The Body at Work: A Treatise on the Principles of Physiology

by Alex Hill

The organ of Corti is spread out on the basilar membrane. It is an epithelial structure of extreme regularity and uniformity. Near to the edge by which the basilar membrane is attached to the spiral lamina rests a double row of rods of Corti, stiff pillars which lean one towards the other, over the tunnel of Corti, the convex head of the outer rod fitting into a concavity in the head of the inner one; in some places one outer rod fits against two inner rods, as the latter are rather the more numerous. On the inner side of the inner rod is seen, in transverse sections a single plump cell filled with cloudy protoplasm, and bearing on its free surface a tuft of very short hairs. On the outer side of the outer rod are three or four hair-cells, each with a cloudy outer segment containing the nucleus, a granular middle segment, and a stiffish stalk, which attaches it to the basilar membrane. Between the hair-cells are supporting cells, thicker below, tapering above, containing in their substance a firm fibre. Still farther to the outer side are epithelial cells, of no special interest. The purpose of the rods of Corti and the supporting cells is to give attachment and support to a reticulated membrane of exquisite delicacy, through the oblong apertures of which the hairs of the hair-cells project into the endolymph. The spiral lamina is traversed by a vast number of fibres of the auditory nerve, which, losing their medullary sheaths, pass across the tunnel of Corti as naked axons, to end amongst the hair-cells. Above the organ of Corti, attached by its edge to the spiral lamina, is a thick, gelatinous, fibrillated structure—membrana tectoria—which rests as a coverlet on the surface of the organ. It has been supposed that it serves to damp the vibrations of the hairs after they have been set in motion by the waves passing across the scala media; but it not impossibly plays a more active part in hearing than this.

Fig. 40.—Organ of Corti.

The spiral lamina, on the left of the drawing, gives attachment to the membrane of Corti, which stretches to the opposite wall. Below the membrane is a bloodvessel which runs its whole length beneath the tunnel of Corti. The tunnel is formed by pillars—the inner on the left, the outer on the right—which meet above it. On the left of the inner pillar is a hair-cell; to the left of this a nerve-cell with two nuclei. To the right of the outer pillar is a space; to the right of this four hair-cells alternating with four supporting cells, which hold up the reticulated membrane through apertures in which the tufts of hairs project. Three nerve-fibres are seen in the spiral lamina; they cross the tunnel to ramify between the rows of outer hair-cells. The lamina tectoria rests upon the tufts of hairs.

The organ of Corti is, beyond doubt, the apparatus which analyses sounds; but the problem of the way in which it responds to tones of different pitch, or analyses compound tones, is not as yet even approximately solved. To escape the acoustic difficulties which have to be faced by anyone who endeavours to expound the theory of the cochlea as a piece of analytical apparatus, various suggestions as to the possibility of an action en masse have been advanced. For example, the basilar membrane has been compared to a telephone-plate which takes up vibrations and transmits them through the auditory nerve to the brain. But if the organ of Corti be the transmitter, there is no ear in the brain to analyse the vibrations given out by a receiving telephone-plate; and without a receiving plate and a listening ear a telephone is purposeless. According to this hypothesis, the basilar membrane vibrates as a whole, moving the hair-cells in various “patterns”; the pressure of the hairs against the tectorial membrane causing irritation of the cells which bear them, and hence producing stimulation of various groups of nerves. Other pattern theories are somewhat similar. But it is obvious that all hypotheses of the vibration of the whole of the basilar membrane, or of large parts of it, simultaneously, leave to the mind the responsibility of reading the pattern which the impulses generated in the organ of Corti make in the brain. It is conceivable that every fraction of a semitone which a musician can discriminate, and every combination of tones which he can analyse, is transmitted to the brain by a large number of co-operating nerve-impulses; but such a theory involves a complexity of mental associations difficult to contemplate.

