The Physiology Of The Ear | Operating Microscopes

The Physiology Of The Ear
By FRANK ALLPORT, M. D., AND R. 0. BEARD, M. D.,
OF CHICAGO, ILL.
OF MINNEAPOLIS, MINN.

THE physiology of the ear is one of those functional problems the solution of which depends upon the application of physical principles to the operations of the living tissue cell. It involves, essentially, the translation of physical phenomena into forms of physiological activity. Nevertheless, a clear distinction must be made between the physical laws under which auditory stimuli are conditioned and the physiological laws under which auditory impressions are developed and interpreted.

The production and propagation of sound waves are governed by these physical laws. Matter, in direct ratio to its elasticity and inversely to its density, is susceptible of vibratory motion. Those forms, phases, or degrees of vibratory motion to which the organs of hearing are responsive are termed sound waves. The limitations of this term are dependent upon the capacity of the auditory apparatus, and vary, therefore, with the degree of auditory development in the particular species or individual.

Waves of sound may be defined, under these limitations, as the to and fro or oscillatory movements of particles of matter, each particle similarly affecting its immediate neighbors, so that alternating condensations and rarefactions of these particles of the sound transmitting medium are produced. These vibratory movements occur in a direction either longitudinal or transversal to the axis of the propagation of the sound, according to the nature and arrangement of the conducting agent.

Particles of matter which are at similar points of condensation or rarefaction are said to be in the same “phase." The distance between such particles in similar phase is termed the wave length. This distance and therefore the wave length varies with the velocity of the wave movement and with the rate of the sound vibrations i. e. the degree of velocity per second, divided by the number of vibrations per second, gives the measure of a particular wave length. The velocity of sound waves is determined by the relative elasticity and density of the transmitting medium.

A wide variance is discovered in the sound propagating qualities of different media, such as air, water, solids, etc.; but the superiority of a medium as a conductor of sound waves does not altogether overcome the difficulty of their transference from one medium to another, as from air to water.

The impact of sound waves upon substances of suitable form and position will cause a reflection of sound ; that is, a reprojection of sound waves of similar character to a distant focus. Echo is an illustration of sound reflection from a reflector so distant that the primary waves die away before the secondary or return waves reach the ear at the focal point.

Sound waves, passing through a substance of biconvex form and of greater density than the air, may be refracted, as light is in passing through a lens, to a focus in front of the refractive body. The expansion or diffusion of sound waves is limited in their conveyance through tubular passages, and thus sound may be said to be susceptible of collection.

Sound waves are possessed of certain physical properties which are the subjects of recognition by the organs of hearing. The accurate analysis of these qualities is dependent upon the degree to which the specialization of auditory function has been carried.

Sound waves are measured (1) by their amplitude; that is, by the energy of the movement of the vibrating particles by the degree of their excursion upon either side of a position of rest. This property marks the force of the stimulus to which the auditory nerve terminals are subjected, and, together with the degree of responsive irritability possessed by these terminals, determines the loudness or intensity of a sound.

Sound waves are measured (2) by that property which is termed pitch a feature determined by the number of vibrations per second which the particles of the sound transmitting medium undergo. The range of variability in this vibration rate possible of appreciation by the human ear is a very wide one, although its limits vary widely with the degree of auditory development. The appreciable extremes of vibration are placed between 24 per second and 40,000 per second,' but the more usual limits of discernment are between 33 and 16,000.

Sound waves are characterized (3) by the presence or absence of rhythm in the recurrence of their vibrations. If the vibrations have a regular periodicity, they are said to give musical sounds; if they are of irregular rhythm, they constitute noises.

Waves of sound, and particularly of musical sound, are distinguished (4) by their quality or timbre, a property which rests upon tile fact that they are usually of a compound character i. e. they are associated or consist not of single, but of several, waves. This association is usually of a fundamental or dominant tone, characteristic of the vibrations of the conducting medium as a whole, with partial or over tones produced by the coincident vibrations at a more rapid rate, and therefore of a higher pitch, of different portions of the conducting medium.

When the vibration rates of associated tones, whether fundamental or partial, are in the same arithmetical relation as small whole numbers are to each other (e. _q. as 4 to 5, or as 6 to 8) that is, when their relationship of rate cannot be expressed in integral multiples the resultant note is termed harmonic.

