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Some Aspects of Mammalian Hearing under WaterAuthor(s): F. W. Reysenbach De HaanSource: Proceedings of the Royal Society of London. Series B, Biological Sciences, Vol. 152, No.946, A Discussion on the 'Ear' Under Water (Apr. 26, 1960), pp. 54-62Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/75363 .

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S. Dijkgraaf (Discussion Meeting) S. Dijkgraaf (Discussion Meeting) with the swimbladder, is exclusively or better developed in the male. In other cases the sounds are only produced during mating or breeding activities. When such sounds were played back to silent fishes of the same species, a general activity increase and sometimes a positive approach towards the sound source (at close

range) or towards fellow fishes was observed (Tavolga I958). In other cases the sound had obviously a threat function (Dijkgraaf 1947b). There are certainly many other possibilities like guarding the nest, defence of territory, and perhaps even echo-location (Griffin I958). More evidence is urgently needed, particularly with

respect to the biological significance of hearing in freshwater fishes.

REFERENCES (Dijkgraaf)

Dijkgraaf, S. I947a Experientia, 3, 206-208. Dijkgraaf, S. 1947b Experientia, 3, 493-494. Dijkgraaf, S. I950 Physiol. comp. Oecol. 2, 81-106. Dijkgraaf, S. I952a Z. vergl. Physiol. 34, 104-122. Dijkgraaf, S. I952b Experientia, 8, 205-216. Dijkgraaf, S. & Verheijen, F. J. 1950 Z. vergl. Physiol. 32, 248-256. Fish, M. P. I954 Bull. Bingham Oceanogr. Coll. 14, 3-109. Frisch, K. v. I936 Biol. Rev. 11, 210-246. Frisch, K. v. 1938 Z. vergl. Physiol. 25, 703-747. Frisch, K. v. & Dijkgraaf, S. I935 Z. vergl. Physiol. 22, 641-655. Griffin, D. R. I958 Listening in the dark. New Haven. Poggendorf, D. I952 Z. vergl. Physiol. 34, 222-257. Pumphrey, R. J. I950 Symp. Soc. Exper. Biol. 4, 3-18. Reinhardt, F. I935 Z. vergl. Physiol. 22, 570-603. Schneider, H. I94I Z. vergl. Physiol. 29, 172-194. Sch6ne, H. 1959 Ergebn. Biol. 21, 161-209. Suckling, E. E. & Suckling, J. A. I950 J. gen. Physiol. 34, 1-8. Tavolga, W. N. I958 Physiol. Zool. 31, 259-271. Wohlfahrt, T. A. I939 Z. vergl. Physiol. 26, 570-604. Wohlfahrt, T. A. 1950 Z. vergl. Physiol. 32, 151-175.

Some aspects of mammalian hearing under water

By F. W. REYSENBACH DE HAAN

Eindhoven, Holland

In all probability the first, most primitive life must have had its origin in the water. When one tries to form an idea of the development of 'hearing under water', it is understandable that the formation of an adequate sensory apparatus for hearing depends on the development of the tactile sense, and later on the coming into

being of a nervous system, lateral line organ, and finally on the formation of the stato-acoustic end-organs of the labyrinth. This gives little cause for wonder, as the reaction toOpressure waves must have been an early felt biological necessity. The step from pressure waves under water to sound waves of very low frequency is neither a great nor a fundamental step; it is merely the addition of sound modality to vibration.

with the swimbladder, is exclusively or better developed in the male. In other cases the sounds are only produced during mating or breeding activities. When such sounds were played back to silent fishes of the same species, a general activity increase and sometimes a positive approach towards the sound source (at close

range) or towards fellow fishes was observed (Tavolga I958). In other cases the sound had obviously a threat function (Dijkgraaf 1947b). There are certainly many other possibilities like guarding the nest, defence of territory, and perhaps even echo-location (Griffin I958). More evidence is urgently needed, particularly with

respect to the biological significance of hearing in freshwater fishes.

