sensation will be that of one point ; there would also be a sensation of one point if they were placed at c XXIIL 6 1 482 TOUCH FIG. 12. Diagram showing overlapping of "Circles of sensibility." (From Beaunis.) and d ; but if the points touch c ande there will be a double sensa tion, because the "circle" d intervenes. Again, in fig. 11, where the " circles " are much smaller and more numerous, the minimum distance at which two sensations are experienced is much less than in fig. 10, for this would happen when the compasses touch a and d. It will also be observed that the same distance d e in fig. 10 would give a single sensation, whilst it would give a double sensation in fig. 11. But c e in fig. 10 gives a double sensation, and yet the same distance would give a single sensation if the points of the com passes touched adjoining "circles." A "circle of sensibility," however, cannot be regarded as an anatomical magnitude or " cutaneous sensory unit," or, in other words, the area of distribu tion of a single nerve-fibre. The extent of any such hypothetical circle can be altered by practice and attention, and we may therefore assume that the circles overlap, and that even the same area of skin receives numerous nerve fila ments, and that consequent ly, when a body is touched, it excites at once many fila ments. This is illustrated by fig. 12. It will be seen that each area receives a certain num ber of nerve fibres and each nerve fibre supplies fibrils that cross the fibrils of ad- j oining nerves. I f the point of the compass touch at a, it will irritate all the fibres from 1 to 7, but these will not be excited with equal in tensity ; the excitation will be at a maximum at 4, more feeble for 3 and 5, and still more feeble for 2 and 6 ; so that the intensity of the excitation may be represented by the curve above a. In this case the sensation will be that of one point, because all the fibrils have been excited. If the other point of the compass be placed at b, there will be an intermediary region not excited, and two points will be felt. Suppose now the second point of the com passes is moved to c, all the fibrils between the two points a and e are excited, and there is likely a sensation of single contact ; but the excitation of the fibrils 7 and 8 is very feeble, and it is possible, by attention and practice, to leave these out, and then there will be a sensation of two contacts (Beaunis). This mechanical theory has no anatomical basis, except it be the statement made by Krause that the distance of the two points of the compasses at which two points are felt includes in the mean 12 tactile corpuscles. Whilst atten tion has been mainly directed to the skin as the locality where an anatomical explanation is to be sought for, it must not be forgotten that processes may be in operation in the nerve centres. It is well known that irradiation of nervous impulses occur in the nerve centres (see PHYSIOLOGY, vol. xix. p. 29), and it is not unlikely that, when a nervous impression reaches the brain from a particular area of skin, this may be diffused to neighbouring nerve-cells, exciting these, and that then the effect on these cells, in accordance with the law that sensations in nerve centres are referred to the origins in the periphery of the sensory nerve fibres reaching them, will be referred to adjoining areas of skin, or, in other words, to adjoining points in the tactile field. Wundt has propounded a psycho-physiological theory that every part of the skin with tactile sensibility always conveys an impres sion of the locality of the sensation. Each area of skin has a " local colour," and this diminishes from area to area. The grada tion is sudden where the sense of locality is acute and gradual where it is obtuse. " A circle of sensation is an area where the local colour changes so little that two separate impressions fuse into one " (Landois). Practice enables one to notice the changes of local colour, and thus more and more accurately to discriminate points closer and closer together. This theory does not appear to explain anything; it simply restates the phenomena for which an explana tion is desired. SENSATIONS OF TEMPERATURE. The skin is not merely the seat of tactile impressions, but also of impressions of temperature. This depends on thermic irritation of the terminal organs, as proved by the following experiment of E. H. Weber: "If the elbow be dipped into a very cold fluid, the cold is only felt at the immersed part of the body (where the fibres terminate) ; pain, however, is felt in the terminal organs of the ulnar nerve, namely, in the finger points ; this pain, at the same time, deadens the local sensation of cold." If the sensation of cold were due to the irritation of a specific-nerve fibre, the sensation of cold would be referred to the tips of the fingers. When any part of the skin is above its normal mean temperature, warmth is felt ; in the opposite case, cold. The normal mean temperature of a given area varies according to the distribution of hot blood in it and to the activity of nutritive changes occurring in it. When the skin is brought into contact with a good conductor of heat there is a sensation of cold. A sensation of heat is experienced when heat is carried to the skin in any way. The following are the chief facts that have been ascertained regarding the temperature sense. (1) E. H. Weber found that, with a skin temperature of from 15 0- 5 C. to 35" C., the tips of the fingers can distinguish a difference of 25 C. to -2 C. Temperatures just below that of the blood (33 C.