§ 53. Theory of the Ampullar Sense. — The three semi- circular canals of each ear are set, approximately, in the three dimensions of space. They are also set symmetri- cally in the two ears: the horizontal canals in the same horizontal plane, and the posterior of the one side and the anterior of the other side in parallel planes. It is thus clear that any movement of the head, whatever its direction, must affect the canals. If it is made in a plane which coincides with the plane of a canal, then that canal alone will be stimulated; otherwise, two or more canals will be involved, in different degrees, on the principle of the parallelogram of forces.
Suppose, then, that we are subjected to a rotary motion in the horizontal plane. When the turning begins, the water in the hori- zontal canal lags behind the containing tube; the back thrust of the water bends the cupula; and the nerves of the crista are stimulated. We have the swimming sensation. As the turning § 53- Theory of the Ampullar Sense continues, the water takes up the motion of the canal, and the cupula recovers its equilibrium. When we stop, the water in the tube shoots forward, bending the cupula in the opposite direction; and we have the reversed swimming sensation, which continues until the ampullar organ comes to rest in its nor- ■•=— mal position.
Precisely the same thing happens if we turn round voluntarily on the heels, and precisely the same sort of stimulation is set up, in the other canals, by movements in other planes. — The quality of the am- pullar sensation, at low intensities, is most like that of a diffused pres- sure. The direction of the swimming is not directly sensed, but depends upon association.
Dizziness usually occurs in a very complicated setting. When we have turned upon the heels, and have come to a sudden stop, the kinaesthetic sensations from the limbs are so disposed as to give the perception of a certain position. On the other hand, the swimming in the head, and the arrest of the soft viscera by the body wall, give the perception of a movement in the opposite di- rection. Further, if the eyes are opened, the objects about us seem to be revolving in the same direction as that of our original movement. Here, then, is a conflict, so to say, of three different perceptions: the limbs are at rest, the body is moving one way, and the outside world is moving another way. Oftentimes, to make things still worse, nausea is added.
Under these conditions, introspection is very difficult. Some psychologists deny the sensory character of dizziness altogether, and regard it as the resultant of the conflicting perceptions. Since, however, it may be set up by such local stimuli as the passage of N Exner's Model of a Semicircular Canal.
178 Kinaesthetic Senses an electric current through the ear, it appears to be referable to the ampullar organs. Moreover, there can be no doubt that the sensation in the head, as we turn it sharply to one side or start to turn round, is from the first swimmy, faintly dizzy, so that dizzi- ness would seem to be the quality natural to it at high intensities.
§ 54. The Vestibular Sense. — The sensory function of the maculae is still somewhat problematical. It appears, however, that they play a part in our perceptions of the position of the body as a whole, and possibly of rectilinear movements of the body in space, more especially when these perceptions refer to the vertical dimension. — We have noticed that the people who ride on merry-go- rounds at fairs lean inwards, as the speed of rotation in- creases; they shift their balance, to avoid flying off at a tangent. This is natural enough. What is curious is that, provided the eyes are closed, the riders think that they are sitting uniformly upright. If they are asked to hold a stick before them, vertically, as they revolve, they hold it at a slant; their perception of the vertical has changed. Again, we have noticed that, when the body is immersed in water, as in diving or in swimming beneath the surface, there is never any doubt as to which direction is up and which down. No one with normal sense-organs has ever been drowned by swimming to the bottom when he in- tended to come to the top of the water.
These perceptions are noteworthy when we compare them with the behaviour of the deaf-mutes who lack the sensation of dizziness. If such persons are placed upon a merry-go-round, they do not adjust themselves to a new vertical. They sit bolt upright, and hold the stick to the true vertical,; they must be tied, or they are thrown off by centrifugal force. And it is dangerous for them to bathe § 54- The Vestibular Sense 179 in deep water, even if they know how to swim, because, once they are immersed, they lose the perceptions of up and down, and may drown from sheer confusion of direc- tions.