When the vibration periods of coactive or associated sound waves are not coincident, or in this relationship of small whole numbers to each other whether they give rise to fundamental or to over tones a phenomenon termed beat ensues. The beat is due to an increased intensity of sound whenever the waves are in the same phase that is, when they are alike in the phase of condensation or alike in the phase of rarefaction and to an interference with or diminution of the sound when the waves are in opposite phases that is, the one in rarefaction and the other in condensation.

The number of these beats depends upon the difference in the vibration rate of the associated waves. When this difference is not great and the beats are therefore few, they are readily appreciated by the ear and do not produce unpleasant effects upon it. As the difference of vibration rate increases and the beats become more numerous, they introduce a discordant element, and at length (when about 33 per second) they produce a sort of vertigo of the auditory sensations which we translate as dissonance. Further increasing in number, the beats become gradually fused and the roughness of sound lessens, until they reach the extreme time limit of distinct sensations (132 per second) and are lost. So that sound waves whose vibration periods are widely different, and which give rise to a very large number of beats very frequently repeated, afford no appreciation of beats whatever to the human ear.

Of especial bearing upon the physiology of hearing are the physical principles of sound selection. Certain substances have a capacity for sympathetic vibration. They are inherently endowed with a definite vibrationperiod, and whenever sound waves of this particular pitch approach them, they are excited to vibrations in harmony with the stimulating waves, and thus serve to swell the volume of the primary sound. To vibrations of period variant from their own they are dumb. So marked is this tendency to sympathetic vibrations in certain media that they are termed resonators, and the quality which they possess is called resonance.

Sympathetic vibration is so acute a quality in some agents as, for instance, in the strings of a piano forte that a complex musical note sounded in their near neighborhood will be resolved into its component tones by their selective power, each string responding to its own intrinsic tone. In this quality lies the foundation of the analysis of sound, unquestionably one of the physico physiological functions of highly specialized organs of bearing.

These functions of the auditory apparatu's will be best understood if the close relationship between physical principles and physiological conditions, which this term implies, is borne in mind.

The Sound collecting Apparatus. The external ear, consisting of the pinna and the meatus, has the primary duty of collecting, reflecting, and perhaps, to a degree, resonating the waves of sound. The auricle with its conch like form and its labyrinthine depressions is essentially a sound gatherer. In this function it is assisted in some animals, although rarely in man, by a group of muscles the attollens auricule, moving the ear upward the attrahens auricula, drawing it forward and upward; and the retrahens auricula pulling it backward. Slighter alterations in the form of the auricle may be effected by a second group viz. the tragicus, the antitragicus, the helices major and minor, the transversus, and the obliquus auricula. By the tragus and by means of the curvature of the meatus the drumhead is protected from the too severe impact of powerful vibrations or currents of air, and the canal from the too easy entrance of insects and foreign bodies. The presence of hairs and of the cerumen in the meatus also guards the ear from these invaders.

The position of the tragus and the form of the curvature of the canal also suggest that from the center of the conch sound waves may be reflected to the inner face of the tragus, from that surface to the roof of the meatus, and thence to the tympanic membrane. The tubular passage of the meatus indicates its sound collecting and, possibly, its sound resonating qualities.

The Sound conducting Apparatus. The middle ear, including the tympanic membrane, the chain of ossicles, the intratympanic muscles, and the fenestre ovalis and rotunda, together with the perilymph enclosed by the bony labyrinth of the internal ear, is pre eminently the organ of sound conduction. To this function the appendages of the middle ear, the Eustachian tube, the antrum, and the mastoid cells indirectly minister. In the process of conduction the sound waves which break u on the tympanic membrane are transmuted, through its agency, into a mechanical movement, a molecular vibration, which involves both the chain of ossicles and the perilympli, and is retransmuted, through the medium of the latter, into sound vibrations in the internal ear.

