By John Denis Macdonald, M.D., F. R. S.,
Staff-surgeon, R.N.
London:Longmans, Green, Reader, and Dyer.
Gosport:Groves, High Street 1869.
SECTION I.
The External Ear and its Optical Analogue.
There seems to be as close a correspondence in the structure of the organs of vision and hearing, as there exists in the general properties of light and sound;while any peculiarity occurring in either will be found to be in accordance with some special requirement of its appropriate stimulus.
Agreeably with the law common to light and sound, that their power and intensity diminish with the square of the distance, nature adopts suitable means for collecting the diffused rays of those agents, preparatory to their further concentration, that an impression of sufficient strength may be conveyed to the seat of perception. Thus, in the eye the collection of the rays of light is effected by dense refracting media, (the cornea and aqueous humour), presenting an expanded convex surface anteriorly, while a funnel-shaped apparatus (the extended and generally concave surface of the auricle, with the external auditory canal) is employed for a similar purpose in the ear.
Now although the auricle and external auditory passage, with its short hairs and glandular apparatus (anatomically considered), correspond with the eye lids, comprehending especially the tersal fibro-catiliges, the cilia and meibomian follicles, yet, the office of the auricle in receiving the rays of sound, and transmitting them to the middle ear, is too important a function to be passed over in the simple notice of the anatomical analogy just mentioned. Indeed, the eyelids rather tend to diminish the quantity of light entering the eye, being watchful guardians against its excess, or the injurious contact of foreign matters. In certain savage tribes, the muscles of the auricular region are remarkably developed, and their action in perking, and otherwise adjusting the auricle, bears relation to its reception of sonorous modulations for transmission to the tympanum.
SECTION II.
The Middle Ear and its Optical Analogue.
A further concentration of the rays of light and sound respectively, is necessary for distinct vision and hearing. In the eye this requirement is fulfilled by a body (the lens) of greater density and refracting power than those employed primarily in collecting the luminous rays;and in the ear, by a membranous expansion, (the membrana tympani) having a conically depressed external surface, beautifully adapted for convening the rays falling upon it.
This membrane is connected with another of much smaller size, (the membrana fenestrae ovalis) through the medium of several small and dense bones, (the assicula auditus) and thus a communication is established with the internal ear. Some consider the membrana tympani overlaid with the lining membrane of the meatus auditorius, to represent the cornea of the eye, with its conjunctival investment;but- Dr. Whaton Jones ingeniously supposes it to represent a natural "mediate anckyloblepliaron" and such appears to be the correct view. The outer cornea in serpents therefore being a natural anchyloblepharon, may be regarded as homologous with the membrana tympani. Between it and the eye ball itself a space exists corresponding with the cavity of the tympanum, and this space is lined by the true conjunctiva, (the oculo-palpebral sac of Cloquet) agreeing with the lining of the tympanum. The lachrymal canals opening into it, and the lachrymal duct communicating with the nose are answerable to the eustachian tube which connects the cavity of the tympanum with the fauces.
SECTION III
The Internal Ear and its Optical Analogue.
The most essential portion of the organ of hearing, or the internal ear, has been appropriately termed the ear bulb, bearing analogy to the eye ball in nearly every particular. There are however certain peculiarities in the figure and repetition of the parts of the former, rendering the subject complex.
The membrana. rotunda, called also the membrana tympani secondarina, has been likened to the cornea. The scala tympani and scala vestibuli of the cochlea, have been respectively conceived to correspond to the anterior and posterior chambers of the eye;the perilymph being analogous to the aqueous humour, and the helicotrema to the pupil. The endolymph is answerable to the vitreous humour;and the otoconia, or caleareous concretions have been compared to the lens;while the several divisions of the auditory nerve taken collectively, represent the retina. A direct currant of sonorous undulations passes along the dense chain of ossicles to the labyrinth, through the membrana fenestra ovalis] and the mere consideration of its yielding character, as bearing an analogy to the thinning of the sclerotica in the eye of the Greenland seal, is not satisfactory, for the functional parallel will distinctly shew that it must be recognised as fulfilling the office of cornea to the ear, as well as the membrana rotunda. The sonorous currant entering by the fenestra rotunda may be strengthened or modified in scala tympani, by that of the scala vestibuli, entering by the fenestra ovalis, and finally descending in the scala tympani. This is an important consideration in relation to the cochlea;and may possibly be connected with the perception of the pitch of musical sounds. The question may be:whether by a property of refrangibility analogous to that of colours, or by the simple law of interference, a collision of the two sonorous currants above noticed may take place at special points of the spiral lamina, according to the pitch, and thus (impressing the visicular matter of the middle scala in different localities) enable the mind to perceive the relation that one note bears to another in the musical scale. This supposition is strengthened by the fact that the most exalted function which the human ear, as well as that of the lower animals possesses, is the discrimination of the pitch of sounds;and the existence of the cochlea is the highest mark of perfection in the organization of the ear.
