TOUCH (derived through Fr. toucher from a common Teutonic and Indo-Germanic root, cf. " tug," " tuck," O. H. Ger. zucchen, to twitch or draw), in physiology, 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 4 Readers of F. Bates's Naturalist on the River Amazons will recollect the account (ii. 344) and illustration there given of his encounter with a flock of this species of toucan. His remarks on the other species with which he met are also excellent.
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 oi temperature, according as the thing touched feels hot or cold These sensations of contact, pressure or temperature are usually referred to the skin or integument covering 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 character of the surface touched. Tactile impressions, properly 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 true sensation of touch.
In the article NERVE (Spinal) the cutaneous distribution of the organs of touch is dealt with.
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). Around the root of the bill in many birds there are special tactile organs, assisting the bird to use it as a kind of sensitive 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 F. S. Merkel, or the corpuscles of Grandry (fig. i). 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 (Ranyier, 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 Pacinian corpuscles ^5^==?^ with thin and very close lamellae. Develop- //^S O\ ments of integument devoid of feathers, / y y^*\\ such as the " wattles " of the cock, the I i J/i-^&ru " caruncles " of the vulture and turkey, \ <TN<!SX'1\ 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. 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 papillae also abound in the naked skin of the nose or snout, as in the shrew, mole, pig, tapir and elephant. In the Ornithorhynchus the skin covering the mandibles is tactile (Owen). In many animals certain hairs acquire great size, length and stiffness. These constitute the vibrissae 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 vibrissae which " act as a staff, in a way analogous to that held and applied by the hand of a blind man (Owen). 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.
FIG. I. Tactile Corpuscles from duck's tongue. n, Nerve.
FIG. 2. Tactile Corpuscle from the hand.
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 lamellae or papillae, contributing no doubt, not only to the growth of the hoof, but also to its sensitiveness. The Cetacea have numerous sensory papillae in the skin. 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. 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. FIG. 3 .-Tactile Corpuscles End-Organs of Touch in Man. In from clitoris of rabbit, man three special forms of tactile "' end-organs have been described, and can be readily demonstrated.
i.^ The End- Bulbs of Krause. These are oval or rounded bodies, from ^J-j to j^j of an inch long. Each consists of a delicate capsule, composed of nucleated connective tissue FIG. 4. End-Bulb from human conjunctiva.
a, Nucleated capsule.
c, Entering nerve-fibre terminating in the core at d.
FIG. 5. End-Bulb from conjunctiva of calf.
mclosing numerous minute cells. On tracing the nerve fibre, t 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. W.. Waldeyer and xmgworth state that the nerve fibrils terminate in the cells, hus making these bodies similar to the cells described by F. S.
Vterkel (ut supra). (See fig. 4.) These bodies are found in the deeper layers of the conjunctiva, margins of the lips, nasal mucous membrane, epiglottis, fungiform and circumvallate >apillae of the tongue, glans penis and clitoris, mucous membrane 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 he mouse, and in the wing of the bat " (Landois and Stirling).
n the genital organs aggregations of end-bulbs occur, known as the " genital corpuscles of Krause " (fig. 3). In the synovial membrane of the joints of the fingers there are larger end-bulbs, :ach connected with three four nerve-filaments.
(2) The Touch Corpuscles of Wagner and Meissner. These are oval bodies, about -j-J^ of an inch long by -gfa of an inch in jreadth. Each consists of a series of layers of connective tissue arranged transversely, and containing in the centre granular matter with nuclei (figs. 2, 3 and 6). One, two or three nerve fibres pass to the lower end of the corpuscle, wind ransversely around it, lose the white substance of Schwann, penetrate into the corpuscle, where the axis cylinders, dividing, :nd in some way unknown. The corpuscles do not contain any soft core, but are apparently built up of irregular septae f connective tissue, in the meshes of which the nerve fibrils end in expansions similar to Merkel's cells. Thin describes imple and compound corpuscles according to the number of nerve fibres entering them. These bodies are found abundantly in the palm of the hand and sole of the foot, where there may be as many as 21 to every square millimetre (i mm. = 5*5 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.
