Cutaneous Sense Organs
The sense organs of the vertebrate skin include a variety of kinds, among which are organs of touch and pressure, temperature, pain, and in some aquatic forms, taste, and lateral-line organs that are stimulated by currents of water.
Some of these cutaneous organs also invade the mucous membranes that line the passage-ways into the body, particularly in the transitional region between the exposed skin on the outside and the moist mucous lining within.
There is an evolutionary tendency for cutaneous receptors to withdraw from their original position directly at the surface of the skin. This is particularly true in land forms whose skin is exposed to dry air, thus necessitating ecdysis or the periodic removal of the dead outer corneal layer. It is an obvious advantage when ecdysis does not disturb sense organs located in the skin, since specialized neurons are not adapted, as are less elaborated cells, for renewal by mitosis. In most aquatic vertebrates where there is not much need for ecdysis the primary superficial location of the sense receptors is common. Upon assuming a deeper position sensory neurons extend their receptive dendritic processes towards the surface and the source of stimulation. These processes may either end freely between the cells of the skin, or they may terminate in intimate relation with some sort of an accessory apparatus that acts as a stimulator, or non-nervous mechanical intermediary between the receptor itself and the stimulus. The stiff “whiskers,” or vibrissae, of a cat, for example, transfer the stimulus of mechanical contact to deep receptor neurons, dendrites of which form a brush or net around the embedded ends of the vibrissae. Cutaneous receptors are the most numerous and widely scattered of any sense organs.
The tactile sense is the most universal and unavoidable of all the means of communication between the organism and its environment. While it is quite possible to close the eyes and ears against sights and sounds, or to avoid the chemical stimuli that lead to taste and smell, it is not so easy for a body of three dimensions to escape from contact with the gases, liquids, and solids surrounding it.
Tangoreceptors give an idea of the weight, size, shape, surface texture, and general character of objects within reach. Touch is not only the most universal of the senses, but it is the great confirmatory sense, bearing supplementary witness particularly to the major sense of sight. A baby, for example, never discovers its toes by simply gazing at them, for it is only when it accidentally grasps them in its exploring fingers that it joyfully recognizes them as its own.
When touch is sustained beyond transient contact, it becomes interpreted as pressure. Although in many instances the same receptors give the sensations of both touch and pressure, the sense of touch is more definitely localized both in time and space and varies in intensity from that of pressure.
While widely scattered, tangoreceptors are by no means equally distributed throughout the skin, but are placed more abundantly in locations where they are most likely to come into contact with external objects. Thus in man the knee is more sensitive to touch or pressure than the thigh, and the friction areas on the inside of the hands and feet than on the scalp. The concave surface of the curving tail of the spider monkey, Ateles, is bare of hairs and richly supplied with organs of touch, because this active animal makes constant tactile use of the tail in its arboreal adventures. The belly of climbing animals like squirrels is particularly well supplied with tangoreceptors, and even on the inside of the forearm of the human fetus there are transient tactile hairs which hark back to the dim past when arboreal ancestors clung to branches of trees.
Scaly reptiles, in addition to the development of a highly tactile tongue, have either non-sensitive corneal “hairs” similar to those on a bee, that act mechanically as stimulators upon the sensory receptors embedded in the skin below, and are shed at ecdysis with the scales on which they grow, or else certain of the scales are penetrated by small pores, through which cutaneous tangoreceptors gain access to outside contacts.
Not much is known about the organs of touch in the skin of fishes and amphibians. Certain sensory terminal buds have been demonstrated in the connective tissue at the base of the fins in elasmobranchs, while there is little doubt that the barbules dangling around the mouth of such bottom-feeding fishes as the catfish, Amiurus, have a cellular equipment which enables them to act as “feelers.” In general the scaly skin of fishes and reptiles excludes tangoreceptors.
In amphibians the mucous membrane covering the tongue is sensitive to touch, and in snakes there are sense cells in the flickering protrusible forked tongue, although specialized tangoreceptors in the skin have not been demonstrated in these animals. It is in mammals that tangoreceptors reach their greatest elaboration and have been most studied.
Free nerve endings may extend between the cells of the epidermis as far as the stratum granulosum. They are especially abundant in the outer root sheaths of hair follicles which are possibly the only tactile organs for much of the integument (Fig. 663).
