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 contact 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 flagellae, and the short rod-like bodies seen on many Infusoria belong to the same class of sensory organs. Among the Coelentera (hydroid polyps, tubularians, Hydromedusae, Medusae, Anthozoa or sea-anemones) tentacles are found, usually arranged in circles around the mouth or on portions of the body engaged in locomotion, 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 Vermes 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 Nemertina, on the tentacles of Bryozoa, on the head segment of Lumbricidae, and on the tentacles and antennae of Chaetopoda. 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 Hirudinea (leeches) complicated 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 Echinodermata have also special parts devoted to touch, and these show their highest differentiation in the tentacles of the Holothuroida. Arthropoda 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 appearance of these rod-like bodies is seen in fig. 1.

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 from cells. These are supplied with nerves, and are touch organs. Such are found on the edge of the mantle in Lamellibranchiata, where they 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 (Triglidae) have three soft flexible rays detached from the fin, and “the filiform radial appendages of the Polynemidae, the prolonged ventral fins of Osphromenus, Trichogaster, and other Labyrinthibranchs, and of the Ophidiidae," are examples of this class of organs (Owen). The barbels are long slender processes of skin, either single or in pairs, found in the Siluridae, loaches, barbels, cods, sturgeons, and in the parasitic Myxinidae. 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 supposed to have found its food by exploring with the symphysial barbule, as well as by the sense of smell” (Owen). Bodies somewhat 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 gran ula r 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 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 Merkel, or the corpuscles of Grandry (fig. 2). 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 Fig. 2. —Tactile Corpuscles from duck's tongue. more flattened nucleated n , nerve.

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 Pacinian 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. 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 wayanalogous 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. 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 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 (Balaenoptera). 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 endorgan has been found in the nose of the mole, while there are “endcapsules” 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 1 / 360 to 1 / 170 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. 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 of an inch long by 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). One, two, or three nerve fibres pass to the lower end of the corpuscle, wind transversely around it, lose the white substance of Schwann, penetrate into the corpuscle, where the axis cylinders, dividing, end in some way unknown. The corpuscles do not contain any soft core, but are apparently built up of irregular septae of connective tissue, in the meshes of which the nerve fibrils end in expansions similar to Merkel’s cells. Dr Thin describes simple 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 (1 mm. = 1 / 25 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 endorgans 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 to of an inch long and 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 cat (fig. 8). 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 connective 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 the axis cylinder passes into the body, and ends in a small knob or in a plexus. Sometimes a bloodvessel 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 (William Stirling).

Physiology of Touch in Man.— Such are the special endorgans 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 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 the two points of a pair of compasses can be felt. This method, first followed by Weber, is employed by physicians 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. [table]

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, 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.

the transverse axis than in the long axis of the limb, to the extent of ⅛ on the flexor surface of the upper limb and ¼ on the extensor surface (Landois). 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...