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crista in the utricle; but while we admit this ingenious suggestion, it does not appear to account for the presence of such bristles in the cristas of the ampullae themselves, nor does it account in any way for the remarkable form of the canals.
Let us consider the base of the stapes moving inwards and outwards with the varying pressures caused by waves of sound. We have already seen how impulses must thus be communicated to the cochlea and the saccule. They must also be communicated to the utricle, into which we find the five openings of the ampulla,' of the canals. It is evident that the pressures must act equally on all the ampullae, and we can conceive the pressure tending to compress or push back the fluid at the mouth of each ampulla. If the pressure were less at the mouth of one ampulla than at the mouth of the other, there would be a flow of fluid in the canal. But as the pressure at the mouth of each canal must be the same, and as the fluid is incompressible, no movement of the fluid can take place, but there will be an increase of pressure on the walls of the ampullae, and the effect of this will be to cause the ampulla and the other end of one canal to separate slightly, and to press the wall bearing the nerve-endings against the osseous wall. If the tube were slightly elastic, such a movement would undoubtedly take place. Thus, with a positive pressure the ampulla and the other end of the canal would tend to move away from each other, thus, -4—..—K and, with a negative pressure towards each other, thus, —^ —. In this way one can conceive of sound-waves acting on the ampullae.1
Again, as the three canals lie in the three directions of space, it has sometimes been held that they may have to do with the appreciation of the direction of sound. An obvious objection to this view is, that as a matter of fact our perception of the direction of sound, or, in other words, of the position in space of the sounding body, is by no means accurate, and that we look with the eyes for the source of the sound, and instinctively direct the ears or the head, or both, in the direction from which the sound appears to proceed. We usually judge of the direction of a sound by making two or more simultaneous or successive observations. If the sound be heard more loudly in the right ear than in the left, we turn the head to the right, and by repeated observations judge of the direction; if the sound be produced at any point equidistant from both ears, we cannot tell its position unless we see what produces it. It is conceivable that we might have been furnished with an organ by which the source of sound might have been at once located, without moving the head, but we do not possess this faculty.2
The relation of the canals in the two ears is remarkable, and must have some physiological significance. This has been carefully investigated in the human subject by Crum Brown,3 who devised a strict method by which the angles formed by the planes of corresponding canals were measured. The results were as follows:—(1) The horizontal (external) canal has its plane sensibly at right angles to the mesial plane, and therefore the two (right and left) horizontal canals are sensibly in the same plane. (2) The planes of the superior (vertical) and posterior (vertical) canals of the same side make nearly the same angle with the
1 Suggestion by Dr. M'Kendrick. Not previously published. 1 See also Preyer, Arch. f. d. ges. Physiol., Bonn, 1887, Bd. xl. S. .186. * Nature, London, 1878, vol. xviii. p. 663; also M'Kendrick's "Physiology," vol. ii. pp. 694-702.
mesial plane, but the right superior and the left posterior canals are not quite parallel—the posterior making with the mesial plane a somewhat greater angle than the superior—as much as 8° more in one instance.
Thus the -j^jr superior canal is nearly parallel to the —--^ posterior,
and in each pair of approximately parallel canals the ampulla of one canal is at the one end, and that of the other at the other end. To sum up—(1) The six canals are sensibly parallel, two and two; and (2) the two horizontal canals are on the same plane, while the superior canal on one side is nearly parallel with the posterior canal of the other. These facts point to the two sets of canals and ampullae acting as one organ, in a manner analogous to the action of two retinae for single vision.
For many years the view has been developing that the semicircular canals communicate nervous impulses, which give a consciousness of the position of the head in space, and thus bring into play the muscular mechanisms necessary for equilibrium. Suppose that the canals and ampullae were free to move in the perilymph surrounding them, a very slight movement of the head in space might cause a small pull or pressure on the nerve-endings in the ampullae, and thus a nervous impulse would be aroused in the nerve fibres distributed to these structures. Such a mechanism would be equivalent to a variation of pressure; but, as we have seen that sound-waves also correspond to variations of pressure, it is evident that such variations might be produced by movements of the stapes or by movements of the head as a whole. Thus the ultimate mechanism of hearing, and that assumed to be the function of the semicircular canals, is essentially the same in' kmc only, according to this view, the pressures communicated by the base of the stapes give rise to sensations of hearing, while those brought about by the varying tensions of the ampullae originate those vague sensations which preside over and regulate the muscular movements required to maintain equilibrium.