3. The Corpuscles of Vater or P acini. These, first described by Vater so long ago as 1741, are small oval bodies, quite visible to the naked eye, from iV to iV of an inch long and (From Landois and Stirling, after Biesiadecki.) FIG. 6. Vertical Section of the Skin of the Palm of the Hand.
b, Papilla of the cutis vera.
d, Nerve-fibre passing to a touch- corpuscle.
, Wagner's touch-corpuscle. /, Nerve-fibre, divided transversely, g, Cells of the Malpighian layer of the skin.
FIG. 7. Vater'sor Pacini's Corpuscle.
b, Nerve-fibre entering it.
c, d, Connective-tissue en- velope.
e, Axis cylinder, with its end divided at /.
fa to -jVf 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. 7). Each corpuscle consists of 40 to 50 lamellae or coats, like the folds of an onion, thinner and closer together on approaching the centre. Each lamella is formed of an elastic material mixed with delicate connectivetissue 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 the axis cylinder passes into the body, and ends in a small knob or in a plexus. Sometimes a blood-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 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.
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 endbulbs " have also been found. A consideration of these various types of structure show that they facilitate intermittent 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 :he two points of a pair of compasses can be felt. This method first followed by Weber, is employed by physicians in the diagnosis Tip of tongue Third phalanx of finger, volar surface FIG. 8. Aesthesiometer of Sieveking.
of nervous affections involving the sensitiveness of the skin. The following table shows the sensitiveness in millimetres for an adult.
Mm. I-I 2-2-3 4-5 4-4-5 5-5-5 6-8 6-8 5-6-8 6-5-7 5-5-6 8-9 Red part ol the lip Second phalanx of finger, volar surface First phalanx of finger, volar surface Third phalanx of finger, dorsal surface Tip of nose Head of metacarpal bone, volar Ball of thumb Ball of little finger Centre of palm Dorsum and side of tongue; white of the lips; metacarpal part of the thumb 9 Third phalanx of the great toe, plantar surface. . . . 11-3 Second phalanx of the fingers, dorsal surface . . . . 11-3 Back n-3 Eyelid 11-3 Centre of hard palate . 13-5 Lower third of the forearm, volar surface 15 In front of the zygoma 15-8 Plantar surface of the great toe 15-8 Inner surface of the lip . 20-3 Behind the zygoma .... 22-6 Forehead 22-6 Occiput 27-1 Back of the hand 31-6 Under the chin 33'8 Vertex 33' 8 Knee 36-1 Sacrum (gluteal region) 44-6 Forearm and leg 45" I Neck 54-1 Back of the fifth dorsal vertebra; lower dorsal and lumbar region 54" r Middle of the neck 67-7 Upper arm ; thigh ; centre of the back 67-7 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. Sensitiveness increases from the joints towards the extremities, and sensitiveness is great in parts of the body that are actively moved. The sensibility of the limbs is finer in the transverse axis than in the long axis of the limb, to the extent of J on the flexor surface of the upper limb and J on the extensor surface. It is doubtful if exercise improves sensitiveness, 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 collateral 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 compasses, it will be found that a smaller distance can be distinguished if one proceeds from greater to smaller distances than in the reverse 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 congestion, 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, resting on the skin, whilst weights were placed on the other end of the beam to equalize the pressure; H. Eulenberg invented an instrument like a spiral spring paper-clip or balance (the baraesthesiometer), having an index showing the pressure in grammes; F. Goltz employed an India-rubber tube filled with water, and this, to ensure a constant 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 ; and L. Landois invented a mercurial balance, enabling him to make rapid variations in the weight without giving rise to any shock. These methods have given the following general results, (i) The greatest acuteness is on the forehead, temples and back of the hand and forearm, which detect a pressure of 0-002 gramme; fingers detect 0-005 to 0-015 gramme; the chin, abdomen and nose 0-04 to 0-05 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 fa to ^ (200: 205 to 300: 310 grammes). The back of the last phalanx of the fingers, the forearm, hand, first and second phalanges, the palmar surface of the hand, forearm and upper arm distinguish differences of A to fa (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 I gramme on the skin, and then determined the least additional weight that could be detected, with this result: third phalanx of finger 0-499 gramme; back of the foot, 0-5 gramme; second phalanx, 0-771 gramme; first phalanx, 0-82 gramme; leg, I gramme; back of hand, 1-156 grammes; palm, 1-108 grammes; patella, 1-5 grammes; forearm, 1-99 grammes; umbilicus, 3-5 grammes; andback,3-8 grammes. (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. (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 blunttoothed wheel, and the wheel were rotated with a certain rapidity, he felt a smooth margin. This was experienced when the intervals of time between the contacts of successive teeth were less than from lio to BJU of 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 detected 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, (i) We note the existence_of something touching the sensory surface. (2) From the intensity of the sensation we determine the weight, tension or intensity of the pressure. This sensation is in the first instance referred to the skin, but after the pressure has reached a certain amount muscular sensations 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 passing 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) such as would take place if the fingers, for purposes of touch, were placed -in that abnormal position. Again, as snowing 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 forehead, 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 individuals. 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 variation 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.
