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See also:TOUCH (derived through Fr. toucher from a See also:common See also:Teutonic and Indo-Germanic See also:root, cf. " tug," " tuck," O. H. Ger. zucchen, to twitch or draw) , in See also:physiology, a sense of pressure, referred usually to the See also:surface of the See also:body. It is often understood as a sensation of contact as distinguished from pressure, but it is evident that, however See also:gentle be the contact, a certain amount of pressure always exists between the sensitive surface and the body touched. See also: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 See also:muscular resistance, as when a See also:weight is supported on the See also:palm of the See also:hand; whilst, finally, the pressure may be so See also:great as to cause a feeling of See also:pain. The force may not be exerted ' Readers of F. See also:Bates's Naturalist on the See also:River See also:Amazons will recollect the See also:account (ii. 344) and See also:illustration there given of his encounter with a See also:flock of this See also:species of See also: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 See also:hair on a sensory surface is pulled or See also:twisted. Touch is therefore the sense by which See also:mechanical force is appreciated, and it presents a strong resemblance to See also:hearing, in which the sensation is excited by intermittent pressures on the auditory See also: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 See also: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 See also:chief sensory surface of touch, it is there that the sense is most highly See also:developed both as to delicacy in detecting See also:minute pressures and as to the See also:character of the surface touched. Tactile impressions, properly so called, are absent from See also: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 See also:article See also:NERVE (See also:Spinal) the cutaneous See also:distribution of the See also:organs of touch is dealt with. The See also:Amphibia and Reptilia do not show any See also:special organs of touch. The lips of tadpoles have tactile papillae. Some See also:snakes have a pair of tentacles on the snout, but the See also:tongue is probably the chief organ of touch in most serpents and lizards. All See also:reptiles possessing climbing See also:powers have the sense of touch highly developed in the feet. Birds have See also: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 See also:security the frosted branches of the See also:Norwegian See also:pine trees " (See also:Owen). Around the root of the See also:bill in many birds there are special tactile organs, assisting the See also:bird to use it as a See also:kind of sensitive probe for the detection in soft ground of the See also:worms, grubs and slugs that constitute its See also:food. Special bodies of this kind have been detected in the See also:beak and tongue of the See also:duck and See also: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 See also:man and mammals, in the See also:outer root-sheath of tactile hairs or feelers. They consist of small bodies composed of a See also:capsule enclosing two or more flattened nucleated cells, piled in a See also:row. Each corpuscle is separated from the others by a transparent protoplasmic disk. Nerve See also:fibres terminate either in the cells (Merkel) or in the protoplasmic intercellular See also:matter (Ranvier, See also:Hesse, Izquierdo). Another See also: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 with thin and very See also:close lamellae. Developments of integument devoid of feathers, such as the " wattles " of the See also:cock, the " caruncles " of the See also:vulture and See also:turkey, are not tactile in their See also:function. In the great See also:majority of See also:Mammalia the See also:general surface of the skin shows sensitive- ness, 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 prehen- sile tails of various species (Ateles). Such papillae also abound in the naked skin of the See also:nose or snout, as in the See also:shrew, See also:mole, See also:pig, See also:tapir and See also:elephant. In the Ornithorhynchus the skin covering the mandibles is tactile (Owen). In many animals certain hairs acquire great See also:size, length and stiffness. These constitute the vibrissae or whiskers. Each large hair grows from a See also:firm capsule sunk deep in the true skin, and the hair bulb is supplied with sensory nerve filaments. In the See also:walrus the capsule is cartilaginous in texture. The marine See also:Carnivora have strong vibrissae which " See also:act as a See also:staff, in a way analogous to that held and applied by the hand of a See also: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 See also:long See also:fine whiskers of the See also:night-prowling felines, and in the aye-aye, a See also:monkey having nocturnal habits. In the See also: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 See also: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 See also:Cetacea have numerous sensory papillae in the skin. Bats have the sense of touch strongly developed in the wings and See also: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 See also:bur-See also:rowing. A See also: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 See also:mouse. FIG.3.—Tactile Corpuscles End-Organs of Touch in Man.—In from clitoris of See also:rabbit. n, Nerve. man three special forms of tactile end-organs have been described, and can be readily demonstrated. 1. The End-Bulbs of See also:Krause.—These are See also:oval or rounded bodies, from a e 6 to i 6- of an See also:inch long. Each consists of a delicate capsule, composed of nucleated connective See also:tissue a, Nucleated capsule. b, Core. c, Entering nerve-fibre FIG.5.