REFERENCES (Dijkgraaf)

Dijkgraaf, S. I947a Experientia, 3, 206-208. Dijkgraaf, S. 1947b Experientia, 3, 493-494. Dijkgraaf, S. I950 Physiol. comp. Oecol. 2, 81-106. Dijkgraaf, S. I952a Z. vergl. Physiol. 34, 104-122. Dijkgraaf, S. I952b Experientia, 8, 205-216. Dijkgraaf, S. & Verheijen, F. J. 1950 Z. vergl. Physiol. 32, 248-256. Fish, M. P. I954 Bull. Bingham Oceanogr. Coll. 14, 3-109. Frisch, K. v. I936 Biol. Rev. 11, 210-246. Frisch, K. v. 1938 Z. vergl. Physiol. 25, 703-747. Frisch, K. v. & Dijkgraaf, S. I935 Z. vergl. Physiol. 22, 641-655. Griffin, D. R. I958 Listening in the dark. New Haven. Poggendorf, D. I952 Z. vergl. Physiol. 34, 222-257. Pumphrey, R. J. I950 Symp. Soc. Exper. Biol. 4, 3-18. Reinhardt, F. I935 Z. vergl. Physiol. 22, 570-603. Schneider, H. I94I Z. vergl. Physiol. 29, 172-194. Sch6ne, H. 1959 Ergebn. Biol. 21, 161-209. Suckling, E. E. & Suckling, J. A. I950 J. gen. Physiol. 34, 1-8. Tavolga, W. N. I958 Physiol. Zool. 31, 259-271. Wohlfahrt, T. A. I939 Z. vergl. Physiol. 26, 570-604. Wohlfahrt, T. A. 1950 Z. vergl. Physiol. 32, 151-175.


By F. W. REYSENBACH DE HAAN

Eindhoven, Holland

In all probability the first, most primitive life must have had its origin in the water. When one tries to form an idea of the development of 'hearing under water', it is understandable that the formation of an adequate sensory apparatus for hearing depends on the development of the tactile sense, and later on the coming into

being of a nervous system, lateral line organ, and finally on the formation of the stato-acoustic end-organs of the labyrinth. This gives little cause for wonder, as the reaction toOpressure waves must have been an early felt biological necessity. The step from pressure waves under water to sound waves of very low frequency is neither a great nor a fundamental step; it is merely the addition of sound modality to vibration.

54 54




Before, in a remote past, dramatic geological changes had created the conditions for the development of life on land and therefore also for life in the air, the fishes were the most highly developed vertebrates. They probably possessed a hearing organ entirely adapted and adjusted to hearing under water. We assume that some of these animals possessed the potency to answer with success the tremendous demands made by the transition to land life. Under-water hearing was transmuted into air-hearing. Air-hearing finally reached its highest degree of development in the mammals.

However interesting this development may be, it lies outside the scope of this lecture. We wish only to call your attention to the behaviour of mammalian

hearing, which is air-hearing par excellence, when it has to function under water. Which ways had nature to follow to readapt this kind of hearing with optimal

results to the second change of environment? Was it a regression or a new step forward?

It is known that there exists a series of mammalian species which have found their way back to aquatic life. These animals lead a life which is partially or entirely readapted to the water. In order to save time while still giving you a reasonable idea of some aspects of mammalian hearing under water, I propose to discuss only the possibilities of two mammalian hearing organs, one which is not at all, and another which is entirely adapted to hearing in this medium: on the one hand the

hearing of man, on the other that of the toothed whale.

Before we enter in detail into these problems, a few essential questions must be answered:

(1) What is sound?

(2) What is the biological significance of sound? Sound is a periodic compression and rarefaction of the medium in which it is

produced, transmitting itself with a speed dependent on this medium, and which, from the biological point of view, may result in an acoustic sensation. We therefore

speak of the sound wave, the sound frequency or pitch, the speed of sound and the intensity of sound.