The tympanic membrane, a small, thin, membranous sheet, tautly stretched across the junction of the external with the middle ear, with its slight irregular convexity, with its radial and circular fibers centering at the umbo and giving it certain fixity of form, with its tensity increased by muscular action, is admirably adapted to its purpose. The longitudinal vibrations of the sound waves which reach it through the column of air in the external meatus excite in it, as they do in other bodies similarly stretched and whose cross section is of similarly small dimension, vibrations of a transversal form. Thus the drumhead vibrates inward and outward between the cavities it divides. Through the attachment of the handle of the malleus to its umbo it is not only put into direct relations with the chain of ossicles, but is controlled by the tensor tympani muscle. This muscle, the tendon of which is attached to the upper third of the handle of the malleus, and traverses a portion of the middle meatus, executing around a bony eminence near the Eustachian canal a turn almost at right angles to the body of the muscle, takes its fixed point in a groove running above the lumen of that canal. The contraction of this muscle, controlled by efferent branches of the fifth nerve, serves, in all probability, a double purpose. It. draws the malleus inward, and thus increases the tensity of the tympanic membrane, rendering it more acutely responsive to sound waves of high pitch. It also increases the contact between the handle of the malleus and the drumhead at the umbo, the former serving, in consequence, as a "damper" by which the fundamental tone of the tympanic membrane which in bodies of such structural form would tend to be over prominent may be diminished.

This suggests the fact that the vibrations of this membrane are of a composite character. It is susceptible of simultaneous response to sound waves having a very wide range of variance both as to pitch and quality.

The Ossicular Chain. These delicate vibrations of the drumhead are brought to bear upon the chain of ossicles (Fig. 457) through the handle of the malleus. The ossicles which, taken as a whole and physiologically, must be regarded as a continuous chain are too minute, in all their dimensions alike, to encourage the theory that they are a medium of sound conduction by molecular vibration. The shortest of wave lengths is long as compared with their greatest measurements. Moreover, the mutual arrangement of the malleus, the incus, and the stapes, and their relations to the drumhead at one extremity of the chain and to the oval window at the other, are such as to indicate their performance of an excursion upon the principle of a lever of the second class. A line drawn from the tip of the horizontal process of the incus through the incudo stapedial joint of the same bone to the end of the handle of the malleus represents this lever (Fig. 458). The handle of the malleus is the point of applied power, begotten by the vibrations of the tympanic membrane; the end of the short process of the incus is the fulcrum, and the incudo stapedial joint is the point of the effect, which is transmitted through the attached stapes and causes its impact upon the oval window. The unity of this lever is secured by the interlocking of the tooth of the incus with the groove of the malleus. At the same time, the ossicular chain is safeguarded from undue rigidity by the loose capsular ligament attaching the head of the malleus to its articulation with the incus.

Thus in the event of excessive pressure developed within the middle ear, pushing out the drumhead and carrying the malleus with it, the ossicles no longer act as a whole, since the reversal of the lever would tend to tear the stapes away from the fenestra ovalis. Instead, a separation occurs between the articular surfaces of the malleus and the incus, the head of the former gliding out of its socket and the tooth of the latter tending to unlock. Should this outward movement of the drumhead be so extreme as to carry a part of the head of the malleus back upon the incus again, the point of most forcible contact would again be at the tooth, which would then serve as a fulcrum, converting the ossicles into a lever of the first class and carrying the stapes back again upon the fenestra.

In the ordinary action of this physiological lever the movement of the short arm is materially less than that of the long arm, while the energy of the movement is multiplied two and a half times between the point of its application and the point of its discharge. As Helmholtz states it: "The mechanical problem which the apparatus within the drum of the ear had to solve was to transform a motion of great amplitude and little force, such as impinges on the drumhead, into a motion of small amplitude and great force, such as had to be communicated to the fluid of the labyrinth." Thus a sharp and relatively forcible blow is struck by the stapes upon the oval window. The effect of' this blow may be accentuated or diminished by the action of the stapedius muscle. This muscle from its origin in the pyramid in the back wall of the tympanic cavity passes to its insertion upon the capitulum of the stapes. It is efferently controlled by fibers of the seventh nerve. Under ordinary circumstances its contraction draws the foot of the stapes outward toward the drumhead, while the heel is thus brought more sharply into contact with the fenestra. A more forcible contraction, which may be excited reflexly by too powerful vibrations of the tympanic membrane, would tend to draw the whole foot plate away from the oval window, and would thus diminish the pressure upon the contents of the labyrinth.

Sound vibrations may reach the middle ear through the bones of the skull instead of by the ordinary path of the meatus, or they may be transferred from one side of the bead to the other; but in either case it appears to be true that the tympanic membrane receives these sound waves and transmits their effects through its own transversal vibrations to the chain of ossicles.