—End-Bulb from c terminating in the conjunctiva of See also:calf. core at d. n, Nerve. enclosing numerous minute cells. On tracing the nerve fibre, it is found that the nerve sheath is continuous with the capsule, whilst the See also:axis See also:cylinder of the nerve divides into branches which lose themselves among the cells. W. Waldeyer and Longworth See also:state that the nerve fibrils terminate in the cells, thus making these bodies similar to the cells described by F. S. Merkel (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 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 See also:guinea-pig, See also:ear and body of the mouse, and in the wing of the See also:bat " (Landois and See also:Stirling). In the genital organs aggregations of end-bulbs occur, known as the " genital corpuscles of Krause " (fig. 3). In the synovial membrane of the See also:joints of the fingers there are larger end-bulbs, each connected with three four nerve-filaments. (2) The Touch Corpuscles of See also:Wagner and Meissner.—These are oval bodies, about -j-~- of an inch long by 5 6 of an inch in breadth. Each consists of a See also:series of layers of connective tissue arranged transversely, and containing in the centre granular matter with nuclei (See also:figs. 2, 3 and 6). One, two or three nerve fibres pass to the See also:lower end of the corpuscle, See also:wind transversely around it, lose the See also: d, Nerve-fibre passing to a touch-corpuscle. e, Wagner's touch-corpuscle. f, Nerve-fibre, divided transversely. g, Cells of the Malpighian layer of the skin. -115- to of an inch in breadth, attached to the nerves of the hands and feet. They can be readily demonstrated in the mesentery of the See also:cat (fig. 7). Each corpuscle consists of 40 to 50 lamellae or coats, like the folds of an See also:onion, thinner and closer together on approaching the centre. Each lamella is formed of an elastic material mixed with delicate connective-tissue fibres, and the inner surface of each is lined by a single continuous layer of endothelial cells. A See also: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. Some-times a blood-vessel also penetrates the Pacinian body, entering along with the nerve. Such bodies are found in the sub-cutaneous tissue on the nerves of the fingers and toes, near joints, attached to the nerves of the abdominal plexuses of the sympathetic, on the See also:coccygeal gland, on the dorsum of the penis and clitoris, in the meso-See also: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 end-bulbs " have also been found. A See also: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 See also: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 whichthe two points of a pair of compasses can be See also:felt. This method first followed by See also:Weber, is employed by physicians in the diagnosis of See also:nervous affections involving the sensitiveness of the skin. The following table shows the sensitiveness in millimetres for an adult. Mm. Tip of tongue 1.1 Third See also:phalanx of See also:finger, volar surface 2–2.3 Red See also:part of the See also:lip 4.5 Second phalanx of finger, volar surface 4–4.5 First phalanx of finger, volar surface 5–5.5 Third phalanx of finger, dorsal surface 6.8 Tip of nose 6.8 See also:Head of metacarpal See also:bone, volar 5–6.8 See also:Ball of thumb 6.5–7 Ball of little finger 5.5–6 Centre of palm 8–9 Dorsum and See also: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 11.3 Eyelid 11.3 Centre of hard See also: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•I Back of the hand 31.6 Under the See also:chin 33.8 Vertex 33.8 See also:Knee 36.1 Sacrum (gluteal region) 44.6 Forearm and See also:leg 45.1 See also:Neck . . . . 54.1 Back of the fifth dorsal vertebra; lower dorsal and lumbar region 54. I See also:Middle of the neck 67.7 Upper See also: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 See also: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 See also: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 See also:limb, to the extent of s on the flexor surface of the upper limb and ; on the extensor surface. It is doubtful if exercise improves sensitiveness, as See also:Francis See also:Galton found that the performances of blind boys were not See also:superior to those of other boys, and he says that " the guidance of the blind depends mainly on the multitude of See also: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 See also: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 See also:baths in See also:water containing carbonic See also:acid or common See also: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 See also: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 See also: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, See also:strychnine, See also:alcohol, bromide of See also:potassium, cannabin and See also:hydrate of See also:chloral See also:blunt sensibility. The only active substance said to increase it is caffein. a, Stalk. b, Nerve-fibre entering it. c, d, Connective-tissue envelope. e, Axis cylinder, with its end divided at f. See also: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 See also:balance, having a blunt point at one end of the See also:beam, resting on the skin, whilst weights were placed on the other end of the beam to equalize the pressure; H. Eulenberg invented an See also:instrument like a See also:spiral See also:spring See also:paper-clip or balance (the baraesthesiometer), having an See also:index showing the pressure in grammes; F. See also:Goltz employed an See also:India-See also:rubber See also:tube filled with water, and this, to ensure a See also:constant surface of contact, See also:bent at one spot over a piece of See also: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 See also:pulse are produced in the tube; and L. Landois invented a See also:mercurial balance, enabling him to make rapid See also: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 0.00 to 0.015 gramme; the chin, See also: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 See also: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 See also:differences of ato to 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 to (200: 220 to 200: 210 grammes). The front of the leg and thigh is similar to the fore-arm. 