To explain the phenomenon of the tactile field, and more specially the_ remarkable variations of tactile sensibility above described, various theories have been advanced, but none are satisfactory. (See article " Cutaneous Sensations " by C. S. Sherrington in Schafer's Physiology, ii. 920). Research shows that the sensation of touch may be referred to parts of the skin which do not contain the special end organs associated with this sense, and that filaments in the Malpighian layer (the layer immediately above the papillae ot the true skin) may form the anatomical basis of the sense. The skin may be regarded, also, as an extensive surface containing nervous arrangements by which we are brought into relation with the outer world. _ Accordingly, touch is not the only sensation referred to the skin, but we also refer sensations of temperature (heat and cold), and often those peculiar sensations which we call pain.
Sensations of Temperature. These depend on thermic irritation of the terminal organs, as proved by the following experiment of E. H. Weber: " If the elbow be dipped into a verv 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: (i) E. H. Weber found that, with a skin temperature of from i5-5C.to35 C., the tips of the fingers can distinguish a difference of 0-25 C. to 0-2 C. Temperatures just below that of the blood (33-27 C.) are distinguished by the most sensitive parts, even to 0-05 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 0-4; back, 0-9; back of hand, 0-3; palm, 0-4 ; arm, 0-2 ; back of foot, 0-4 ; thigh, 0-5 ; leg, 0-6 to 0-2; cheek, 0-4; temple, 0-3. (3) If two different temperatures are applied side by side and simultaneously, the impressions of ten 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. (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 compass points are felt is diminished when one point is wanner 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 differences of temperature is noticed after removal, alteration by vesicants, 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 hyperpselaphesia, and is a rare phenomenon in nervous diseases. Paralysis of the tactile sense is called hypopselaphesia, whilst its entire loss is apselaphesia. 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.
Sensations of heat and cold are chiefly referred to the skin, and only partially to some mucous membranes, such as those of the alimentary canal. Direct irritation of a nerve does not give rise to these sensations. The exposed pulp of a diseased tooth, when irritated by hot or cold fluids, gives rise to pain, not to sensations of temperature. It has now been ascertained that there are minute areas on the skin in which sensations of heat and cold may be more acutely felt than in adjoining areas; and, further, that there are points stimulated by addition of heat, hot spots, while others are stimulated by withdrawal of heat, cold spots.
A simple method of demonstrating this phenomenon is to use a solid cylinder of copper, 8 in. in length by in. in thickness, and sharpened at one end to a fine pencil-like point. Dip the pointed end into very hot water, close the eyes, and touch parts of the skin. When a hot spot is touched, there is an acute sensation of burning. Such a spot is often near a hair. Again, in another set of experiments, dip the copper pencil into ice-cold water and search for cold spots. When one of these is touched, a sensation of cold, as if concentrated on a point, is experienced. Thus it may be demonstrated that in a given area of skin there may be hot spots, cold spots and touch spots.
