§ 46. The Tendinous Sense. — In all cases of severe or prolonged muscular work, we get a sensation which can- not be identified with any one of those derived from muscle. Where we are ourselves active, as in wrestling, pushing, pulling, lifting, we term it effort or exertion; where we are passive, as in supporting a weight, or stand- ing for a long time on one leg, we term it strain. The quality is the same throughout.
This sensation of strain appears to come from the ten- dons, and to have its organs in the spindles of Golgi. Like the dragging sensation from the muscles, it passes over, at high intensities of stimulus, into dull pain.
Tendinous tissue is, as we have said, directly continuous with the muscle fascia and with the periosteum. It is therefore im- possible to isolate the tendon for separate stimulation. In seek- ing to discover the nature of tendinous sensation, we can only rule out the qualities that come from skin, muscle and joint, and note what is left. The remainder turns out to be the sensation of strain. Having reached this result, we find it confirmed by the intimate connection of strain with muscular fatigue and muscular pain, and by the tendency to localise these sensations together in the substance of the limb. — There are, further, certain experiences which seem to depend upon the cooperation of end-organs in muscle and tendon. When we are feeling particularly well, we move lightly, springily, 164 Kinaesthetic Senses jauntily; and if we try to analyse the feeling, we notice light, thrilling sensations, which appear to come from the skeletal mus- cles, and are most marked in the calves of the legs. Again, if we are feeling excited, and try to analyse that feeling, we soon come upon similar bright sensations, most marked in the thighs. Now corpuscles, of the same kind as those of the muscle fasciae and the capsules of the joints, are found in the sheaths and substance of the tendons, and sparsely in the substance of the muscles. They occur also in the sheaths of certain nerve-trunks, and near large vessels. In a word, they represent a widely distributed type of sense-organ. Since the sensations mentioned above are also distributed over a wide area, and since they closely resemble the sensations set up by movement in the joints, we may suppose that they are due to a weak stimulation of the corpuscles. Against this conclusion stands the dull, dead character of the sensation which is produced by pressure on the body of the mus- cle, and which we have ascribed to the corpuscles of the fasciae. It must, however, be remembered that the stimulus here employed is both unnatural and severe. As the pressure spots of the skin give first a bright sensation of contact, and then a sensation of more solid pressure, so may these corpuscular end-organs give first the light, thrilling experiences of health and excitement, and at higher intensities of stimulus something that is duller and harder.
§ 47. The Articular Sense. — If the hand is moved slowly to and fro at the wrist with fingers outspread, while the eyes remain closed, we have, besides the visual image of the movement, various sensations from the skin. There is probably a sense of coolness over the palm; and there are waves of diffused pressure, now across the knuckles, now down the front of the forefinger, now on the sides of the fingers, as the tension of the skin changes. Subcu- taneous sensations from the body of the hand, if re- marked at all, are exceedingly faint: there is no trace of § 47- The Articular Sense 165 strain, and hardly a trace of muscular sensation. We notice, however, a rather massive complex of sensations in the wrist-joint, whose quality is not distinguishable from that of cutaneous pressure.
These sensations come, predominantly, from the end- organs of the articular ligaments. The general impres- sion is the same as that produced by moving the finger over a greasy surface of indifferent temperature; or, bet- ter,, by smearing a finger of the right hand with vaseline and turning it in the loosely closed left hand. Sensations of like quality are derived from the sensitive surfaces of the bones, around or beneath the articular cartilages. They may be brought out by pressing a finger strongly down into its socket, and in this position moving it back and forth.
The corpuscles are distributed most thickly on the flexor side of the articular capsule, and it is easy to see that they must be stim- ulated by the tensions and compressions of the tissue as the limb is moved. The corpuscles of the synovial membrane and of the ligaments that, in some joints, run between the articular surfaces may be stimulated either by movement or, like the surfaces of the bones, by pressure and counterpressure within the joint itself. — The last few sections have raised a question which recurs throughout the study of organic sensation: the question whether we may assume that every sensory end-organ is an organ of sense. It seems natural to reply in the affirmative. If the hair-bulbs, and Meissner's corpuscles, and the free nerve-endings of the epidermis, and such and such other structures furnish sensations, why not all the rest of them? Nevertheless, a decision is not easy. In the skin, for instance, there are many such organs — Ruffini's plumes, Tomsa's knots, Merkel's cells — which cannot with cer- tainty, even with probability, be brought into connection with sensations. As histological research advances, more and more of these structures are brought to light. Either they are, in large 1 66 Kinaestlietic Setts es measure, mere reflex mechanisms, or they are sense-organs which, despite differences of form, yield the same quality of sensation. The second of these hypotheses is, perhaps, the more probable. Differences of form may be attributable to local differences of nutrition and of the conditions of development at large; indeed, several of the corpuscles figured by histologists seem to be related as terms of a single developmental process.