- 27 C.) are distinguished by the most sensitive parts, even to 05 C. (2) The thermal sense varies in different regions as follows : tip of tongue, eyelids, cheeks, lips, neck, belly. The "perceptible minimum "was found to be, in degrees C. : breast, 4; back, 9; back of hand, 3 ; palm, 4 ; arm, 2 ; back of foot, 4 ; thigh, 5 ; leg, 6 to "2 ; cheek, 4 ; temple, "3. (3) If two different temperatures are applied side by side and simultaneously, the impressions often fuse, especially if the areas are close together. (4) Practice is said to improve the thermal sense. (5) Sensations of heat and cold may curiously alternate ; thus " when the skin is dipped first into water at 10 C. we feel cold, and if it be then dipped into water at 16 C. we have at first a feeling of warmth, but soon again of cold " (Landois). (6) The same temperature applied to a large area is not appreciated in the same way as when applied to a small one ; thus " the whole hand when placed in water at 29 5 C. feels warmer than when a finger is dipped into water at 32 C. There is every reason to hold that there are different nerve fibres and different central organs for the tactile and thermal sensations, but nothing definite is known. The one sensation undoubtedly affects the other. Thus the minimum distance at which two com pass points are felt is diminished when one point is warmer than the other. Again, a colder weight is felt as heavier, " so that the apparent difference of pressure becomes greater when the heavier weight is at the same time colder, and less when the lighter weight is colder, and difference of pressure is felt with equal weights of unequal temperature" (E. H. Weber). Great sensibility to differ ences of temperature is noticed after removal, alteration by vesi cants, or destruction of the epidermis, and in .the skin affection called herpes zoster. The same occurs in some cases of locomotor ataxy. Removal of the epidermis, as a rule, increases tactile sensibility and the sense of locality. Increased tactile sensibility is termed 7iypcrpselaphesia, and is a rare phenomenon in nervous diseases. Paralysis of the tactile sense is called liyj>opselaphesia t whilst its entire loss is apsdaphcsia. Brown-Sequard mentions a case in which contact of two points gave rise to a sense of a third point of contact. Certain conditions of the nerve centres affect the senses both of touch and temperature. Under the influence of morphia the person may feel abnormally enlarged or diminished in size. As a rule the senses are affected simultaneously, but cases occur where one may be affected more than the other. Herzen states that "limbs which are sleeping" feel heat and not cold (Landois). PAIN. In addition to sensations of touch and of temperature referred to the skin, there is still a third kind of sensation unlike either, namely, pain. This sensation cannot be supposed to be excited by irritations of the end-organs of touch, or of specific thermal end-organs (if there be such), but rather to irritation of ordinary sensory nerves, and there is every reason to believe that painful impressions make their way to the brain along spinal tracks in the spinal cord. If we consider our mental condition as regards sensation at any moment, we notice numerous sensations more or less definite, not referred directly to the surface, nor to external objects, such as a feeling of general comfort, free or im peded breathing, hunger, thirst, malaise, horror, fatigue, and pain. These are all caused by the irritation of ordinary sensory nerves in different localities, and if the irritation of such nerves, by chemical, thermal, mechanical, or nutritional stimuli, passes beyond a certain maximum point of intensity the result is pain. Irritation of a nerve, in accordance with the law of "peripheral reference of sensation," will cause pain. Sometimes the irritation applied to the trunk of a sensory nerve may be so intense as to destroy its normal function, and loss of sensation or anaesthesia results. If then the stimulus be increased further, pain is excited which is referred to the end of the nerve, with the result of producing what has been called anaesthesia dolorosa. Pains frequently cannot be distinctly located, probably owing to the fact of irradiation in the nerve centres and subsequent reference to areas of the body which are not really the seat of irritations. The intensity of pain depends on the degree of excitability of the sensory nerves, whilst its mas- siveness depends on the number of nerve fibres affected. The quality of the pain is probably produced by the kind of irritation of the nerve, as affected by the structure of the part and the greater or less continuance of severe pressure. Thus there are piercing, cutting, boring, burning, throbbing, pressing, gnawing, dull, and acute varieties of pain. Sometimes the excitability of the cutaneous nerves is so great that a breath of air or a delicate touch may give rise to suffering. This hyperalgia is found in inflammatory affections of the skin. In neuralgia the pain is characterized by its character of shooting along the course of the nerve and by severe exacerbations. In many nervous diseases there are dis ordered sensations referred to the skin, such as alternations of heat and cold, burning, creeping, itching, and a feeling as if insects were crawling on the surface (formication). This condition is termed paralgia. The term hypalgia is applied to a diminution and analgia to paralysis of pain, as is produced by anæsthetics.
Muscular Sense.—The sensory impressions considered in this article are closely related to the so-called muscular sense, or that sense or feeling by which we are aware of the state of the muscles of a limb as regards contraction or relaxation. Some have held that the muscular sense is really due to greater or less stretching of the skin and therefore to irritation of the nerves of that organ. That this is not the case is evident from, the fact that disordered movements indicating perversion or loss of this sense are not affected by removal of the skin (Claude Bernard). Further, cases in the human being have been noticed where there was an entire loss of cutaneous sensibility whilst the muscular sense was unimpaired. It is also known that muscles possess sensory nerves, giving rise, in certain circumstances, to fatigue, and, when strongly irritated, to the pain of cramp. Muscular sensations are really excited by irritation of sensory nerves passing from the muscles themselves. We are thus made conscious of whether or not the muscles are contracted, and of the amount of contraction necessary to overcome resistance, and this knowledge enables us to judge of the amount of voluntary impulse. Loss or diminution of the muscular sense is seen in chorea and especially in locomotor ataxy. Increase of it is rare, but it is seen in the curious affection called anxietas tibiarum, "a painful condition of unrest, which leads to a continual change in the position of the limbs" (Landois). See also Physiology. (J. G. M.)