It is possible that to some small degree and especially in the event of fixity of the ossicles the air contained in the tympanic cavity may be thrown into vibrations, and that these may affect the perilymph through the oval or round window.

The Appendages of the Middle Ear. A thin mucoid fluid is secreted by glands imbedded in the submucous lining of the tympanic cavity, or more probably formed by the deliquescence of its effete cells. The ciliated epithelium, which constitutes the mucous membrane of the cavity, excepting upon the surface of the ossicles and the tympanic surface of the drumhead, and is found also in the Eustachian tube, with which the tympanic cavity is continuous, serves to carry the excess of fluid toward and through the tube into the pharynx.

The Eustachian tube has an irregular lumen, and in its lower portion its walls are in somewhat loose contact, and appear to be, as a usual thing, closed. The tube opens for the discharge of the mucous secretion of the middle ear into the pharynx; it is opened also during the act of deglutition, when air finds its way into the middle ear. Its most important and, perhaps its sole, functions are thus to drain the tympanic cavity and to preserve an equilibrium of pressure between the gaseous contents of the cavity and the atmosphere. Should the contained gases become absorbed and the tube be impermeable, a vacuum results which may cause retraction of the drumhead and disease of the intratympanic tissues. The opening of the tube during acts of deglutition is sufficient, as a rule, to maintain this equilibrium of intratympanic and extra tympanic pressure.

The antrum and the mastoid cells are, physiologically, extensions of the tympanic cavity, although their communications with that cavity are not always patent. Their functions are still a matter of conjecture. They probably serve as pneumatic spaces within which a supply of air may be retained as an additional means of maintaining the air pressure within the tympanum. They have been supposed also to serve as diffusion chambers for excessive sound vibrations, which may be communicated to the* air in the tympanic cavity, and which might otherwise fall with undue energy upon the windows of the labyrinth. There is little evidence, however, in support of this view, since sound waves, within ordinarily wide limits, and whether conveyed through the external meatus or through the bones of the head, are transmitted to the tympanic membrane, and, centering at the umbo, are forwarded through the movements of the ossicular chain rather than through the air of the cavity.

The Bony Labyrinth and the Perilymph. By means of the fenestra ovalis and the fenestra rotunda, the windows of the bony labyrinth, increase and decrease of pressure in the perilymph are provided for. The influence of the sound vibrations of the drumhead, through what may be called the sound¬ movement of the ossicles, is conveyed to the perilymph by the impact of the stapes upon the membrane which curtains the oval window and divides the tympanic cavity from the vestibule. The shock which is thus transmitted to the fluid of the bony labyrinth follows the course of its cavity, and is finally ex¬pended upon the membrane of the round window, which curtains the cochlear canal from the middle ear. Thus in the round window a safety valve is afforded for any excess of pressure.

What has been said of the chain of ossicles with reference to their insusceptibility to molecular vibrations is equally true of the perilymph, enclosed as it is in a bony cavity of minute dimensions, of labyrinthine form, and with resistant walls. The impact of the stapes upon the oval window produces, not waves of sound traveling through the particles of this fluid, but a wave movement which involves the perilymph as a whole. So difficult is the transference of sound vibrations from one kind of medium to another that the vibratory movement of the perilymph more readily develops sound waves in the walls of the membranous labyrinth than would a series of molecular vibrations passing through the particles of this fluid. Such a movement has, in fact, an advantage over sound vibrations of the molecular form as a means of communicating to the sensitive structures of the internal ear the influence of the sound waves which break upon the drumhead.

The Sound recording Apparatus. The utricle, the semicircular canals, the saccule, and the cochlear canal make up the membranous labyrinth, enclosing the endolymph and surrounded by the perilymph within its bony sac. These organs are concerned not merely with the receipt of auditory impressions in general, but with the analysis and synthesis of sound. The impressions which they record are destined for the development of auditory sensations, which ' in their turn, give rise to auditory perceptions and judgments relating to intensity, rhythm, pitch, quality, distance, location, etc. The part, which each portion of the membranous labyrinth plays in the attainment of these physiological ends, is not yet sufficiently well worked out to justify much in the way of precise statement. Certain propositions may be established, however, with some measure of confidence, and these form the basis for certain safe conclusions.