It seems, then, that we must have some source of per- ception which the deaf-mutes are without; and it is natural to look for it in the vestibular organs. Many psychologists believe that the maculae furnish sensations of pressure, which, under favourable conditions, may be separately dis- tinguished by introspection. — Most of us have noticed, again, as we lay in a sleeping car or were carried in an express lift to the top of a high building, that there were parts of the journey during which the perception of movement altogether disappeared. The eyes must, of course, be closed; there must be no draught of air to suggest that we are moving; and the motion itself must be smooth and uninterrupted. Under these condi- tions, a movement of the body forwards or backwards, up or down, is not perceived, so long as its rate remains the same. Let the rate change, however: let the car slow or quicken its speed: and we are at once aware of our pas- sage through space.
Some psychologists believe that whenever there is accel- eration, positive or negative, of the rate at which the body is travelling, the maculae are brought into action; while, so long as the rate is uniform, they adapt themselves to it, as the ampullar organs adapt themselves to uniform move- ment of rotation. If the movement is in the horizontal plane, forward, backward or sidewise, the vestibular sensa- tions are ordinarily swamped in the mass of sensations from skin, muscle, tendon and joint. If, on the other hand, it is in the vertical plane, — as in going up or down in a 180 KinaestJictic Senses lift, tobogganing, dropping off a high place, swinging,— the sensations appear as a compression or lightness in the head. Like the ampullar sensations, they are often clearest in the form of negative after-images: when a rising lift suddenly comes to a standstill, we have a distinct feeling of squeeze in the region of the ears.
§ 55. Theory of the Vestibular Sense. — The maculae func- tion in much the same way as the cristae. The otoliths which are suspended above the fields of hair-cells corre- spond to the water in the canals; they lag behind, as the supporting structure moves away, and they overshoot or press down upon their support when the movement is arrested. The two maculae of each ear are so situated that the lines of displacement of the otoliths are at right angles to each other; the one moves in a horizontal plane, which slopes at an angle of some 450 from before back- ward, and the other in a sagittal plane, which slopes at the same angle from behind forward. They are thus affected, on the principle of the parallelogram of forces, by move- ments in any spatial dimension.
Applied to the changed perception of the vertical on the merry- go-round, this theory means that the centrifugal force shifts the otoliths outwards; they glide into the position that they would naturally assume if one were really leaning outwards with the body at rest. The true vertical thus seems to slant out, and an effort to restore it means a bend inwards. A like explanation holds of the perception of direction under water; the pressure or pull of the otoliths tells us whether we are head-up or head-down. Destruction or atrophy of the organs would then lead to those insufficiencies of perception which we note in the deaf-mutes.
Whether we may appeal to displacement of the otoliths for the perception of acceleration of movement in the horizontal § 55- Theory of the Vestibular Sense 181 plane depends upon the degree of inertia which they possess. Since this is not known, any hypothesis would be merely guess- work. There is, however, no introspective evidence of their function, as there seems to be in the case of rectilinear vertical movements of the body. — Otolith organs have a very wide range in the animal kingdom, a range that extends from the jelly-fish to man. Their functions appear to be, from the first, tonic and static. It has been found possible, for instance, to replace the otolith of a crustacean, which sheds the contents of the otocyst at the same time that it sheds the skin, by a mass of fine iron-filings. When the artificial otolith is approached by an electromagnet, and the circuit closed, the animal assumes a peculiar position which is directly due to the stimulation of the organ. On the other hand, the otolith sac is the precursor of the organ of hearing, and this fact has been made much of in theories of its function. It has been suggested, for instance, that the maculae of our own ears give the sensations of noise, and possibly of shrill, chirping tones, and that the cristae give us the perception of the direction from which sounds come. There is no evidence for either view. Again, it has been found that the hairs which support the otolith are, at least in some forms, selective: if tones are sounded in their neighbourhood, certain hairs remain passive, while others — varying with variation of the stimulus — are thrown into violent vibration. In the same way, the hairs of the feathered antennae of the male mosquito vibrate selectively to notes which correspond in pitch to the hum of the female insect. It is tempting, in such cases, to regard the vibratile organ as auditory, and to conclude that the stridulating, rasping and tapping sounds of the invertebrates are heard by others of their kind as they are by ourselves. It must, however, be remem- bered that sympathetic vibration is a purely mechanical matter: the excitation set up by the moving hairs might release a reflex, or might be sensed as jar or quiver. Indeed, the close relation of the otolith organ to the organ of touch makes it probable that their sensations would be closely alike. Even the fish, in which the membranous labyrinth has begun to divide into a vestibular and a cochlear portion, do not appear to possess sensations of [82 Kinaesthetic Senses hearing, while they are extremely sensitive to jar of the water in which they swim. On the whole, then, and with the reserve which is due to the obscurity of the subject, we may say that true hearing probably appears at a point fairly high up in the verte- brate series.