Of the types of tangoreceptors that make use of accessory non-nervous structures the simplest are Merkel’s corpuscles (Fig. 664) in which the exposed ends of the dendrites form cups, each fitting under an epithelial stimulator cell. Whenever such cells are agitated by contact, the stimulus is received by the cuplike enfolding tips of the nerve fibers and transferred to the brain, with the resultant sensation of touch or pressure. In this case the non-nervous stimulator cells are larger and more likely to effect a contact with something outside than free intercellular nerve endings would be. Merkel’s corpuscles are particularly abundant on the exploring tip of a mole’s nose or the rooting snout of a pig.
In hairless portions of the skin there are encapsulated tactile endings, located chiefly in the outermost region of the derma. Included among these are: the end bulbs of Krause, the Vater-Pacinian corpuscles, and Meissner’s corpuscles. Krause end bulbs are found in the sensitive mucous membranes of the mammalian tongue and lips, in mammary glands, in the conjunctiva of the eye, in the corium of the finger tips, in the external genitalia of man, and in the moist snouts of grazing cattle (Fig. 665). They may be cold-receptors instead of tactile organs.
Vater-Pacinian corpuscles occur in the subcutaneous layers of the skin, as well as even deeper in tendons and joints, in the pleural walls, around the larger blood vessels in the diaphragm, and in the peritoneum and mesentery of the body cavity (Fig. 666). The Vater-Pacinian corpuscles are the largest and microscopically the most elaborate of all tangoreceptors. The terminal branches of the receptor neuron are enwrapped by successive layers of connective tissue, like a Dutch vrouw’s petticoats, the whole structure being easily visible to the naked eye.
A similar but less elaborate mechanism, Grandry’s corpuscles (Fig. 667), found only along the margins of the beaks of birds like sandpipers and ducks, consists of two large, disclike, non-nervous stimulator cells with nerve endings from a sensory neuron sandwiched in between them, the whole packet being surrounded by an envelope of connective tissue.
The so-called Herbst corpuscles (Fig. 668), likewise found only in the mouthparts of birds, consist of two rows of non-nervous stimulator cells arranged on either side of a neural core within a capsular sheath. They are distributed not only within the mouth cavity but also between certain muscles and in areas of the skin that are comparatively free from feathers.
Finally, Meissnefs corpuscles (Fig. 669), which occur in the friction areas of the skin of primates, resemble highly elaborated Grandry corpuscles, but instead of two, there is an irregular pyramid of non-nervous stimulator cells, interlaced by the branching ends of sensory receptor neurons, the entire mass being enclosed in a sheath. When pressure is applied to a Meissner corpuscle the whole apparatus compresses like an accordion and the squeezed arborizations of the tangoreceptor send along to the brain the sensation which is translated as touch or pressure. Meissner corpuscles are also the sense buds that form the sensory papillae just below the epidermis along the friction ridges of the finger tips, which make the fingers such delicate and effective organs of touch. Removing a glove before shaking hands has a physiological as well as a social reason back of it.
While tangoreceptors are usually concerned only with actual contact, there is some evidence that they aid in determining the presence of nearby objects, possibly through pressure resulting from intervening air currents. A blind person, for example, whose tactile senses have become compensatingly sharpened, is uncannily aware of the neighborhood of a wall which he may be approaching before he bumps into direct contact with it.
The fact that the delicate skin of infants is evidently more sensitive than that of adults is partly due to the stretching of the growing skin, resulting in an increase of the spaces between the tangoreceptors.
Discrimination in touch varies in different regions of the skin. It may be measured by recording the minimal distance at which the fine points of a lightly placed pair of calipers give the impression of two points of stimulation instead of one. See Table XIV.
Every animal and plant has an optimal range of temperature in which it can most successfully carry on its activities and upon which its distribution over the face of the earth very largely depends. Whenever the temperature of the environment departs from this optimum the organism becomes increasingly handicapped, until finally extremes may be reached that make life no longer possible. The range of temperature in which organisms can remain active is somewhat less than that in which they can remain alive. Thus, hibernating animals recover from the torpor produced by extreme cold, as well as from the rigor of inactivity resulting from extreme heat, if cold and heat are not too prolonged or excessive.