The principles of sound conduction indicate that the walls of the membranous labyrinth, with their fibrous structure, are a better medium for the development and transmission of sound waves, as the result of the impact of the perilymph upon them, than the endolymph contained within these membranous walls can possibly be. The endolymph is a viscid fluid whose density would prove an obstacle to acute vibratory motion. In variable quantity it bathes the specialized auditory epithelium of the criste, the macule, and the cochlear spiral (Fig. 458). From its contact with a highly vascular membrane, the stria vascularis, from its identity with the cerebral fluid and the continuity of its channels with those of the brain, from its homology with nutrient fluids in other localities, it may fairly be considered as an agent of nutrition to these epithelial cells, rather than as a medium through which sound waves are conveyed to them. A difficulty, too, and a quite unnecessary one, is involved in the idea of the transmission of vibrations through the walls of the membranous labyrinth to a medium of so markedly different a character and vibratory quality as the endolymph.

It would seem that the auditory epithelium resting upon the inner surface of these membranous walls must be more readily affected by sound vibrations, directly transmitted to it from beneath than by vibratory movements in the endolymph above (Fig. 459). The peculiar form of the bony labyrinth, as related to the points at which the sweep of the perilymph begins and ends viz. at the two fenestra indicates that the force of the movement of the perilymph is probably conveyed across the membranous labyrinth, and bears strongly upon the ampulla, the utricle and saccule, and the walls of the cochlear canal.

The functions of the internal ear are of a more varied character than is suggested by the general term auditory impressions. There are reasons, still under debate, but perhaps sufficiently conclusive, for regarding the semi-circular canals, or the terminals of the vestibular nerve in their criste, as well as in the macula of the utricle and saccule, as the source of afferent impressions which assist in the preservation of both static and dynamic equilibrium.' Whether these impressions arise from the movements of the endolymph within the semi circular canals, and are therefore dependent upon position, or whether they are the effect of vibrations transmitted through the walls of the ampulla to the vestibular terminals, is a question still sub judice; but there remains little doubt that these terminals are, in one way or the other, concerned in the development of equilibriar impressions. The presence of the so called otoliths or otoconia in the walls of the labyrinth has given rise to the suggestion that they are concerned in the causation of these impressions. Recent experiments tend, however, to prove that the vestibular portion of the labyrinth is not, in an exclusive sense, an organ of equilibration. It is simply an afferent field from which the centers of co ordination receive a certain measure of instruction! In the event of its injury or removal, leading to temporary symptoms of vertigo, compensatory phenomena have been developed, which, in their readiness of appearance and their measure of substitutive function, are in direct ratio to the degree of cerebral development.' One distinct phase of the equilibriar functions of the internal ear is observed in its afferent regulation of compensatory movements in the eyeball.' But while the evidence holds good that the auditory epithelium and the nerve terminals of the criste and the macula are the recipients of other than purely auditory impressions, it is not necessary to dissociate the equilibriar from the auditory functions of the vestibule, or to consider it exempt from auditory duties. The fact that this organ is of some physiological service in co ordinating the movements of the body does not even argue a separative function for this purpose. The sense of equilibrium is not wholly independent of the sense of hearing. Loud or peculiarly harsh noises, and those extreme disturbances of rhythm which are incident to the occurrence of numerous beats in musical sounds, often beget sensations of a vertiginous character. Extremely deaf persons have a characteristic uncertainty of gait, which in deaf mutes often amounts to actual insecurity. Forty per cent., of the unfortunates of this class who have been examined have been found faulty in co ordination.1 an absence of nystagmus is frequently observed in such persons (Crum Brown). While these facts do not conclusively prove the interdependence of equilibriar and auditory functions, they suggest a very close relationship between them.

Furthermore, the absence of the cochlea or its very rudimentary form in certain animals who possess the sense Of hearing to a marked degree, compels the recognition of the vestibular portion of the internal ear as a receiver of auditory stimuli of at least certain kinds. Conversely, the form and the arrangement of the cochlea (Fig. 460) indicate unmistakably that it is an organ of sound analysis and perhaps of sound synthesis, but do not offer equally good evidence of its capacity to develop those auditory impressions which create sensations and judgments relating to intensity, rhythm, dissonance, etc.