References for Further Reading J. Lubbock. On the Senses, Instincts and Intel. \ f Animals, 1SS0.
chs. iv.. v.; K. M. Yerkes. The Dm % Mouse. 1907. ch. v.; M. F. Washburn. The Animal Afind, iqoS, ch. vi.
OTHER ORGANIC SENSATIONS § 56. The Sensitivity of the Abdominal Organs. — We may group together, under the name of visceral sensations, all the sensations, except those of sex, which are derived from the internal organs of the body below the diaphragm. The evidence for the nature and origin of these sensations, and for the part that they play in the make-up of conscious- ness, comes from various sources, surgical, physiological, pathological, psychological. It is not all of the same tenor, and no single line is at all complete.
Let us take, first, the testimony of the surgeon. Opera- tions upon the abdominal organs are now fairly common, and in many cases the anaesthetic employed affects only the outer skin and the underlying connective tissue. Under these circumstances, it has been found that stomach, intes- tine, liver, gall bladder, kidney, — with the investing and interstitial tissues, — as well as the rectal mucous mem- brane, the anterior wall of the vagina, the uterus, the ovaries, the Fallopian tubes with the adjacent portions of the broad ligaments, and probably the part of the testes that is covered by serous membrane, are one and all insen- sitive. The organs may be pinched, stretched, cut, pricked, burned, cooled, and the patient knows nothing about it. On the other hand, the external or parietal peritoneum, which lines the abdominal and pelvic walls, the muscular and serous layers of the diaphragm, and the tunica vagina- lis, are extremely sensitive: the first and third, apparently, 184 Other Organic Sensations to pain alone, the second to pressure and pain. From the surgical point of view, then, the whole mass of visceral sensations must be referred to these three tissues. The sense of satiety, of a full stomach, would come from an up- ward pressure against the diaphragm, colic pains would be due to the pressure or pull of the distended intestine upon the peritoneum, and so on.
On the whole, the results of direct physiological experi- ment support this conclusion. Nevertheless, physiology seems to take away with the one hand only to give with the other. We learn, for instance, that there are enough sensory mechanisms in the abdominal cavity to furnish any number of sensations. We are reminded, also, of the dif- ference between adequate and inadequate stimuli. The inroads of a surgical operation are not natural or normal to the organism; and it is entirely possible, from the physiological standpoint, that sensations may be aroused, by natural processes, in organs which fail to respond to external attack. In particular, we are confronted with the law of reflex pain. Wherever two regions, of low and high sensitivity respectively, stand in close nervous con- nection, an injury done to the former is referred, as sen- sation of pain, to the latter. Now viscera and skin are thus connected. Hence a visceral disturbance might be sensed, locally, as mere vague oppression, while the related area of the skin was the seat of sharp, cutting pain. In other words, the skin may steal from the viscera as taste steals from smell.