The mutual relationship between the first three is indicated by the equation

A = v/v

The wavelength of sound A is equal to the speed of sound v divided by the

frequency v. The higher the frequency, the shorter the wavelength and the more the waves are bundled and obtain the nature of rays, as is the case with light. According to the above definition, the wavelength is, however, also dependent on the speed of sound, which in turn depends on the medium. In the air this

velocity is about 300 m/s, in sea water 1200 m/s, or four times greater. A sound wave of the same frequency is therefore four times as long in water as in air. This

implies that in water the bundle of ray character can only occur with much higher frequencies, and this is again decisive for the degree of directional power of sound.

Finally, it is of interest to know that as the frequency of the sound becomes

higher, its power of penetration decreases: the long waves of the low frequencies

55



F. W. Reysenbach de Haan (Discussion Meeting)

penetrate very far in all directions, the very short waves of supersonic frequencies radiate as a bundle in one direction, but do not reach far. They are absorbed in the medium to a much greater extent than the longer waves.

The most important biological significance of sound is the information given by it. This information is directly dependent on its directional nature. Only then follow the penetrative power, the intensity, and the emitted or subsequently reflected frequency spectrum and the variation of sound in time.

What use can the hearing organ make of these data ? Its function is to offer the sound waves it receives to the sensory epithelium of the inner ear with the least possible loss. This is the task of the reception and transmission apparatus of the external ear and middle ear, which is essentially a mechanical transmission system for bringing about the transition of sound from one medium into another, without loss and distortion, and for each of the two ears separately.

The hearing organ must be able to determine as quickly and faultlessly as possible the direction and nature of the sound. The determination of direction mainly depends on the external ear and the shortest distance in the sound-transmitting medium between the two independently working ears (compare, for example, with binocular vision), and on the architecture of the central nervous pathways for sound and their correlation.

The nature of the sound is determined by a frequency-analysis in the greatest possible detail, that is by an inner ear as finely differentiated as possible.

A comparison with the eye shows that this effectively works over a much smaller distance than the ear, because its receptive field is limited by optical obstacles which would not affect the propagation of sound. The eye is, moreover, dependent on the presence of sunlight or derivatives of it. The olfactory sense depends to a high extent on prevailing conditions such as direction of wind or degree of humidity. The tactile sense does not serve any purposes of telereception, apart from the lateral line organ of the fishes. For the mammals the ear remains the telereceptor par excellence. It gives information first and most rapidly, and is therefore the most efficient warning organ.

What is to be expected of the auditory function in man when he is to hear under water ? The external ear has then lost its function; for sound it is physically identical to water. The middle ear will no longer be able to display its function. When sound is transmitted from the water medium to the air medium, 99-9 %, or more than 60 db, is lost. An even more important feature is that underwater sound- we are speaking here of the lower frequencies-causes a vibration of the whole skull including the two hearing organs, and not of the two middle ears separately with respect to inner ear and skull, as is the case in air.

This leads us to expect: (1) That a loss of sound-intensity must arise as occurs in human subjects with an eliminated middle ear as a result of chronic otitis media or a radical operation, a loss of hearing, therefore, of about 60 db, (2) that there can be no question of a directional-sensation in hearing under water.

I have been in the position to make audiograms of myself and other test subjects under water; these confirmed that the loss of intensity was indeed 60 db.

56




Determination of direction proved entirely impossible. Neither the distance between observer and source of sound, nor the type of sound, namely short pulses or sweep tones, made any difference. In these experiments the frequency range of 1000 to 16000 c/s was investigated (Reysenbach de Haan 1956). About a year later Hamilton, in the U.S.A., who did not know of our investigation, carried out more or less the same experiments and arrived at the same results and conclusions.