The cause of death is not the same at the two extremes of excessive temperature. An Amoeba when subjected to gradually increasing temperatures at first becomes more active, but later entirely withdraws its extended pseudopods until it exposes the least possible surface, finally dying from the coagulation of its protoplasm. If the temperature is slowly lowered from the optimum, the pseudopods remain extended and movement becomes more and more sluggish until in the end the animal freezes in an expanded condition.
Organisms that contain very little water, like seeds and highly desiccated animals, can withstand very low temperatures, even freezing, without losing the ability to resume activity eventually. On the other hand some algae and insect larvae, probably as the result of long periods of continuous adaptation and selection, are able to exist under extremes of high temperature such as occur in hot springs, which are ordinarily fatal to most forms of life. For many years Brues has carried on special observations on the curious and unusual inhabitants of hot springs. He reports some astonishing instances of the Dantesque organisms that thrive in these aqueous infernos.
Davenport says of the distinction between endurance of extreme cold and extreme heat, “the effect of high temperatures is principally chemical, involving the living plasma; that of low temperatures is principally mechanical, involving the water of the body.”
Changes in environmental temperature act as stimuli to organisms, by means of which their behavior is largely regulated. Frogs, which ordinarily swim at the surface of the water, take to the bottom when the temperature drops to about 50° Fahr. Dolbear discovered that tree crickets, Oecanthus, at 60° Fahr. chirp 90 times per minute, varying four chirps to every degree of deviation from that standard. At 55° the number of chirps is 71, and at 75°, 150 chirps per minute, showing that these vociferous insects are so absolutely dependent upon the exact degree of temperature in which they find themselves, that they constitute an audible thermometer of considerable accuracy.
A similar responsiveness to shades of temperature is exhibited by those amphibian Romeos, frogs, toads, and hylas, when they are staging their amorous serenades in the spring of the year.
Thermoreceptors are almost all restricted to the skin. Variations of heat and cold are not felt in the digestive tube except in the transitional mucous membrane at either end. Thermoreceptors are in most cases free nerve endings that are confined to the skin, being absent in the viscera. When a gulp of hot coffee “feels hot all the way down,” it is because thermoreceptors in the skin are stimulated from the inside so that the sensation is erroneously referred to the esophagus.
There are two types of thermoreceptors, namely, caloreceptors and frigidoreceptors, which transmit stimuli resulting in the sensations of warmth and cold respectively. The former lie deeper in the skin and are less numerous than the latter. Degrees of temperature are registered by a thermometer as a continuous series, but there is a point of demarcation between thermoreceptors at which caloreceptors hand over the reception of stimuli giving rise to temperature sensation to frigidoreceptors.
The caloreceptors upon the cheeks and forehead are highly receptive, as are those on the palms of the hands, which are naturally spread out towards an open fire on a cold day. They, as well as frigidoreceptors, are practically absent from the front face of the eyeballs, although receptors for pain, algesireceptors, are abundant there. In fact caloreceptors are frequently confused with algesireceptors, although frigidoreceptors and tangoreceptors are apt to be more closely associated together.
Owing to the proximity of dermal sense organs to each other, and the irregular way in which they are intermingled over the surface of the skin, accurate discrimination between the sensations produced by their stimulation is very difficult.
In the case of thermoreceptors the response resulting from stimulation is particularly conditioned by the number of receptors involved. The total effect experienced, for example, when an exploratory foot is plunged into a cold bath is quite different than when the whole body is submerged at once.
Confusion may also arise with respect to the stimuli that excite the temperature receptors, since the previous condition within the body immediately before change in temperature occurs plays a hangover part in determining the resultant feeling. For example, if one hand is immersed for a few moments in ice water at the same time that the other is thrust into hot water, and then both are withdrawn and plunged together into tepid water which is intermediate between the extremes, the former hand will “feel” warm and the latter cold, although both are being subjected to the same thermal stimulus.
Goldschneider, one of the first to study thermoreceptors thoroughly, located with meticulous exactness in repeated trials on his own skin warm and cold “spots” over definite small areas (Fig. 670), and then, with commendable scientific zeal, cut out pieces of this carefully charted skin which, after sectioning and staining, he subjected to detailed microscopic examination. He found separate distinct nerve terminals that corresponded to the warm and cold spots, and drew the conclusion that the real thermoreceptors were distinct from each other.