These views may be traced to two important researches, the one by Purkinje1 in 1820, and the other by Flourens2 in 1828. Purkinje directed attention to the well-known vertigo that follows rapid rotation of the body in the erect position in a vertical axis. The vertigo is felt, often with nausea, on suddenly stopping the rotatory motion. DuriE rotation, if the eyes are open, objects appear to be moving in an opposite direction to that of the real movement; and when the movement of the body is arrested, a sensation of movement in the same direction continues for a few instants, and external objects still appear to be moving in the opposite direction. This may be termed visual vertigo. It has its counterpart in tactile sensations, when there is a tactile vertigo, on touching bodies during the rotation. In these experiments we may suppose external objects to be rotating round an imaginary axis, and, as have seen, the rotation may be in the same direction as, or in the opposite direction to, that round the vertical axis of the body. Purkinje made the important observation, that the position of the imaginary axis of rotation for external objects depends on the axis of rotation executed at the head; and that if we change the position of the head, after having arrested the movement of the body, we find that the axis of imaginary rotation
1 Med. Jahrb., Wien, 1820, Bd. vi. ; also Bull, der schlcsuchen Gescllsek., 1825-1826.
2 Mem. Acad. d. sc. de I'Inst. de France, Paris, 1828, tome ix. p. 5; also " Recherche* experimentales sur les proprie^s du systeme nerveux," Paris, 1842, p. 488.
remains unaltered. It was also noted that the sensation of movement was experienced even when the eyes were closed, and when no tactile sensations were possible; nor could Purkinje avoid observing the disturbance of equilibrium all have experienced, and the involuntary tendency to steady one's-self by seizing hold of external objects. Purkinje explained the phenomena by supposing that when rotation takes place, the brain, of soft consistence, and surrounded by a thin layer of fluid, has a tendency to drag behind the movements of the walls of the cranium, and that the dragging might, without tearing the brain substance, exercise tension on the cerebellum and cerebral peduncles. He did not associate the phenomena with the semicircular canals; but he supposed that, as during rotation the images of external objects flash across the retinae, the objects appear to be in movement, and the ocular muscles contract spasmodically or intermittently to secure fixation. A struggle takes place between the sensations excited by retinal stimulation and the nervous impulses connected with the muscles. This struggle, in the language of Purkinje, along with the tension on the brain itself, and especially on its peduncles, gives rise to vertigo and the peculiar sensations of movement.
As bearing on this subject, the following interesting experiments, first studied by Purkinje, may be readily performed. Stand erect and rotate round a vertical axis for some minutes, and then stop. Surrounding objects appear still to move round, and there may be vertigo. The direction of the apparent motion depends on the direction of the preceding real motion, and is always opposite to it, and the axis about which the apparent motion takes place is always that line in the head which was the axis of the preceding real rotation. Bend the head forwards, rotate, and then stop. In this case the apparent motion is round a vertical axis. Again, bend the head forwards, rotate, stop, and then raise the head so as to look forwards, and the apparent motion will take place round a horizontal axis, the horizontal axis having been vertical when the real rotation took place, and the floor will seem to rise on one side and to fall on the other. Again, certain relations of these movements to visual phenomena are of interest. During real rotation round a vertical axis, at first the eyes move by a series of jerks; that is to say, they remain fixed for an instant, then follow the head, again they remain fixed, again move with a jerk, and so on. After a little time, however, if the rotation is continued, the eye does not continue fixed for even an instant, but moves more slowly than the head, then quickly makes up to it in speed, then falls behind. At last both head and eye move together. If, then, the rotation stops, the movements of the eyeballs are renewed, and gradually become less and less until they cease. The oscillatory movements of the eyeballs are the effect of the visual vertigo. When the real rotation ceases, the person has still a sensation that his head is turning round, but he feels also that his body is at rest relatively to his head; he sees that objects are at rest relatively to his head, and he concludes that as his head is turning round, his body, and external objects, must be rotating also, and his eyes oscillate exactly as they would do if these movements were real. That these oscillations of the eyeball may be related to irritations of the semicircular canals, is rendered probable by the observation of Cyon, that irritation of the canals excites oscillatory spasms of the ocular muscles at the rate of from 20 to 150 per minute. The eyeballs oscillate about an axis perpendicular to the plane of the irritated canal. Oscillatory movements of the eyeballs of a similar kind may be observed in rapid railway travelling when the person looks at near objects, and probably they form no inconsiderable part of the cause of the fatigue in such circumstances.1 1 M'Kendrick's "Physiology," vol. ii. p. 701.
Flourens, in 1828, announced that injury to the semicircular canals causes motor inco-ordination and disturbance of equilibrium, and that section of the canals produces a rotatory movement of a kind corresponding to the canal that had been divided; in other words, that division of a membranous canal causes rotatory movements round an axis at right angles to the plane of the divided canal; that is to say, the body of the animal always moves in the direction of the cut canal Further, he was led to suggest that, as section of the canals produces disturbances of the same kind as those following injury of the cerebellum, they also have to do with co-ordination of movement.
These observations of Flourens have been corroborated by mamother observers. Vulpian1 thought that the phenomena are due to an auditive vertigo, which .acts on all the organs. Brown-Sequard state! that the same phenomena are caused by irritation of the auditory nerve, but this was denied by Schiff.2 In 1869, Lowenberg,3 after numerous experiments, more especially on the horizontal and vertical canals, in which they were not only divided but subjected to mechanical and chemical excitation, concluded that the difficulties of co-ordination of movement are due, not to paralysis, but to excitation, that these movements are reflex and unconscious, and that the centres for the reflex excitations are in the optic lobes (bird).