Whales and dolphins are the mammals most adapted to submarine life, also with regard to their sense organs. This implies a regression of the visual sense and the almost total disappearance of their olfactory sense, and has been substituted

by a very particular development of the auditory sense. This is illustrated by the ear of a toothed whale, for example, the pilot whale, and of the common porpoise. The external ear is absent. In view of what was said above, this should be regarded as a very understandable adaptation. Under water a pinna would be without function as a sound receiver, and would moreover be in conflict with the stream- lined form. The auditory opening in the skin is so small (about half a millimetre in diameter) that it is practically not to be found. This made the investigators of past time exclaim, with Pliny: 'Delphinos audire manifestum est quanam audiunt mirum' (it is certain that the dolphins hear, but how they hear is a riddle).

The external auditory canal runs with an S-shaped bend first through the blubber and then through more compact connective tissue. It begins, as stated before, with a very small lumen and ends as an inverted funnel with a somewhat increasing diameter-about 5 mm-against the basis of the tympanic conus, which is contained in the meatus acusticus externus of the bulla tympanica. It is filled with water and partially with desquamated epithelium. In a proximal direction it is surrounded by some rudimentary muscles and cartilaginous plates.

This is somewhat different in whalebone whales. They also have a narrow external

opening, but then the canal ends in the blubber and changes into a strand of connective tissue. Then this strand changes again into a canal with a rather wide

funnel-shaped lumen, entirely filled with a wax plug extending as far as the tympanic membrane (Fraser & Purves 1954).

All these structures cannot have any function in the reception, concentration, or conduction of sound, as the rate of sound transmission in the tissues in and around the external auditory canal is practically equal to that in water. Every- where the sound radiates equally through these tissues. We were able to confirm this by experiments on dead material (Reysenbach de Haan 1956).

It is, however, of the greatest importance that the auditory canal be closed for

great variations in pressure, as behind the tympanic conus in the bulla there is the air of the middle ear into which absolutely no water should penetrate from the outside.

And now some words on the middle ear:

During the embryonic development the whole middle and inner ear, forming a dual structure, namely, the os petrotympanicum or auditory bone, has dropped out of the skull and hangs loosely underneath it. This petrotympanicum consists of: (1) the petromastoid, in the compact bone mass of which the inner ear is contained, and (2) the bulla tympanica in which the middle ear mechanism is

57



F. W. Reysenbach de Haan (Discussion Meeting)

suspended. This bulla itself is suspended underneath the petromastoid by fragile bony connexions. The petromastoid is almost entirely surrounded by air-filled, i.e. foam-filled sinuses. These sinuses are in communication with the air in the

bulla, and via the eustachian tube with the air in the nose and other respiratory passages. Thus the inner ear is entirely air-insulated from the surrounding tissue and skull and therefore from the other inner ear and the water. This also holds

good for the middle ear mechanism with one exception, however. This is the base of the tympanic conus, which faces the external auditory canal filled with water or tissue. Sound waves, therefore, cannot reach directly the inner ear system or the middle ear system (which here, like in nearly all mammals, consists of hammer, anvil, stapes and two middle ear muscles), otherwise than via the tympanic conus.

As said before, sound is reflected for 99-9 % at the boundary surface between water or tissue and air. Sound waves, therefore, can reach the inner ear only via the middle ear system. A very important consequence of this is that sound is never capable of stimulating the two ears at the same time by causing to vibrate the whole skull plus the middle ears usually contained in it, as is the case in man under water. In the whales and dolphins the organs of hearing have developed into two receptors independent of each other. Differences in time, intensity and phase of the sound under water can, therefore, fully be employed for directional hearing.