If we turn to pathology, we find the facts as definite as those offered by surgery, but of an opposite character. There are cases of visceral anaesthesia, in which the patients cannot tell whether they have eaten enough, but § 56. The Sensitivity of the Abdominal Organs 185 must measure out the amount of their food; in which they do not feel the need of evacuating the bladder and bowels, etc. Now in these cases there is loss, not only of such things as appetite, nausea, repugnance to food, the sense of renewed vigour after sleep, but also of feeling and of perception. Feeling may be lost entirely; the patients are apathetic, incapable of most if not of all the emotions; the agreeable and the disagreeable aspects of experience dis- appear together. We return to these phenomena in § 74. More to the present point is the loss of perception. The patients have no knowledge of the flight of time; in the daytime they depend solely upon the clock, and on wak- ing in the morning they are not aware that they have slept. Plainly, then, if we may trust pathology, visceral sensa- tions exist in some variety, and serve as the raw material of certain complex perceptual processes.
All these differences may be reconciled, though as yet only in a broad and general way, by an appeal to psychol- ogy. The most obvious thing about visceral sensations is their periodical recurrence. For the greater part of the day we know nothing whatever, if we are in good health, of the state of the abdominal organs; introspection of visceral sensation is impossible, because there is no sen- sation to introspect. But for short times, at fairly definite intervals, they loom large in consciousness; either as hunger and repletion, thirst and its satisfaction, or as the characteristic feels that precede, accompany and follow urination and defaecation. It is, evidently, this character of recurrence that fits them for the part they play in per- ception. It proves, further, that they are aroused only under certain peculiar circumstances, on the occurrence of certain changes in their organs. Since surgical operations 1 86 Other Organic Sensations do not reproduce these circumstances or bring about these changes, it is not surprising that the organs should show themselves insensitive to knife and cautery. The negative results are facts, but from the standpoint of psychology they are irrelevant facts. The same thing holds of the results of physiological experiment. The physiologist as- serts that the stomach is insensitive to temperature; if we drink hot or ice-cold water, we have at most a dull heaviness in the stomach, while the warmth or cold is referred by introspection to the body wall. But there may be a law of reflex temperature sensation, as there is a law of reflex pain. And in any case, we have no proof, in such results, that the stomach is altogether insensitive, when stimulated adequately, in the appropriate way.
In general, then, we may say that the external peritoneum is probably responsible for colic pains; that the diaphragm furnishes both muscular sensation and pain; and that there are other sensations, peculiar to the alimentary canal, which are aroused by special stimuli at recurring intervals of time. We have now to examine these latter in more detail.
We distinguish, in ordinary speech, many different sorts of pain: we speak of head-ache, tooth-ache, stomach-ache, ear-ache; of rheu- matic, sciatic, gouty, neuralgic, anginal, labour pains; and, more generally, of dull, acute, fine, massive, throbbing, lancing, gnaw- ing, cutting, aching, boring, shooting, stabbing, rending, chafing, smarting, burning, scalding, racking pains. It is a disputed ques- tion whether pain, as used in these phrases, is like colour a general name for a number of sensible qualities, or like cold the name of a single quality. On the whole, it seems necessary to distinguish two ultimate pain sensations, a fine bright and a heavy dull pain (§ 41); all other differences, however, appear to be reducible to differences of intensity, of diffusion, and of duration or periodicity.
§57- Sensations of Digestive and Urinary Systems 187 Thus, a stabbing pain is a pain of limited area, definitely localised, which suddenly attains a high degree of intensity; a boring pain oscillates between certain limits of intensity; a racking pain swells gradually to its maximum, and then sinks again; and so on.
§ 5 7. The Sensations of the Digestive and Urinary Systems — Thirst is localised in the soft palate, and appears as a diffuse pressure or as a blend of pressure and warmth, — dryness and feverishness. It may be quenched, for a time, by painting the soft palate with acid, or by rinsing the mouth with water, or even by wetting the skin of face and neck: that is to say, by stimuli which cool and moisten the tissues, and start the salivary glands to action. It soon recurs, however, as what Helmholtz called a general feel of the lack of water in the body, though it is still referred predominantly to the soft palate; in this form, it may be assuaged by the injection of fluid into a vein. We may, perhaps, suppose that a deficiency of lymph in the lymph- spaces of the mucosa of the soft palate brings about a relaxation of the membrane, which serves to stimulate Pacinian or kindred organs. Since the alimentary canal is the normal channel by which the body is supplied with water, it is natural that a regulative organ of especial sensi- tiveness should be placed at its entrance.