An important contribution was made to the subject by Goltz,4 who. although he devised no new experiments of importance, and relied mainly on the observations of Flourens, was the first to clearly formulate the conditions necessary for co-ordination, namely—(1) A central co-ordinating organ, (2) centripetal fibres with their peripheral terminations, and (3) centrifugal fibres with their terminal organs. A lesion of any one of these portions of the mechanism produces inco-ordination. Goltz admitted the contention of Flourens, that the canals are necessary for the equilibration of the body, but he went farther, and contended that they have mainly to do with the equilibrium of the head. The mechanism was stated to be as follows:—The endolymph exercises a stronger pressure on the walls of the ampulla? when the movements of the head bring these to a low level; this pressure irritates the nerves of the ampulla? and excites centripetal impulses, which reflexly cause movements resulting in the equilibration of the head. If, then, the canals are divided or injured, disordered movements ensue from the head losing equilibrium. This theory is often termed the hydrostatic theory of Goltz, and it assumes that the movements of the body are regulated entirely by the more or less conscious appreciation we have of the position of the head in space.
The subject was next investigated by Cyon,5 who found the general loss of co-ordination of movement after section of a canal to be due to the disordered movements of the head. Thus, when the head of a pigeon was fixed so that the beak was directed upwards and the
1 "Lecons sur la physiologie du systeme nerveux," Paris, 1866, p. 600.
2 "Lehrbuch der Pliysiologie," 1858.
3 Arch. f. Augcn- u. Ohrenh., Bd. iii.
1 Arch. f. d. (jcs. Physiol., Bonn, Bd. iii. S. 172.
5 Ibid., 1873, Bd. viii.; "Cours de pliysiologie," St. Petcrsbourg, 1873-74, tonic ii.; "MethodikderphysiologischcnExperimente,"St. Petersburg, 1876, S. 540-547 ; "Rapports physiologiques cntre le nerf acoustique," etc., Compl. rend. Acad. d. sc., Paris, 1876, tome lxxxii. p. 856; "Les organes periphenques du sens dc l'espace," ibid., Paris, 1877, tonie lxxxv. p. 1284; also "Rccherches cxperimentales sur les fonctions des causux semiciretilaires," These, 1878.
occiput downwards, the bird showed irregularity of movement and a loss of equilibrium, as if one of its canals had been divided. If the apparent position of surrounding objects is determined by the position of the head, any sudden change in position might cause for a moment disturbance of motion, and the same result might be expected to follow a strabismus, at all events for the first few minutes. Cyon adjusted glass prisms before the eyes of birds, so as to cause a squint, and the result was loss, for a short time, of co-ordinating power. He also found that after the disturbance of motion following section of the horizontal canal on one side, consisting of oscillations in a horizontal plane and round the vertical axis of the head, and of loss of equilibrium, supporting the head, by placing the finger below the beak, at once caused the movements to disappear, and the bird became calm. Flight after such an injury was almost impossible. At the end of eight or ten days, all the symptoms described disappeared, except that there was still some awkwardness in flying, but eventually this also disappeared, and the movements of the bird were normal. The phenomena following section of the vertical canals were of the same nature, the only difference being as regards direction of movements. The movements of the head were now from below upwards or from above downwards in a vertical plane and around a horizontal axis, and the movements of the body, instead of being mouvements de manage, or round a vertical axis, as after division of a horizontal canal, were now backwards and forwards, as if around a horizontal axis passing through the body. As Cyon held that the perception of the position of bodies in space depends on the position of the head, his general conclusions rest largely on this view. Thus (1) to maintain equilibrium, we must have an accurate notion of the position of the head in space; (2) the function of the semicircular canals is to communicate impressions that give an accurate representation of this position—each canal having a relation to one of the dimensions of space; (3) disturbance of equilibrium follows section; (4) involuntary movements following section are due to abnormal excitations; (0) abnormal movements occurring a few days after the operation, are caused by irritation of the cerebellum.
It might be argued, as indeed was done by Bottcher,1 that the phenomena of Flourens were secondary, and that they were caused by irritation of neighbouring parts of the brain, more especially of the cerebellum. This view, however, is disproved by the fact that the phenomena of Flourens appear immediately after section, whereas similar phenomena, caused by irritation of the cerebellum, do not appear until several days after the irritation. It is contradicted also by the observation that if we divide, not two symmetrical canals, but on the one side a horizontal and on the other a vertical canal, or if we cut two canals on the same side, the phenomena of Flourens do not appear. It is clear, therefore, that secondary lesions of the brain have nothing to do with the phenomena.
Hitherto we have considered the results obtained by experimental physiologists. The functions of the canals have, however, been investigated in another way, by physical experiment and by the application of purely theoretical considerations. This has been done more especially
1 Arch. /. Ohrtnh., Bd. ix.; see also Baginsky, Biol. Centralbl., Bd. i. S. 438; Jacobson, Arch, f, Ohrenh., 1881-2, Bd. i.; 1894, Bd. xxii.; Hogyes, Arch. /. d. gcs. Physiol., Bouu, 1881, Bd. xxvi. S. 658.