SECTION IV,
Perception of the Pitch, Distance and Position of Sounds by the Ear;and analogous faculties of Vision.
The perception of the distance and of the locality from whence sound originates must be regarded as two special functions of audition, and quite distinct from the sense of pitch which discriminates the relative graveness or acuteness of sounds. In the eye we also notice similar endowments, for while we form a judgment of the distance and position of objects by the light which they reflect, we can also appreciate their colours, but individuals are to be found in whom one or other of these functions is defective, either as regards the eye or ear. Thus, a person may have no power of judging the distance through which a sound may have travelled in order to reach his ear j or, what is analogous with respect to the organ of vision, he may not be able to conceive how far an object is distant from him by its image depicted on his retina. Again, he may have acute hearing in every respect, but no power of ascertaining the pitch of a musical sound, or its relative position in a scale of eight notes;or what is similar with regard to the eye, he may not be able to discriminate between one colour and another, although every object as to outline, light and shade, is distinctly perceived;so that the power of distinguishing the pitch of sounds by the ear, and the species
of colour by the eye are analogous functions.
SECTION V.
Sympathetic Vibration.
When the active motion of a sonorous body excites the passing vibration of surrounding substances, they are said to vibrate sympathetically. This sympathetic vibration may be of two kinds, viz.:1st., when the tremors of a sounding body are communicated by the atmosphere or other means, to an adjacent surface or substance, in the particles of which a corresponding motion is excited, and secondly, where sonorous bodies give out their own peculiar sound, when the vibrations of others are conducted to them in the manner just named. The motion of the fore part of the violin, occasioned by the tremor of the strings, is an instance of the former;and the excitement of one string on a harp by the vibration of another would illustrate the latter. Here the string sympathetically affected sounds its own note, and not that of the string which thus excited it.
It is probable that the membrana tympani (while it collects and transmits the sounds that subsequently impinge on the auditory nerve), may be sympathetically affected in both these ways, for in its passive state it is susceptible of the vibrations of any note falling upon it within a certain sphere, but, by its muscular adaptation it may commingle, its own proper vibration with that of the sound affecting it, and thus augment the sonorous impression.
Tbe strings of a piano-forte communicate their vibration to the sound board so accurately that their notes may be reproduced by the percussions of any loose dense body, as a small piece of steel placed upon it;and that the same particles may be simultaneously affected by two or more notes, is proved by the fact that the separate intervals of any chord, C E G, for example, may be distinctly traced in what we may call the compound percussions of the dense body. This experiment shows how the membrana tympani at the same degree of tension may transmit to the internal ear many different sounds, either simultaneously, as in a harmonious chord, or in succession, as in a melody. But as in the case of the soundboard just instanced, the drum of the ear will respond in some situations better than in others, according to the pitch of the note, suggesting the inference that all parts of the surface are not alike susceptible of the same vibration, and this we shall presently see more fully illustrated in the drum head. The fact also indicates that a certain change in the tension of the membrana tympani is necessary for the more efficient reception of sounds when their pitch extends beyond a certain range, so that by a very few stages of tension it may suit all the sounds coming within the scope of audition, without adapting itself to eacli respective sound, as some physiologists suppose.
SECTION VI.
Acoustic properties of the ordinary Drum Head, as supplied to the physiology of the membrana tympani.
Well taught drummers are aware thai there are three notes on the drum head, agreeing with the principal harmonics of a musical string. See diagram XVI. One note is limited to the centre, a second to the circumference, and a third to the intermediate portion. The central point gives the tonic or key note of the instrument, say C;that of the circumference being a fifth or five notes higher than the central, is equivalent to G;while the note between these, which is a third above the key note, corresponds to E;thus the constituents of the perfect chord or triad of C may be produced by striking those several parts successively. Here also we perceive that the laws of vibrating chords are extended to membraneous surfaces, harmonic vibrations being developed in both. Thus, a string sounding C may have secondary or harmonic vibrations excited in several of its parts, making up the elements of the perfect chord, and this is also well exemplified in the drum head.