The middle ear mechanism in itself also shows spectacular changes. It is heavy and very compact, like, as a matter of fact, the osseous tissue of the whole tym- panopetromastoid. The malleus adheres to the bulla by the bony processus gracilis. The manubrium mallei has been reduced to a tiny ridge on which the point of the tympanic conus ends. The tensor tympani muscle is well developed; when con- tracting, it can stretch the tympanic conus longitudinally, thus giving the whole middle ear apparatus an even greater rigidity. The structure of the articular surfaces of the anvil with the hammer is such that only the physiological movement of the hammer-oscillating around the bony processus gracilis--can be transmitted. Fraser & Purves (I954) demonstrated this by an ingenious experiment on models. The compact stapes, is almost immovably inserted into the oval window of the

petromastoid. There is a well-functioning stapedius muscle, as also a round window. This heavy, massive, and very rigid, seemingly completely fused middle ear

system has caused most investigators in this field to deny any functional significance to the middle ear. I believe, however, that Fraser & Purves were the first to under- stand the real significance of this middle ear mechanism in the Cetacea.

I had arrived at the same conclusion in a different way. I assumed the middle ear had to have a function, and its special construction to be of special audio- physiological significance. When examining the middle ear mechanisms of mammals already partially adapted to aquatic life, for example the Pinnipedia and even more characteristically the Sirenia, one is struck by their heaviness and massiveness. This must be associated with the adaptation of hearing to underwater sound. Physical calculations show that to obtain sound in water of the same intensity as sound in air, the sound presssure must be sixty times as strong as in air. The amplitude of the particles in the wave in water is then sixty times smaller than in air (Reysenbach de Haan I956). Water is indeed a much heavier and more

58




rigid medium than air. To effect a good transmission of sound the middle ear system of marine mammals must, therefore, be much heavier than that of land mammals, which is as light as possible. Its mass must be proportionately greater and its

suspension must be much more rigid. Its movements or deflexions need not be as great as in land mammals.

The middle ear of the Cetacea is certainly much heavier than that of land mam-

mals, but undeniably lighter than that of mammals less adapted to water. Its construction is, however, much more rigid than for example in Pinnepedia and Sirenia. This marked rigidity explains this seeming contradition: it serves for the

particularly developed supersonic hearing of these animals.

Many observations have shown that Cetacea are sensitive to supersonic fre-

quencies, even much higher than 100 000 c/s, and it is also known that these animals are capable of producing very high tones. This is quite understandable from the physical and physiological points of view. When an animal has to find its

way (hearing its way) or find its food under water or in the dark, in other words without the help of small, vision or lateral line organ, it will have to do so by means of echo-location. The farther away the objects to be explored, the lower the

frequencies and therefore the longer the wavelengths it has to use (these longer waves are less absorbed and spread in all directions). Conversely, the closer and the smaller the objects, the shorter the waves and the higher the frequencies of the emitted sound vibrations have to be (increasing bundle character, decreasing penetrative power of the sound). A bat does the same, but in the air. In water, with the four times longer wavelength, echo-location has therefore to work with

very high frequencies. What are the consequences of this for the middle ear? We know that the thinner

the string of a violin, or the smaller its mass and the greater the tension, the

higher the tone it produces. The same holds good for a sound receptor; the smaller its mass and the greater its rigidity, the higher the tones it can receive and transmit. This can very simply be represented in the equation:

I / /i idity v? =- \ mass /

in which v. is the natural frequency of the system under consideration itself: the natural frequency. v0 is directly proportional to the rigidity of suspension of the middle ear system and inversely proportional to its mass. Supersonic fre-

quencies therefore demand a great rigidity and a small mass. However, on other

grounds, underwater hearing demands a large mass again. The whale middle ear is therefore a compromise: it must be as stiff as possible and only as heavy as is

necessary to achieve this. For this reason it is light compared with that of the Pinnipedia and Sirenia and

heavy compared with that of land mammals. A comparative anatomical examination of the middle ear in man, monkeys,

cats, mice, and bats is therefore very instructive. The ability to hear supersonic sounds increases markedly, in this order. The mass of the middle ear decreases

strongly in this order, but the stiffness increases enormously. In the bats the

59



F. W. Reysenbach de Haan (Discussion Meeting) hammer adheres completely to the bulla and the stapes is just as difficult to