According to the general principles enunciated in this book, analysis of stimuli is the function of sense-organs. It cannot in all cases be compared with the analysis effected in a physical laboratory; nor is this necessary; but it must be carried so far that nerve-impulses which have no specific qualities apart from their source shall give rise to effects in consciousness which have no basis other than the topographical distribution of the said impulses in the brain. There may be sensory impulses of different orders; there may be in the brain psycho-physical substances which react to impulses of various orders in various ways; but until we have some hint of the existence of specific impulses and specific psycho-physical substances, we are not justified in postulating their existence simply in order that we may escape from physiological embarrassments.

The organ of Corti has in the highest degree the appearance of a piece of apparatus for the analysis of sound. If the basilar membrane, with the cells which rest upon it, be cut out and laid flat, the suggestion of some kind of instrument is very strong. It is a long narrow ribbon, narrowest at the bottom of the spiral, increasing to about twice the width at the apex. It is crossed by radiating fibres, presumably elastic. The cells which rest upon it carry vibrating hairs, and are supplied with nerves. The rods of Corti hold up the reticulated membrane, which keeps the hair-cells in place. It is not to be wondered at that when its structure was first discovered it was thought that the problem of the analysis of musical tones was solved. If two pianos in perfect tune are in the same room, when one is played the corresponding wires of the other twang. Anyone who sings into a piano, whilst the loud pedal raises the dampers, feels an increased fulness in his voice. This is the familiar phenomenon of resonance. Why should not the fibres of the basilar membrane resonate to the tones conveyed to the ear—the shorter ones at the base of the cochlea to high tones, the longer ones at the apex to low tones? This is the order in which we should expect the pulsations of sound which ascend the scala vestibuli to be taken up—the more rapid, near its commencement, the less rapid farther up it. But an explanation of the physics of the selection of vibrations of different frequencies by different sets of the elements which make up the organ of Corti, if such selection occurs, is still to seek. In the first place, the fibres of the basilar membrane are so exceedingly short. What could a fibre less than 0·5 millimetre in length make of the vibrations of a 36-foot organ-pipe? Even if this objection be waived, as certain eminent physicists hold that it may be, there is not a sufficient difference in length between the longest and the shortest fibres to account for the great range of tones which we are able to discriminate; nor is there any evidence that some fibres are more tightly stretched than others.

A further consideration which tempts physiologists to look upon the organ of Corti (including the basilar membrane) as a series of resonators is the somewhat remarkable agreement between the number of separate pieces of apparatus of which it appears to be composed and the number of different musical sounds which, if it were a series of resonators, it might be called upon to discriminate.

The squeak given by a bat at each turn in its flight has a pitch of about 11,000 vibrations to the second—the sixth E above the middle C (Tyndall). In a group of persons listening for the squeak there are usually some who cannot hear it. Above this the range of hearing is very variable. The suddenness of transition from perfect hearing to total want of perception makes experiments with small pipes or with a siren somewhat amusing, when a number of persons are tested at the same time. One complains that the note is intolerably loud and shrill, whilst others assert that there is perfect silence. Thirty-three thousand vibrations is usually regarded as the upper limit for the human ear, but certain physiologists place it at 40,000, or even higher. The upper limit is of little consequence, since there is very little power of discriminating rapidities above the highest note used in music—the piccolo stop of the organ, with a pitch of 4,096. It is possible that a sound with a lower frequency than 27 (the contra-bassoon) may be heard as a tone—16 according to certain writers; but again our power of discriminating very low notes is small. Over a certain range a skilled musician can tell that a note is out of tune when it is one sixty-fourth of a semitone higher or lower than it ought to be. If we assume that by allowing equal sensitiveness for a range of seven octaves, the excess of the allowance over the actual sensitiveness towards either end of this stretch would compensate for the comparatively few distinctions which the ear can make either below or above it—64 × 12 × 7 = 5,376. A much higher estimate, based upon observations which seem to show that the ear can distinguish sounds less than one sixty-fourth of a semitone apart, places the total number at 11,000.