Little is known of thermoreceptors aside from free nerve endings, particularly in vertebrates other than man. Almost nothing is known of these organs in non-vertebrates. In the cornea and conjunctiva of the eye, which are sensitive to cold but not to heat, numerous end bulbs of Krause are found. These are suspected of being frigidoreceptors. Ruffini endings, each the interlaced minute terminal branches of a single nerve fiber, are abundant in the eyelids, which are particularly receptive of heat, and probably are rightly to be classified as caloreceptors.
That thermoreceptors are distinct from other cutaneous sense organs is well established, since they not only have a differential distribution, but also behave differently upon being subjected to anaesthesia. Cocaine applied to the skin temporarily destroys the effectiveness of tangoreceptors and algesireceptors, but leaves thermoreceptors unaffected. Following transplantation operations too, the invasion of the newly-formed skin by different receptors is not simultaneous, the sense of touch being the first to be reestablished, followed by the sense of pain, and finally by the temperature senses.
Temperature discrimination in different parts of the body has been tested with the conclusion that caloreceptors of the eyelids are able to register a difference of 1/20 of a degree centigrade, the red lips, 1/10, the outside of the arm, 1/4, and the palm of the hand, 1/2 of a degree centigrade.
Pain is felt through definite specialized free nerve endings termed algesireceptors. Painful sensations are also experienced whenever excessive stimulation is applied to sense organs other than algesireceptors. “The constantly smouldering embers of sensibility,” says Foster, “may at any moment be fanned into the flame of pain.” Thus one speaks of a “piercing tone” as painful, as well as a too brilliant light, immersion in ice water or scalding water, or too strongly applied pressure. In all these cases the painful sensation is received through stimulatory channels already established for other uses than that of giving warning of unpleasantness by pain.
Messages of pain usually demand corrective attention, since they indicate that something has gone wrong. In this sense Sherrington’s term of nocireceptors, or receptors concerned with injuries, is particularly applicable. It is far more important, much as we dislike pain, to be warned of injury in time than to be regaled uninterruptedly with delights optical, auditory, tactile, or chemical, as the case may be, that constantly assail other sense organs, and which rarely if ever admit stimuli that are fatal to life. Algesireceptors, therefore, play a peculiarly important role in safeguarding the well-being of the individual. They are in consequence very numerous and widespread, not only over the skin but also in underlying organs.
It has been estimated from careful mapping of certain cutaneous regions that in the entire skin of a normal human adult there are present 30,000 caloreceptors; 250,000 frigidoreceptors; 500,000 tangoreceptors; and 4,000,000 algesireceptors. This is a ratio of approximately 2:13:25:200, indicating how far algesireceptors outnumber all the others.
When it is remembered that excessive stimulation of any sensory receptor may register warning pain, it will be seen that the provision for pain sensations is beneficently generous. So elaborate an equipment for the detection of pain is far more effective in man than in other animals because the central nervous system where painful stimuli are registered is much more highly developed than in other vertebrates. The fact that fishes probably feel the hook very little allows Isaac Walton to rest in peace and not turn over in his grave.
Sometimes a part of the body, while retaining a high degree of touch or pressure receptivity, is comparatively non-sensitive to pain, as for example the inner wall of the cheek. It is not easy to localize pain at the exact point of stimulation. A toothache may seem to involve the entire jaw, or a cinder the whole eyeball, while lame chest muscles frequently are interpreted as sore lungs.
Fortunately many injuries like wounds do not result in pain in proportion to their extent. For instance the chief pain from the amputation of a leg or arm has its origin in the stimulation caused by cutting the algesireceptors located in the skin. The cortical cells of the brain itself are insensitive to mechanical contact. There are no specific stimuli for pain or localized centers in the brain for painful response alone, as are known to exist for the senses of sight, hearing, smell and taste.
Certain cutaneous sense organs, arranged around the head and down the sides of the bodies of fishes and aquatic amphibians, are termed rheoreceptors, or water-current receptors, which aid the animal in orientation to flowing water.
In embryonic development and adult structure these organs appear to be related to the organs of hearing. Further they probably detect vibrations in the water. Consequently they are often classified with the ears as parts of the acoustico-lateral system.