dislodge from the oval window as in whales. A few words on the inner ear of whales: This is contained in the stone-heavy compact petromastoid, which hangs rela-

tively loosely underneath the skull. Why is this bony substance and that of the bulla so extremely heavy in contrast to the other bones in the Cetacea? The answer is that it must be the static, the fixed point for the sound. It should not be capable of being set in vibration together with the whole whale or its skull by frequencies of physiological order for the whale; in a way it must be inert for these

frequencies. It is suspended from the skull as the heavy weight of a pendulum. Only by this

method of suspension and by the special air-insulation can the middle ear mechanism make the sensory apparatus contained in the petromastoid vibrate independently of the other structures. It must, therefore, be loosely connected, acoustically insulated and as heavy as possible. This implies that cochlea and vestibular

apparatus should occupy as little space as possible. This was very easy in the case of the vestibular apparatus, because it can be of very small dimensions (provided the mutual proportions are correct) without losing anything of its function. It is, therefore, very difficult to be found in the glass-like bone (Camper 1767, 1785, I82o).

The robust cochlea requires a certain minimum size, in connexion with its

qualitative function. The tones to be heard range from very low to very high, which demands an elongated organ. In the mammalian ear the cochlea contains the sensory hearing apparatus: the organ of Corti. The sensory epithelium present in the basal turn of the cochlea closest to the round window is adapted to the

reception of the highest sound frequencies. Higher up in the more apical turns, lower frequency tones are effective. If, therefore, one wishes to have more

certainty about supersonic hearing in Cetacea, special structures should be sought for in the lower part of the basal turn.

A comparative functional-anatomic investigation of the cochleae of man, monkey, cat, mouse, and bat, all of them mammals in which the highest audible pitch is known to be 16, 30, 50, 90 and 120 kc/s, respectively, yielded striking results. The relationship between the mass of the organ of Corti and the rigidity of its

suspension and its surroundings showed remarkable differences in the various animals.

The higher the frequencies an animal can hear, the smaller the mass of the organ of Corti, the stiffer its suspension, that is the basilar membrane, and the higher the cells which form its boundary-the cells of Claudius. This is a differentiation in development present in every cochlea: from the upper regions where the lowest tones are heard down to the lower regions, i.e. at the end of the basal turn near the round window, where the sensory cells react to the highest frequencies perceptible for the organism.

This development assumes extreme forms in those mammals that can hear the

highest frequencies. The organ of Corti becomes very small, the basilar membrane on which it lies becomes very short and tautly stretched by the particularly

60



Some aspects of mammalian hearing under water 61

developed ligamentum spirale, which in the mammals sensitive to sound of the highest pitch is also almost entirely supported by a bony lamella. The cells of Claudius tower far above those of the organ of Corti and they are the highest epithelial cells known in the mammals (Kolmer I908). Mouse, bat and toothed whale, in this order, show these particular features in an increasing degree.

The adequate impulse for the organ of Corti is effected by a movement of the basilar membrane under the influence of pressure waves in the perilymph, caused by stamping movements of the stapes in the oval window. This gives rise to friction, or shearing forces, between the organ of Corti lying on this membrane and the tectorial membrane, into which the offshoots of the hearing hairs of Corti protrude. One may imagine that here also, for the perception of the highest sound frequencies, selection has favoured the smallest possible mass suspended as rigidly as possible.

Finally, a brief remark on the so very interesting central acoustic system in toothed whales. We may put it that here the brain is mainly built around the central acoustic system, which is a very remarkable feature in the mammalian kingdom. The nuclei, reflex and co-ordination centres, which possess a special significance for the interpretation and elaboration of information supplied by supersonic frequencies, are particularly well developed; this suggests a very striking parallelism with the central acoustic system of bats.

REFERENCES (Reysenbach de Haan)

Camper, P. 1767 Verhandelingen over het gehoor van den Cachelot of Pot-Walvisch. Verhandelingen uitg. door de Hollandsche Maatschappye der Weetenschappen, Dl. IX, st. 3, pp. 193-229. Haarlem by J. Bosch.