§ 48. Movement and Position, Resistance and Weight. — - We are able, with closed eyes, to tell pretty accurately in which direction a limb moves and how far it travels. We are also able, as a rule, to describe the position of an un- moved limb. These perceptions of movement and position are based upon the articular sensations discussed in the preceding section.
It seems clear, on general principles, that the perception of movement cannot come by way of muscle and tendon. For movements of equal range and of like direction may be made with the limb bent in or stretched out, heavily weighted or held free: that is to say, similar movements may involve very different de- grees of muscular and tendinous sensation. It is hardly possible that a reliable set of perceptions of movement could be built upon so instable a foundation. Moreover, experiment shows that we estimate passive movements as correctly as active; it makes no difference whether the arm, for instance, is laid on a rest and moved by someone else or whether we hold it out and make the movement for ourselves. The perception of movement is, then, as a matter of fact, independent of changes in muscle and tendon.
There is also positive evidence to connect the perception of movement with the joints. In the first place, skin, muscle and joint may be rendered partially anaesthetic by faradisation, that is, by the repeated passage of electrical shocks through them. If, now, the skin and muscles are thus treated, the perception of movement is not affected; if, however, the joints are anaesthetised, it is very considerably impaired. Secondly, there are diseases §48. Movement and Position, Resistance and Weight 167 which bring with them anaesthesia of the skin, or of skin and muscles together, or of the whole surface and substance of a limb. In the former, the perceptions of movement and position are nor- mal; where the muscles are involved, they are not inadequate; but where the joints are also insensitive, the patients can neither adjust their movements nor judge of the position of the diseased limb without the aid of sight.
It may be objected to this view that we are aware of certain movements — movements of tongue, lips, eyeballs — in which the joints are not concerned. That is true. It must, however, be remembered that tongue and lips play against fixed structures, the roof of the mouth and the teeth; and it is, in fact, mainly by ref- erence to these that their movements are estimated. One has only to hold the tongue free in the mouth cavity, and to watch its movements in a mirror, to be convinced that an organ which has only skin and muscle to rely upon is exceedingly obtuse as regards the perception of movement. With the eye things are different. The eyeball turns on the fatty cushion of the orbit very much as the ball of a joint turns in its socket. We thus get a true percep- tion of movement of the eyes, although the sensations are dulled and weakened by the yielding nature of the tissues. — We have spoken throughout of the perception of movement: there is no such thing as a specific movement sensation. What happens is that a complex of articular sensations becomes asso- ciated, with constant repetition, to a visual perception of a move- ment. The association is, in course of time, so firmly established that the occurrence of the articular complex calls up, even with closed eyes, a visual idea of the displacement of the limb. Posi- tion is perceived in the same way. When a limb comes to rest, there is a certain final distribution of tensions and compressions in the ligaments of the joints, which gives rise to a complex of sensations. So long as these persist, we can call up a visual idea of the position of the limb. When they fade out, by adaptation, we lose the visual idea along with them, and can recover the per- ception of position only by making movements which bring the articular end-organs into renewed function. We have all had the experience, on waking from a sound sleep, of a blank loss of arm' 1 68 KinaestJictic Senses or leg: for a moment, we cannot imagine where the thing is. A slight shift of position puts us to rights again.
When we lift a weight, we are working against the force of gravitation; when we overcome a resistance, we are working against mechanical forces in some other direc- tion. The perceptions of weight and resistance seem to be of the same order, psychologically, as their objects are of the same order physically. Their organs are, in the first instance, the sensitive surfaces of the joints. When strain or exertion is involved, the spindles of the tendons also come into play.
The perception of weight may be either passive or active. If the arm is laid out upon a table, and a heavy object placed upon the skin, we have the passive perception: the organs affected are the pressure spots of the skin, and the Pacinian corpuscles of the subcutaneous connective tissue and the muscle fasciae. Under these circumstances, our discrimination of weight is inaccurate; it corresponds to the perception of movement by tongue or lips. When the weight is lifted, and the perception thus becomes ac- tive, discrimination is much more delicate; it corresponds to the articular perception of movement.