It is altogether probable that the auditory epithelium and the nerve terminals of the macula, and perhaps of the criste, are the media by which are appreciated those qualities which pertain to so called noises, and which establish the differentiation between rhythmic and arhythmic sounds (Howell). It is, in fact, these primary auditory functions with which those animals are conspicuously endowed who have only the vestibular portions of the internal ear, while we have little or no evidence that they are possessed of the faculties of sound analysis and synthesis.

These most highly specialized of auditory functions by which the variations in pitch and quality of sound waves are recognized, by which composite notes are resolved into their component tones, and by which individual tones are fused into complex sensations, are unquestionably possessed by the cochlea.

In the basilar membrane (Fig. 460), upon which the organ of Corti rests, is found the only structure in the highly developed ear which satisfactorily accounts for the faculty of tone selection. Although in man it is of small dimensions as a whole, its radial tensity, together with its longitudinal laxity, the sufficiently wide range of difference in the radial lengths of its fibers, and the number of these radial fibers, estimated at 24,000, are qualities which suggest its vibratory function and endow it with ample possibilities of selective vibration. By selective or sympathetic vibration is meant the possession by its individual fibers of intrinsic pitch, in consequence of which each will vibrate only in harmony with a sound wave whose vibration period is identical with its own.

To the rods and cells of the organ of Corti these vibrations are certainly transmitted; in them they are intensified perhaps, and by them are conveyed as impressions of sound to the terminals of the auditory nerve. Physiology has not yet gone so far as to differentiate the several functions of the rods, 9450 in number, of the inner and outer hair cells, numbering 15,500 (Howell), of the twin cells of Deiters, or of the cells of Hensen and of Claudius, which all enter into the delicate structure of this organ. They are doubtless the media of communication between the basilar membrane and the terminals of the auditory nerve, but they are probably far more than this. Their structure and mutual arrangement suggest a mechanism for the execution of vibrations of rapid period or high pitch, and for the differentiation of varying vibration rates. They may serve not only as a means of analyzing composite sound waves, but as a means of synthesizing complex auditory impressions.

In the tectorial membrane exists, seemingly, a physiological "damper" by which excessive vibrations or too dominant tones are diminished. While it is difficult to demonstrate its possession of this function, its form, situation, and relations to the organ of Corti and to the overarching membrane of Reissner justify the conclusion. Excessive wave movements within the vestibtilar scala would necessarily bear upon the stretched membrane of Reissner, and would subject the endolymph beneath it to a pressure which, operating upon the upper surface of the tectorial membrane, would depress its free extremity toward or upon the delicate hair cells which it surmounts.

That such highly elaborated functions as these attributed to the organ of Corti exist in the human ear is predicated on the remarkable development of many individuals in the faculties of sound analysis and synthesis faculties which, while resting lastly upon the possession of specialized nerve centers which develop sensations and beget auditory perceptions and judgments, must needs require some mechanism upon which the sound waves may be registered and in which these varying auditory impressions arise.

The Mechanisms of Auditory Sensation, Perception, and judgment. The specialized auditory epithelia of the cochlea, the macula, and the crista are the media of communication between the recording apparatus of the ear and the terminations of the auditory nerve.

There is neither satisfactory evidence nor physiological analogy in support of the theory that auditory impressions are developed elsewhere than in these nerve terminals or conveyed to the nerve centers by other than auditory nerve paths. The apparent reaction to high notes or to loud a low tone which has been observed in animals which have been deprived of the membranous labyrinth is doubtless a matter of general sensation rather than audition (Bernstein).

In view of the varied character and location of the auditory epithelium, and the finely specialized quality of these nerve terminals, it cannot be doubted that they, in common with other special sense nerve endings, have a selective action upon auditory stimuli. They must have something to do with determining the nature of the impression which a given stimulus excites. Conversely, their responses must be conditioned, as are those of other nerve terminals, by the character and the mode of application of the stimulus.

Not only with the recognition of the qualities of intensity, periodicity, and pitch, but with the fixation of the limits of this recognition, must they 'be partially concerned. That such limits of function exist has been clearly shown. Fatigue phenomena, incident to excessive intensity, too rapid repetitions, and extremes of vibration in sound are shared by the auditory terminals. Wundt has successfully disputed the doctrine of the specific nerve energy of the conducting fibers of the auditory nerve; but to carry this contention down to a denial of the specific functions of the terminals would be a physiological reductio ad absurdum, since it would deny all utility to the highly differentiated structural forms of these receiving cells.