Camper, P. I785 Abhandlung fiber den Sitz des beinernen Gehororgans und uiber einen der Wichtigsten Theile dieses Werkzeugs selbst bei den Wallfischen. Petrus Campers Sdmmtliche Kleinere Schriften. Leipzig: F. C. W. Vogel, Band II, st. 1, 1-40.

Camper, P. i820 Observations anatomiques sur la structure interieure et le squelette de plusieurs especes de Cetaces. (Publies par A. G. Camper avec des notes par G. Cuvier.) Paris: Gabr. Dufour.

Egmond, A. A. J. van, Groen, J. J. & Jongkees, L. B. W. I949 The mechanics of the semi- circular canal. J. Physiol. 110, 1-17.

Fraser, F. C. 1947 Sounds emitted by dolphins. Nature, Lond. 160, 759. Fraser, F. C. & Purves, P. E. 1954 Hearing in cetaceans. Bull. Brit. Mus. (Nat. Hist.) Zool.

2, 103-113. Groen, J. J. I949 De evenwichtszintuigen. Medische Physica, onder red, van H. C. Burger en

G. C. E. Burger, pp. 510-561. Amsterdam: N. V. Noord-Holl. Uitg. Mij. Groen, J. J. 1955 5 Leerboek der Audiologie D 1. I (stencil). Hamilton, P. M. I957 Noise masked thresholds as a function of tonal duration and masking

noise band width. J. Acoust. Soc. Amer. 29, 506-511. Kellogg, W. N. I958 Echo ranging in the porpoise. Science, 128, 982-988. Kellogg, W. N. 1959 Auditory perception of submerged objects by porpoises. J. Acoust.

Soc. Amer. 31, 1-6. Kellogg, W. N., Kohler, R. & Morris, H. N. I953 Porpoise sounds as sonar signals. Science,

117, 239-243. Kolmer, W. I908 Ueber das hautige Labyrinth des Delphinus. Anat. Anz. 32, 295-300. Lowenstein, 0. & Roberts, T. D. M. I951 The localization and analysis of the responses to

vibration from the isolated elasmobranch labyrinth. J. Physiol. 114, 471-489. Purves, P. E. 1955 The wax plug in the external auditory meatus of the mysticeti. Discovery

reports of the National Institute of Oceanography, Cambridge. Vol. xxvii.



62 F. W. Reysenbach de Haan (Discussion Meeting)

Reysenbach de Haan, F. W. 1956 De Ceti Auditu, Over de Gehoorzin bij de walvissen (with a summary in English). Diss. Utrecht.

Reysenbach de Haan, F. W. I957 De Gehoorzin van Cetacea. Vakbl. Biol. 37, 117-127. Reysenbach de Haan, F. W. I957 Hearing in whales. Acta otolaryng. Stockh. Suppl. 134. Schevill, W. E. & Lawrence, Barbara 1949 Underwater listening to the white porpoise.

Science, 109, 143-146. Schevill, W. E. & Lawrence, Barbara. 1953 Auditory response of a bottle-nosed porpoise,

Tursiops truncatus, to frequencies about 100 ke. J. Exp. Zool. 124, 147-165.

Anatomy and function of the cetacean ear

BY F. C. FRASER AND P. E. PURVES

British Museum (Natural History), London

[Plates 10 and 11]

At the present time the sounds emitted by cetaceans and consequently their sense of hearing are the subject of a considerable amount of investigation. The remarks that follow are in anticipation of a more detailed account of the cetacean ear which the writers have completed for the Bulletin of the British Museum (Natural History).

In order to clarify the hypothesis arrived at, a brief summary of the anatomical features involved is necessary (in connexion with which reference to figures 88 to 90, plates 10 and 11 should be made).