Some psychologists separate the perception of weight from that of resistance, and refer the former to the tendons and the latter alone to the articular surfaces. And indeed it seems natural, at first thought, to say that the pull of a weight must draw the sur- faces of the joint apart, while the resistance of an inert body must jam them together. Really, however, the very fact that the arm is braced and set for lifting means that it is strongly bound at the joints; and the heavier the weight to be raised, the greater is the articular pressure. Moreover, it makes little difference in the perception of lifted weights whether the arm is flexed or ex- tended, whether the hand grasps the object loosely or tightly; so that the perception is, at least in some degree, independent of the state of the tendons.
§ 49- The Alleged Sensation of Innervation 169 Again, some psychologists hold that the sensitive surfaces of the bones play a large part in the perception of movement. The surfaces must, of course, rub against each other as the limb is moved; and we know that patients who suffer from anaesthesia of skin and muscles perceive movement and position more accu- rately when the joints are pressed together than when they are pulled apart. On the other hand, the rubbing can be but light in the case of passive movements, where we are relieved of the weight of the limb. And it is possible, in the pathological con- ditions, that the sensations from the bones serve, by adding to the sum of articular sensation in general, simply to call the pa- tients' attention to the diseased limb, without contributing directly to their perception of its movement. The strongest bit of nega- tive evidence lies, however, in the fact that we may get the same perception of movement with very different accompaniments of resistance. — Here as before it is important to remember that we are deal- ing not with sensations but with perceptions. In movement, we have a complex of sensations from the joint capsule, along with varying sensations from skin, muscle, tendon and articular sur- face. In weight and resistance, we have a complex of sensations from the articular surfaces, coloured at high intensities of stimulus by tendinous strain, along with varying sensations from skin, muscle and joint capsule. There is no specific sensation of weight or of resistance.
§ 49. The Alleged Sensation of Innervation. — We have so far taken it for granted that sensations are due to the action of stimuli upon a sense-organ. Light falls on the eye, or a contraction of the muscle fibres squeezes the muscle spindles; the excitation thus started is carried, by afferent sensory nerves, to the brain; and we get the sensation of colour or of fatigue. The course of the nerve process which arouses sensation is always from without inwards, from periphery to centre.
170 KinaestJictic Senses There is, however, one case — that of the sensation of effort or exertion — in which this view has been chal- lenged. We have ascribed the sensation to the Golgi spindles (§ 46), and have thus put it upon the same plane with all other sensations. Some psychologists believe, on the contrary, that it is an outgoing sensation, due to the discharge of motor excitations from brain to muscle. Its nerve process would then run from within outwards, from centre to periphery.
The arguments, are drawn, in the main, from pathology. A patient who cannot move a leg, or who cannot turn his right eye outwards, may nevertheless believe that he has made these movements; he will assure his physician that he feels the weight of the moved limb, or the turn of the eyeball in the orbit. Since no movement is made, these sensations must, apparently, come from the centre, must accompany the outgoing current of innervation.
We notice, however, in observing such patients, that the effort to move the diseased leg always means a shift of the hips, and various jerks and twitches in the sound leg; and that the effort to move the right eye always means actual movement of the left. Here, then, are sources of kinaes- thetic sensation which might easily give the illusion of movement in the unmoved part. Besides, there are patho- logical facts to be quoted on the other side. In certain diseases,- a patient may make quite extensive movements of the limbs, without being aware of the fact; indeed, he is surprised, when his glance falls upon arm or leg, to find that it has changed its position. Since these movements are made, they must have been innervated; since they are made unconsciously, the innervation cannot have aroused any sensation of effort.
The following experiment tells very strongly against the exist- ence of the innervation sensation. If two objects of the same weight but of different size are lifted successively in the closed hand, or even if they are lifted by a string attached to the finger, the smaller appears the heavier. The observer may have seen them weighed, and may be convinced that the weights are physi- cally equal: nevertheless, the illusion persists. If, now, the judg- ment of weight depended upon a sensation of innervation, this result would be impossible: the observer, knowing that the same amount of energy is required to raise both objects, would inner- vate his muscles to the same degree.