Cold spots are more abundant than hot spots. The spots are arranged in curved lines, but the curve uniting a number of cold spots does not coincide with the curve forming a chain of hot spots. By Weber's method it will be found that we can discriminate cold spots at a shorter distance from each other than hot spots. Thus on the forehead cold spots have a minimum distance of 8 mm., and hot spots 4 mm.; on the skin of the breast, cold spots 2 mm., and hot spots 5 mm.; on the back, cold spots 1-5 mm., and hot spots 4 to 6 mm.; on the back of the hand, cold spots 3 mm., and hot spots 4 mm. ; on the palm, cold spots 8 mm., and hot spots 2 mm. ; and on the thigh and leg, cold spots 3 mm., and hot spots 3-5 mm. Electrical and mechanical stimulation of the hot or cold spots call forth the corresponding sensation. No terminal organ for discrimination of temperature has yet been found. It will be observed that the sensation of heat or cold is excited by change of temperature, and that it is more acute and definite the more sudden the change. Thus discrimination of temperature is similar to discrimination of touch, which depends on more or less sudden change of pressure. The term cold means, physiologically, the sensation we experience when heat is abstracted, and the term heat, the sensation felt when heat is added to the part. Thus we are led to consider that the skin contains at least two kinds of specific terminal organs for sensations of touch and temperature, and two sets of nerve fibres which carry the nervous impulses to the brain. In all probability, also, these fibres have different central endings, and in their course to the brain run in different tracts in the spinal cord. This will explain cases of disease of the central nervous system in which, over certain areas of skin, sensations of touch have been lost while sensations of temperature and pain remain, or vice versa. Tactile and thermal impressions may influence each other. Thus a leg sent to "sleep" by pressure on the sciatic nerve will be found to be less sensitive to heat, but distinctly sensitive to cold. In some cases of disease it has been noticed that the skin is sensitive to a temperature above that of the limb, but insensitive to cold. It is highly probable that just as we found in the case of touch (pressure), the terminal organs connected with the sense of temperature are the fine nerve filaments that have been detected in the deeper strata of the Malpighian region of the epidermis, immediately above the true skin, and it is also probable that certain epidermic (epithelial) cells in that region play their part in the mechanism. Sensations of a painful character may also, in certain circumstances, be referred to the viscera, and to mucous and serous surfaces. Pain is not a sensation excited by irritating the end organs either of touch or of temperature, nor even by irritating directly the filaments of a sensory nerve. Even if sensory nerves are cut or bruised, as in surgical operations, there may be no sensations of pain; and it has been found that muscles, vessels and even the viscera, such as the heart, stomach, liver or kidneys, may be freely handled without giving rise to any feeling of pain, or indeed to any kind of sensation. These parts, in ordinary circumstances appear to be insensitive, and yet they contain afferent nerves. If the sensibility of these nerves is heightened, or possibly if the sensitiveness of the central terminations of the nerves is raised, then we may have sensations to which we give the name of pain. In like manner the skin is endowed with afferent nerves, distinct from those ministering to touch and to temperature, along which nervous impulses are constantly flowing. When these nervous impulses reach the central nervous system in ordinary circumstances they do not give rise to changes that reach the level of consciousness, but they form, as it were, the warp and woof of our mental life, and they also affect metabolisms, that is to say, nutritive changes in many parts of the body. They may also, as is well known, affect unconsciously such mechanisms as those of the action of the heart, the calibre of the blood-vessels and the movements of respiration.
XXVII. 4 If, however, this plane of activity is raised, as by intermittent pressure, or by inflammatory action, or by sudden changes of temperature, as in burning, scalding, etc., such nervous impulses give rise to pain. Sometimes pain is distinctly located, and in other cases it may be irradiated in the nerve centres, and referred to areas of skin or to regions of the body which are not really the seat of the irritation. Thus irritation of the liver may cause pain in the shoulder; disease of the hip-joint often gives rise to pain in the knee; and renal colic, due to_ the passage of a calculus down the ureter, to severe pain even in the abdominal walls. These are often termed reflex pains and their interpretation is of great importance to physicians in the diagnosis of disease. Their frequent occurrence has also directed attention to the distribution in the skin and termination in the brain of the sensory nerves. It is also noticeable that a sensation of pain gives us no information as to its cause; we simply have an agonizing sensation in a part to which, hitherto, we probably referred no sensations. The acuteness or intensity of pain depends partly on the intensity of the irritation, and partly on the degree of excitability of the sensory nerves at the time.
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 special 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 impeded 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 massiveness 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 disordered 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 parafgia. The term hypalgia is applied to a diminution and analgia to paralysis of pain, as is produced by anaesthetics.
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. There are specialized spindle-like bodies in many muscles, and there are organs connected with tendons which are regarded as sensory organs by which pressures are communicated to sensory nerve-filaments. 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 (see EQUILIBRIUM).
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)