Primitively a rheoreceptor organ, known as a neuromast, consists of a group of sensory cells lying at the surface of the integument and accompanied by supporting cells (Fig. 671). Each sensory cell has a “hair” projecting from its exposed surface and is innervated by one of the terminal branches of a nerve fiber.
Rheoreceptors are entirely absent in land vertebrates, but four general kinds have been recognized in fishes and aquatic amphibians, namely: (1) lateral-line organs; (2) scattered pit organs; (3) ampullae of Lorenzini; and (4) vesicles of Savi.
Neuromasts of the lateral-line organs tend to be arranged in rows extending from one end of the body to the other. In cyclostomes, some bony fishes, and all aquatic amphibians (including larval stages of anurans) they remain at the surface of the body. Usually, however, rows of neuromasts sink down to form grooves which then close over to form canals embedded in the skin but with numerous pores opening to the outside. In some cases, for example Chimaera, the grooves remain open. These canals have a fairly constant arrangement as seen in Squalus (Fig. 672). The lateral-line canal runs from the head to the posterior end of the body along the outer edge of the horizontal septum which divides epaxial musculature from hypaxial. In the head region there are three pairs of longitudinal canals: a supraorbital above the eye; an infraorbital below the eye; and a hyomandibular extending from the infraorbital posteriorly along the upper jaw region. There may also be a small pair of mandibular canals on the lower jaw and a supratemporal over the top of the head connecting the two sides just posterior to the spiracle. The lateral line canals and the supratemporal are innervated by the vagus (Xth) nerve, except for a small part near their junction to which the glossopharyngeal (IXth) nerve goes. All the other canals are supplied by the facial (VIIth) nerve.
Scattered pit organs are isolated neuromasts, each sunk into a separate pit. In some fishes certain of these pits may be arranged in rows which occupy the same position as a groove or canal in another species.
Whenever lateral fins are so placed that they would naturally extend over a portion of the lateral line, or by their movement cause an agitation in the water which would interfere with the reception of the stimulus produced by external currents of water, the part of the lateral-line canal interfered with may curve out of the way in a detour, thus avoiding interference (Fig. 673).
Various stages of elaboration of the lateral line may occur at the same time in different regions of the same fish, or at different times in the course of development. The eel, Anguilla, for example, during the early “leptocephalus stage,” has only isolated pit organs present on the head region. Young fingerlings develop a lateral line closed anteriorly but open posteriorly, while the adult has the entire system insunken and closed except for craterlike pores along the line.
Among amphibians rheoreceptors are always present in larval and perennibranchiate forms, the lateral line being usually represented by three parallel rows of isolated clusters. Traces of these structures remain in some land amphibians in the form of spots on the skin, indicating the former locality of sensory clusters which have changed in function from rheoreceptors into tactile organs.
The ampullae of Lorenzini, as well as the vesicles of Savi, are highly modified pit organs that occur only in cartilaginous fishes. They are deeply sunken below the surface, the former enlarging into a bulblike cavity at the bottom of an elongated duct (Fig. 674), and the latter becoming entirely closed off from the outside. Both are equipped at the bottom with receptor cells, surrounded by abundant mucus, and terminating with sensory hairs.
The vesicles of Savi are found only in the aberrant electric fish, Torpedo, where they lie along the outer edge of the electric organ on either side, and around the border of the nasal pits. They are supplied by a branch of the trigeminal (Vth) nerve, but their function is unknown.
The presence of abundant mucus in the lateral-line system and related pit organs led formerly to the idea that these organs were glandular and primarily secretory in function, but the discovery by Leydig in 1850 that they have a sensory rather than a motor nerve supply placed them definitely in the category of sense organs. Subsequent experimentation on fishes, by cutting different nerve supplies, showed that these sense organs make the animal receptive to coarse vibrations agitating the surrounding medium, as well as to the sustained impact of water in the form of currents. By means of such organs the fish is enabled to face against the water flow, even in darkness or in turbid waters when no “landmarks” are visible, and so to maintain its position without being constantly carried down-stream, or away with flowing tides and currents.
Rheoreceptors no doubt also help in making a fish aware of the approach of enemies which agitate the water by swimming movements, producing low vibrations that act as warning stimuli.