The external auditory meatus is a continuous narrow tube in the toothed cetaceans (figure 88). In the whalebone whales (figure 89) it is usually closed for some part of its length immediately internal to the blubber layer, The raeatus is lined by a pigmented extension of the epidermis. Surrounding the lining layer is a fibro-elastic sheath, the fibres of which run predominantly along the length of the meatus. Surrounding this sheath again is a fibro-cellular structure in which a thin stratum of circular constrictor muscles has been observed. Enveloping the whole of this area and adhering to the bones is a great mass of dense, white fibrous tissue about 30 cm in thickness with tough, unyielding fibres forming a close randomly orientated reticulum throughout the entire mass. The meatus is thus enclosed in a tunnel formed dorsally of bone and ventrally of white fibrous tissue.

Associated with the tube are incompletely investing cartilages into which auricular muscles are inserted (figure 89). In the toothed cetaceans the meatus has a distinctive sigmoid flexure which is differently orientated in different species (figure 88).

In both toothed and whalebone whales the meatus widens out mesially to terminate at the tympanic membrane. In whalebone whales, deep to the blind external portion, the mesial distended portion of the meatus is filled by a lami- nated accumulation of keratinized epitheliuim and cholesterol-the ear plug, which has been used in age determination. The ear drum is not as in terrestrial

62 F. W. Reysenbach de Haan (Discussion Meeting)

Reysenbach de Haan, F. W. 1956 De Ceti Auditu, Over de Gehoorzin bij de walvissen (with a summary in English). Diss. Utrecht.

Reysenbach de Haan, F. W. I957 De Gehoorzin van Cetacea. Vakbl. Biol. 37, 117-127. Reysenbach de Haan, F. W. I957 Hearing in whales. Acta otolaryng. Stockh. Suppl. 134. Schevill, W. E. & Lawrence, Barbara 1949 Underwater listening to the white porpoise.

Science, 109, 143-146. Schevill, W. E. & Lawrence, Barbara. 1953 Auditory response of a bottle-nosed porpoise,

Tursiops truncatus, to frequencies about 100 ke. J. Exp. Zool. 124, 147-165.

Anatomy and function of the cetacean ear

BY F. C. FRASER AND P. E. PURVES

British Museum (Natural History), London

[Plates 10 and 11]

At the present time the sounds emitted by cetaceans and consequently their sense of hearing are the subject of a considerable amount of investigation. The remarks that follow are in anticipation of a more detailed account of the cetacean ear which the writers have completed for the Bulletin of the British Museum (Natural History).

In order to clarify the hypothesis arrived at, a brief summary of the anatomical features involved is necessary (in connexion with which reference to figures 88 to 90, plates 10 and 11 should be made).

The external auditory meatus is a continuous narrow tube in the toothed cetaceans (figure 88). In the whalebone whales (figure 89) it is usually closed for some part of its length immediately internal to the blubber layer, The raeatus is lined by a pigmented extension of the epidermis. Surrounding the lining layer is a fibro-elastic sheath, the fibres of which run predominantly along the length of the meatus. Surrounding this sheath again is a fibro-cellular structure in which a thin stratum of circular constrictor muscles has been observed. Enveloping the whole of this area and adhering to the bones is a great mass of dense, white fibrous tissue about 30 cm in thickness with tough, unyielding fibres forming a close randomly orientated reticulum throughout the entire mass. The meatus is thus enclosed in a tunnel formed dorsally of bone and ventrally of white fibrous tissue.

Associated with the tube are incompletely investing cartilages into which auricular muscles are inserted (figure 89). In the toothed cetaceans the meatus has a distinctive sigmoid flexure which is differently orientated in different species (figure 88).

In both toothed and whalebone whales the meatus widens out mesially to terminate at the tympanic membrane. In whalebone whales, deep to the blind external portion, the mesial distended portion of the meatus is filled by a lami- nated accumulation of keratinized epitheliuim and cholesterol-the ear plug, which has been used in age determination. The ear drum is not as in terrestrial