The consideration of the qualities of the drum head (above referred to) accounts for the extension of the handle of the malleus to the apex or centre of the membrana tympani, (as indicated by the perpendicular in the diagram) in order that it might convey to the seat of impression, the segmental vibrations, whether harmonically or sympathetically excited.
The high notes affecting the circumference of the membrane take but a short course along the handle of the malleus, agreeably with their pitch and intensity, while the vibrations of the centre, being of a grave and penetrating character, take the tragit of the entire manubrium j thus all inequality is compensated, and an important physiological end answered. Hence, it may be inferred, that the handle of the malleus, in correcting the unequal effects that would necessarily result from the diversity of the pitch and power of musical intervals, takes an analogous office to that fulfilled by the refracting media of the eye, whereby the contending refrangibilities of different colours are reconciled, and all are brought as nearly as possible to impinge upon the retina in the same plane.
It is an interesting fact that in order to establish any note as a new key, a correct conception of its fifth is necessary, as any other interval will not determine it. This constitutes the difficulty in following abrupt modulations or sudden changes of key, without notes of transition;just as the educated eye feels hurt when discordant colorific intervals are ignorantly associated, for the impression of one colour must be artfully neutralized by the juxtaposition of others having a sympathy with it. Now the three vibrating segments of the membrana tyrapani respecting to a key note, its third, and fifth, embrace all the requisites for determining the key;and this natural or physical division of the membraneous surface producing the perfect chord above mentioned, explains the necessity of every musical composition both commencing and ending with, this combination of sounds. Moreover, on applying the laws of musical harmony to the association of colours, we shall find it necessary to establish some one colour as a key note in pictures, and upon this we may construct a colorific chord. Again, on the principle of harmonic vibration, if we suppose the note C alone to impinge on the membrana tympani, the notes E and Gr will be harmonically excited. Thus, the physical change affecting the membrane being communicated to the auditory nerve, induces the mental conception of the natural agreements;and this we may conclude is the mode of instruction which the mind undergoes in the formation of what is called "the musical ear," a faculty enjoyed by some favoured individuals, independently of any musical education.
The laws of the complementary colours have a similar bearing on the visual faculty, thus:the impression of red (equivalent to C) on the retina induces the perception of green, composed of yellow and blue, (third and fifth), answerable to E and G respectively, to the complementaries of which (purple and orange) the same principle will apply. The membrana tympani and the malleus accurately represent the sound board and steel rod of the experiment previously alluded to, and we may fairly infer that while the membrana communicates its vibrations to the ossicles, it also causes the malleus to percuss the face of the incus, responsively to the rapid and varied impressions made upon it. The accentuations, piano, forte, &c, in musical pieces will be marked with extreme accuracy, and should the parts of the music be deranged by a defect in time, an uneasy jog will be produced upon the auditory apparatus. Hence the antipathy of the mind to any erratic deviation in this respect. It is very remarkable that the malleus and incus (the hammer and anvil), should correspond, not only in figure, but also in function, to the objects from which their respective names are derived, for as we have already seen, the uses of the hammer and anvil as employed in mechanics are literally fulfilled by the malleus and incus, answering very important ends in the faculty of audition. By the action of one upon the other, sounds are not only correctly transmitted to the auditory nerve, but an accurate register of time is effected in the manner above explained. Thus, the mind is impressed with a faithful transcript of the harmony, style and general effect of musical pieces.
SECTION VII.
Laws of Rhythm and Time in relation to organic structure.
With regard to rhythm, it is in music what symmetry is in the arts, for as the lateral parts of symmetrical bodies or their pictorial representations must be similar, in order to maintain their special character, so the parts of a melody, although consisting of a series of intervals impinging successively on the ear, mtistbe equal in duration is though symmetrically disposed, on the one hand preceding, and on the other following, a centre of time, for equal breadth in visual impressions is analogous to equal continuance in those of audition, space having the same relation to the organ of vision that time has to the organ of hearing. In this point of light the difficulty of philosophers in explaining the nature of rhythm appears to be in some measure cleared away, for it is no more wonderful to conceive the idea of equal partions of time, as presented to the ear in the several parts of a melody than that of similar portions of space, either as occupied by natural objects or their outlines in pictures. Tt is well understood that the most difficult thing to be attained in the study of music is the correct appreciation of the value of the notes composing the bars and parts of musical pieces as regulated by a determinate time. And it is not unreasonable to presume, that the physical impulses of the malleus, whereby it percusses the incus in accordance with the measure of the music, are the very means employed by nature in thus instructing the mind to appreciate musical symmetry in rhythm.
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