We explained a similar illusion of the resting skin (§ 42) by the difference in the slope of the pressure gradient. This factor is here replaced by visual association. In the vast majority of cases, the larger of two like objects is also the heavier. Hence we have learned to interpret size as weight; when we see a large thing, we unconsciously innervate the muscles for a heavy thing. The association holds, in spite of our knowledge that the weights are equal: we lift the larger object as if it were heavy, the smaller as if it were light. The former then flies up, giving us the kinaes- thetic sensations that light things arouse, and we judge it to be lighter than its companion.
§ 50. Some Touch-blends. — We are now able to analyse the touch-blends mentioned in § 39. The difference between hard and soft, for instance, is mainly a difference in degree of resistance offered to the hand; and this means a difference in the degree of pressure exerted by the one articular surface upon the other. The distinction thus belongs to the joints rather than to the skin. Again, the difference between smooth and rough is a difference, first, between continuous and interrupted movement, and sec- ondly between uniform and variable stimulation of the pressure spots of the skin. The distinction thus belongs to joints and skin together.
172 Kinaestlietic Senses Sharp and blunt differ, primarily, as pain and pressure: a thing is sharp if it pricks or cuts, blunt if it sets up diffuse pressure sensations. Here, however, as in all the touch- blends, visual association plays a very large part.
Wetness is a complex of pressure and temperature. It is possible, under experimental conditions, to evoke the perception of wetness from perfectly dry things, — flour, lycopodium powder, cotton wool, discs of metal; and it is possible, on the other hand, to wet the skin with water and to evoke the perception of a dry pressure or a dry tempera- ture. Not the moistening of the skin, but the fitting dis- tribution of pressure and temperature sensations, gives rise to the perception of wetness. Other modes of distribution of the same sensations produce the perception of dryness.
Clamminess is a mixture of cold and soft: the cold sen- sations and the pressure elements in the softness must be so distributed as to give the perception of moisture. The clammy feel of a wet cloth may be got by laying the finger on a loosely stretched rubber membrane, and sending a puff of cold air over it at the moment of contact. Oiliness is probably due to a certain combination of smoothness and resistance; movement seems to be necessary to its percep- tion. Clinging, sticky feels may be obtained from dry cotton wool.
References for Further Reading §§44-50. A. Goldscheider, Gesammelte Abhandlungen, ii., 1898; V. Henri, Revue genirale sur le sens musculaire, in D Annie psycholo- gique, 5?ne anuee, 1899, 399; C. S. Sherrington, The Muscular Sense, in Schafer's Text-book, ii., 1900, 1002 ff.; I. M. Bentley, The Synthetic Experiment, in American Journal of Psychology, xi., 1900, 414 ff.; R. S. Woodworth, Le Mouvement, 1903; W. Nagel, Die Lage-, Bewe- gungs- umi Wider standsempfindungen, in Nagel's Handbuch, iii., 1905, § 5^- Kinaesthetic Organs of the Internal Ear 173 §51. The Kinaesthetic Organs of the Internal Ear. — We have found, in the capsules of the joints, organs which re- ceive their stimulation from movement of the limbs, and which give us the perceptions of movement and position of these members. We have now to consider certain struc- tures of the internal ear, which represent kinaesthetic organs of a different kind. They are stimulated mechan- ically, by the acceleration of a mass-movement through gravitation, inertia or centrifugal force, and they give us the perceptions of movement and position of the head and, perhaps, of the whole body. They are known as the cris- tae ampullares of the semicircular canals, and the maculae acusticae of the vestibule.
In § 28 we dealt with the cochlea of the internal ear, the part of the membranous labyrinth which forms the end-organ of the cochlear nerve and furnishes sensations of hearing. We are now to discuss the function of the remaining portion, the vestibule and the semicircular canals, which together form the end-organ of the vestibular nerve.1 There are five cell-groups, in each ear, to which the fibres of this nerve are distributed: the maculae of the utri- cle and saccule, the two divisions of the vestibule; and the cristae in the ampullar enlargements of the three semicircular canals. Maculae and cristae are all of the same general type: there is a local thickening of the membranous wall, upon which rests a little field of hair-cells. In the maculae, however, the hairs support a mass of tiny crystals of carbonate of lime, the otolith; in the cristae, they project freely, like a camel's-hair brush, into the ampullar cavities. The otoliths are enclosed in a homogeneous, viscous substance, which also interpenetrates and surrounds the ampullar hairs. The mass which is moved in the maculae is, therefore, the otolith; in the cristae it is the cemented brush-like structure, which has been termed the cupula. — 1 In addition to the models of the internal ear mentioned on p. 109, Exner's Bogengangmodell (shown in Fig. 24, p. 177) and Otolithenmodell will be found useful for demonstration.