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 t 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, 1 gramme; back of hand, 1.156 grammes; palm, 1.108 grammes; patella, 1.5 grammes; fore-arm, 1.99 grammes; umbilicus, 3.5 grammes; and back, 3.8 grammes. (4) In passing from See also: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) See also:Valentine noticed that, if the finger were held against a blunt-toothed See also:wheel, and the wheel were rotated with a certain rapidity, he felt a smooth margin. This was experienced when the intervals of See also:time between the contacts of successive See also:teeth were less than from to to of a 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. See also:Information from Tactile Impressions.—These enable us to come to the following conclusions. (I) We See also: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 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 See also: (See article " Cutaneous Sensations " by C. S. Sherrington in Schafer's Physiology, ii. 920). See also: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 of 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 See also:world. Accordingly, touch is not the only sensation referred to the skin, but we also refer sensations of temperature (See also:heat and cold), and often those peculiar sensations which we See also: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 See also: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 See also: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 See also: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 15.5° C. to 35°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.: See also: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°; See also:temple, 0.3°. (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 to° 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 See also: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 See also:compass 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 differences of temperature is noticed after removal, alteration by vesicants, or destruction of the epidermis, and in the skin See also:affection called See also:herpes zoster. The same occurs in some cases of locomotor ataxy. Removal of the epidermis, as a See also:rule, increases tactile sensibility and the sense of locality. Increased tactile 'sensibility is termed hyperpselaphesia, and is a rare phenomenon in nervous diseases. See also:Paralysis of the tactile sense is called hypopselaphesia, whilst its entire loss is apselaphesia. See also: Certain conditions of the nerve centres affect the senses both of touch and temperature. Under the See also:influence of morphia the See also: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-See also:canal. See also: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 See also:copper, 8 in. in length by 2 in. in thickness, and sharpened at one end to a fine See also:pencil-like point. See also: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 See also:ice-cold water and See also: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 See also:curve uniting a number of cold spots does not coincide with the curve forming a See also: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 i.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. See also: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 See also: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 See also: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 See also:brain. In all See also:probability, also, these fibres have different central endings, and in their course to the brain run in different tracts in the spinal See also:cord. This will explain cases of disease of the central nervous See also:system in which, over certain areas of skin, sensations of touch have been lost while sensations of temperature and pain remain, or See also:vice versa. Tactile and thermal impressions may influence each other. Thus a leg sent to "See also: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 See also: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 See also:heart, See also:stomach, See also: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 See also: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 See also:mental See also: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. AXvrr. 4If, however, this See also:plane of activity is raised, as by intermittent pressure, or by inflammatory action, or by sudden changes of temperature, as in burning, scalding, &c., 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 See also:shoulder; disease of the See also:hip-See also:joint often gives rise to pain in the knee; and renal See also: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 See also:interpretation is of great importance to physicians in the diagnosis of disease. Their frequent occurrence has also directed See also:attention to the distribution in the skin and termination in the brain of the sensory nerves. It is also See also:notice-able 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 See also:objects, such as a feeling of general comfort, See also:free or impeded breathing, See also: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 See also:law of " peripheral reference of sensation," will cause pain. Sometimes the irritation applied to the See also:trunk of a sensory nerve may be so intense as to destroy its normal function, and loss of sensation or See also: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, See also: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 See also:air or a delicate touch may give rise to suffering. This hyperalgia is found in inflammatory affections of the skin. In See also:neuralgia the pain is characterized by its character of See also: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 See also: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 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 (See also:Claude See also: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 See also: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 anxsetas tibiaruna, a painful condition of unrest, which leads to a continual change in the position of the limbs (see See also:EQUILIBRIUM). (J. G. Additional information and CommentsThere are no comments yet for this article.
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