Next in order, if we travel from above downwards, is the sensation of hard pressure that results from the hasty swallowing of too large a morsel or too large a gulp of liquid. Unlike most of the alimentary sensations, this pressure is localised towards the back. Its downward course can be followed by introspection. It often has upon it a suggestion of nausea, and undoubtedly comes from the oesophagus; but whether from the free nerve-endings of the mucosa or from the layers of striped muscle, we do not know.
1 88 Other Organic Sensatiojis Nausea itself is usually preceded and accompanied by cold sweat and copious salivation. Besides the sensations thus arising, it is sometimes complicated by a bitter taste at the back of the mouth, by sensations of taste and smell from contents returned by the stomach, and by dizziness. Intrinsically, it appears as a sensation of pressure-like quality, localised at the lower end of the oesophagus, and probably due to muscular constriction. — The act of vomit- ing introduces sensations from the muscles of the abdomi- nal wall and the diaphragm, and from the pharynx, all of familiar quality.
Hunger, like nausea, is a complex experience. It is characterised by a dull ache, extending throughout the lower jaw; by pressure in the pharynx; and by the sensa- tions accompanying salivation. Its specific sensation is a dull pressure in the stomach; this rises, through a gnawing soreness, to positive aching pain. The reference of hunger to the stomach is as unhesitating as that of thirst to the soft palate, and its localisation may be made definite by palpation of the skin. The sensation may, perhaps, be ascribed to tension of the stomach, caused by the engorge- ment of the mucosa with the digestive granules developed in the cells. — The sense of repletion, and the oppression of a too hearty meal, are also referred to the stomach, and may be definitely localised by palpation. We can only guess that they are due to a tension of the walls of the stomach, possibly complicated, in the case of overloading, by pressure upon the abdominal wall and the diaphragm.
The intestinal tract from stomach to rectum is usually free from sensation, with the exception of occasional colic pain. Persons with weak digestion, however, report that they generally find, after eating, stationary or travelling § 58. Sensations of Circulatory and Respiratory Systems 189 pressure sensations in this region. The sensations are re- ferred to the front of the body, and may be localised by palpation.
The sensations which appear before, during, and after defaecation are those of pressure and dull pain. In urina- tion, the sensation of warmth is added. The pressure in both cases is somewhat sore, tinged with an ache; it is not unlike the sore muscular sensation (§ 45). The relief that follows evacuation is partly a negative matter; we are freed from a mass of insistent sensation and can turn our attention elsewhere. Oftentimes, however, we get a posi- tive sense of lightness, akin to the bright diffused pressures of health and excitement (§ 46) and perhaps referable to the same organs.
It is tempting to regard the specific sensations of thirst, nausea, hunger, etc., as novel qualities. Closely analysed, however, all the experiences above described seem to reduce to two familiar con- tinua: the light, thrilling sensations which pass into dull, hard pressure, and the sore, achy sensations which pass into dull pain. It would be overhasty to assert that the pressure quality in hunger and nausea, for instance, is precisely the same; but, at any rate, analysis reveals a likeness which is surprising in view of the gross difference between the hungry and the nauseated consciousness. This perceptual difference will occupy us later (§ 104), as will also the question of internal localisation (§ 88).
§ 58. The Sensations of the Circulatory and Respiratory Systems. — For the most part, the action of heart and lungs is not accompanied by sensation. There are times, however, — after severe exertion, or during transient dis- turbance of function, — when the separate heart-beats are clearly sensed as a dull throbbing pressure: it is not easy to say whether the sensations are localised in the body 190 Other Organic Sensations wall or in the heart itself. Further, when we are anxious or worried or apprehensive, we get a characteristic sense of oppression from the carcjiac region. This may appear alone, or in connection with nausea or choking sensations of muscular pressure from the pharynx. The two latter experiences are so well-marked that they have received names in everyday speech; in the first case, we say that we have a sinking of the stomach, in the latter that the heart has come up to the mouth.