174 Kinaesthetic Senses The study of the semicircular canals and the vestibule presents a curious difficulty to psychology, a difficulty the reverse of that which we have just met in our discussion of the sensitivity of mus- cle, tendon and joint. There, we had a tangled complex of sensations, and the problem was to distribute them among the available end-organs. Here, we have highly developed end-organs, but no very obvious group of sensations to refer to them. More- over, there can be no doubt that the functions of the vestibular nerve are, in large measure, reflex: by virtue of its cerebellar con- nections it plays a large part in the regulation of what one may call the tone of the muscular system; the impulses normally pro- ceeding from it keep the muscles trim and braced, while the cutting off of these impulses has an atonic effect similar to that produced by section of the dorsal roots of the myel. Under these circumstances, it is not surprising that different investigators should take very different views of the kinaesthetic importance of cristae and maculae. The following sections, however, represent the gen- eral trend of current psychological opinion.
§ 52. The Ampullar Sense. — If you turn round rapidly upon the heels several times in succession, and then come to rest with closed eyes, you have a sensation which can only be described as a swimming in the head. Its apparent direction is opposed to the direction of the actual move- ment, so that it wears the appearance of a negative after- image. Having once noticed it, you are afterwards able to notice, as you begin to turn, a swimming whose di- rection is the same as that of the movement of rotation. The sensation seems to circle through the head, and its plane changes with change of the head's position. If, for instance, you turn round with the head bent forwards on the chest, and suddenly raise it to the normal attitude after you have stood still, the plane of the swimming changes, as suddenly, from horizontal to transverse vertical; if you § 52. The Ampullar Sense 175 turn with the head inclined on the shoulder, and raise it afterwards in the same way, the plane changes from horizontal to sagittal.
This swimming sensation, which with practice may be observed to follow a quick movement of the head in any direction, comes from the cristae of the semicircular canals. At high intensities, it passes into dizziness or vertigo.
It is significant that we find the canals fully formed, and con- joined with a merely rudimentary cochlea, in animals, such as birds and fishes, which have to balance in the surrounding medium. Their size and accessibility, in these lower vertebrates, makes ex- periment easy: the canals may be severed, plugged, or extirpated without further injury to the organism. If, now, a single canal is cut, say, in a pigeon, we note, as the result of the operation, a general slackening of the whole muscular system, and also a dis- turbance of movement in the plane of the severed canal. On the one hand, the bird seems weakened: its flight is feeble, its legs bend inwards; on the other, it is subject to certain forced move- ments. If, for instance, the right horizontal canal is cut, the pig- eon keeps up a pendular motion of the head sidevvise, to the right and back again; it also tends, in walking, to bend to the right and so to circle round and round, instead of moving straight forward. These symptoms vary with the extent and the standing of the injury. Where the lesion is one-sided, there may presently be complete recovery; if both sets of canals are extirpated, the muscular weakness may end in general muscular atrophy, and all coordinated movements are thrown into confusion.
It is clear, then, that the canals constitute an organ which serves to regulate the tone of the muscular system. But it seems clear, also, that they stand in a special relation to movements of the head. Injury done them not only cuts off the tonic impulses, but in addition gives rise to abnormal impulses which arouse the abnormal movements. This dynamic function might, like the tonic, be reflex. Probably, however, it is attended by sensation, the quality of which is lost to ordinary observation in the complex 176 KinaestJietic Senses of cutaneous and kinaesthetic sensations which we have described in previous sections.
The evidence for sensation comes partly from experiments made upon the normal human subject, and partly from pathology. Experiments upon rotation, carried out under strict conditions, reveal the swimming and dizziness of which we have spoken. Sim- ilar sensations are evoked by syringing the ear, or passing an electric current through it. Further, it is found that about half of the deaf-mutes in our large institutions cannot be made dizzy by rota- tion; they do not stagger when thfe movement ceases, nor do they show the compensatory twitching of the eyes which is normally a symptom of dizziness. Now, autopsy proves that in about 50 per cent, of deaf-mute ears there is lesion or degeneration, not of the cochlea alone, but of the whole internal ear. This correspondence furnishes a strong argument for referring the sensation of dizziness to the canals.