The degree of irritability manifested by the auditory terminals varies physiologically with hereditary conditions, age, training, and functional fatigue. An illustration of this variation with age is seen in the marked contraction of the compass of the human bearing incident to advanced years.'

The specific functions of the nerve centers of the bulb, of the basal ganglia, and of the cortex, which are in anatomical relations with the fibers of the cochlear and vestibular branches of the auditory nerve, are not, as yet, well understood. The fact that a portion of the vestibular division is traceable to the cerebellum reemphasizes the probability of an equilibriar function in the vestibule. The deep centers of the bulb and of the lateral nucleus, to which the cochlear and vestibular nerves are primarily traced, are possibly of purely trophic function.

The decussation of the auditory fibers in large part, by which the trapezoid bodies are formed, is suggestive of a fusion of the binaural auditory impressions in the nerve centers of the two sides an event which Schafer, however, denies.

In the posterior quadrigeminal body and the internal geniculate body we find evidence of the existence of auditory centers to which the major portion of the auditory fibers pass from the olivary body through the fillet. These are, clearly, the seats of auditory sensation. In this localization there is a striking homology to the visual sensory centers of the anterior quadrigeminal body.3 The posterior nucleus of the thalamus is possibly involved also in the registration of sensations of hearing.'

Of the manner in which auditory sensations are developed in response to a varied range of auditory impressions but little can be said at present. It is unlikely that each vibration wave which produces an impression upon the nerve terminals is represented by a separate and distinct sensation. In all probability, certain fusions of sound wave impressions are received by the nerve endings, having been synthesized perhaps in the cochlea, and these are translated into composite primary sensations analogous to the primary visual impulses, and then out of these integers of sensation, as it were, other and more complex sensory groups are developed. These centers are susceptible of an increase of irritability dependent upon stimulation. By the receipt of an impression, or perhaps of a series of similar impressions, the auditory centers are awakened to the appreciation of a succeeding and dissimilar impression. With the binaural conduction of sound there appears to be an. alternating centric increase of sensation upon the two sides.'

There is not only a close homology, but a functional relationship, between the auditory and the visual centers of the quadrigemina. Acoustic stimulation of the posterior body leads to a quite apparent increase of irritability in the cells of the anterior body, and to such a degree that more distinct visual sensations, especially in the color field, are induced.'

The functions of sound perception, of auditory judgment, and of auditory memory are localized in the cerebral cortex. In a portion of the first and second temporal convolutions lying ventral to the Sylvian fissure, and in direct communication with the auditory sensation centers of the basal ganglia 3 by fiber tracts which pass in both directions lie the centers which constitute the auditory brain.

These cortical functions have to do with the analysis and synthesis of sound, with the recognition of rhythm, with the determination of distance, and sound location, and with the recollection and recreation of sounds previously registered. The remarkable development of the faculties of sound analysis and synthesis in certain individuals predicates a high order of specialization in this seat of the musical mind. The judgments of sound distance and location are largely instructed by a comparison of the sensations begotten of impressions made, simultaneously or alternatingly, upon the two sides. In the estimate of distance the intensity of a sound is a governing and an often misleading guide. Thus a low, feeble sound produced in the near neighborhood will often convey the impression of distance, and vice versa In making up the judgment of distance it is not so much the total intensity as the intensity of the component elements of a sound which gives the most correct conclusions (Bloch).

The location of a sound is almost wholly dependent upon binaural hearing. Bloch has shown that it is more readily determined in the horizontal a frontal planes than in the sagittal plane. A comparison by the nerve centers of the several characteristics of intensity, continuance, pitch, and quality in the sounds received by the organs of hearing upon the two sides is the major factor in the case. The degree of sound collection achieved by the two auricles is a minor influence in informing the judgment of the locality of a sound.

The function of sound memory is but imperfectly developed in the majority of persons, while in a very few individuals it reaches a high degree of perfection. The existence of a memory center for auditory perceptions, apart from the temporal centers of sound perception and judgment, is undemonstrated.

Any tendency to dogmatic statement in regard to the specific functions of auditory centers is 'arrested by the promise of new light which is suggested by the recent investigations of Kolliker, v. Lenhossek, and Held into the origins of the auditory nerve fibers, and by the late demonstration by Ramon y Cajal of a new auditory nucleus in front of the convexity of the upper olivary body.

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