Again, it is supposed that circulatory sensations, due to the contraction of the walls of the blood-vessels, play a part in the experiences of shudder, shiver, and goose-flesh. Analysis is here very difficult. In shuddering, however, there is certainly a muscular element; in shivering, this is complicated by sensations of cold; and in goose-flesh we have, perhaps, besides pressure from the hair-bulbs, sensa- tions from the contraction of the unstriped muscles. The tingling sensations that occur when circulation is suddenly restored to a benumbed limb — pins and needles, as they are called — are also, in all probability, to be ascribed to nerve-endings in the walls of the vessels.
A sense of oppression, not unlike that which comes from the cardiac region and often associated to it, appears in the chest in connection with disturbances of breathing. It may be induced by a cramped position of the body, as when one sits for a long time bent over a desk; or by bad air, as in a lecture or concert room; or by an unusual effort of respiration, as in the first stages of running, be- fore the respiratory mechanism has become adapted and the runner, as we say, has got his second wind. At low intensities we speak of it as stuffiness; at the highest, as suffocation; when it is fused with cardiac oppression, as dis- § 59- The Sensations of the Genital System 191 tress. It is probably to be referred to nerve-endings in the alveoli of the lungs, and is prominent in asthma and other dyspnoeic conditions. The bracing sense of fresh air, on the other hand, is due to sensations from the respiratory muscles.
§ 59. The Sensations of the Genital System. — Repro- duction is one of the supreme vital functions; and the study of the reproductive organs, their development and mechanism, belongs accordingly to all divisions of the science of life, — to biology in the narrower sense, to com- parative anatomy, to embryology and histology, and to physiology. These sciences have, as a matter of fact, devoted much attention to the various phases of the problem; they have ascertained facts, established laws, and worked out homologies and correlations. Especial interest has been taken, of recent years, in the questions of sexual pathology, not only in their medical, but also in their moral, social and legal aspects. It is the more sur- prising, then, that very little is known, psychologically, of sexual sensation.
There seems to be no doubt that all the reproductive functions may run their course reflexly, without any sort of conscious con- comitant. Normally, however, the train of reflexes is under cere- bral control. Sexual stimulation implies, besides sensations from the sex-organs themselves, a wide-spread arousal of other organic sensations, and a play of perceptions and ideas, visual, tactual and kinaesthetic. The special sensations of the genital system appear to occur in three stages: first as an irritation or excitement, which accompanies the tumescence of certain erectile tissues; then as sexual gratification, which culminates in the orgasm accompany- ing ejaculation or the consummation of the sexual act; and thirdly as relief, the sexual analepsis which follows coition.
192 Other Organic Sensations Sexual excitement, so far as it is a matter of specific sensation, is usually described as a need of evacuation. Thus, Bain writes that "the appetite that brings the sexes together is founded on peculiar secretions which periodically accumulate within the sys- tem, producing a feeling of oppression until they are either dis- charged or absorbed." * This view, however, is negatived by many lines of evidence. Sexual appetite and its satisfaction may persist after excision of the testes in the male and the ovaries, Fallopian tubes and uterus in the female; in children there frequently exists a well-defined sexual excitement long before there is any true sexual secretion; in adults the sensations may continue to appear long after the sexual glands have discontinued their functions; and, finally, there may be an intensive sexual life in the congenital absence of any sexual glands at all. Moreover, sexual irritability is localised, in the male on the surface of the glans penis, and in the female in the clitoris and the adjacent erectile parts. These organs are sexually sensitive even in the flaccid state, though the degree of sensitivity differs greatly in different individuals and even for the same individual at different times. Both in quality and in its irradiating character the sensation of sexual excitement resem- bles tickling. We do not know how it is aroused: there are no lust spots, akin to the sensitive spots of the skin, and the organs sometimes described as genital corpuscles are certainly not sexual in function.