IMAGE EVALUATION TEST TARGET (MT-3) 5, Z, 1.0 I.I Z m 1.8 1.25 1 ^ 16 ^ 6" ^ ,^. .%^^ / y ^ Photographic Sciences Corporation 1 \ .V N? :\ \ ^^ V '^>. 6^ # ^!;. 23 WEST MAIN STREET WEBSTER, N.Y. 14580 (716) 872-4503 CIHM/ICMH Microfiche Series. CIHM/ICMH Collection de microfiches. Canadian institute for Historical Microreproductions / Institut Canadian de microreproductions historiques Tschnical and Bibliographic Notaa/Notas tachniquaa at bibiiographiquas Tha Instituta has attamptad to obtain tha bast original copy availabia for filming. Faaturas of this copy which may ba bibiiographicaily uniqua, which may altar any of the imagas in tha raproduction, or which may significantly changa tha usual mathod of filming, ara chackad balow. □ Colourad covars/ Couvartura da coulaur I I Covars damariad/ D D D D D D Couvartura ar^dommagia Covars rastorad and/or laminatad/ Couvartura rastaurte at/ou palliculte □ Covar titia missing/ La D titra da couvartura manqua I I Coloured maps/ Cartes gAographiquas an couleur Coloured ink (i.e. other than blue or black)/ Encra da coulaur (i.e. autre que bleue ou noire) Coloured plataa and/or illustrations/ Planchns at/ou illustrations en coulaur Bound with other material/ ReliA avec d'autre« documents Tight binding may cause shadows or distortion along interior margin/ La re liure serrie peut causer de I'ombre ou de la distorsion le long de la marge intArieure Blank leaves added during restoration may appear within the text. Whenever possible, these have been omitted from filming/ II se peut que certainas pages blanches ajoutias lors d'une restauration apparaissent dans le texte, mais. lorsque cela itait possible, ces pages n'ont pa!! 4t6 fiimies. Additional comments:/ Commentaires suppl^mentaires: L'Institut a microfilm* la meilleur examplaire qu'il lui a itt possible de se procurer. Las details de cet exempiaira qui sont paut-Atre uniques du poim de vue bibliographique, qui peuvent modifier une image reproduite, ou qui peuvent exiger una modification dans la mithoda normale de fiimage sont indiqute ci-dessous. □ Coloured pages/ Pagaa da coulaur □ Pages damaged/ Pages endommagias |~n Pages restored and/or laminated/ D D Pages restaur^es at/ou peilicuiias Page? discoloured, stained or foxed/ Pages dAcolories, tachaties ou piquies □Pages detached/ Pages ditachies EShowthrough/ Transparence Transparence Quality of prin Quality inigale de I'impression Includes supplementary materii Comprend du material supplimentaire [~n Quality of print varies/ I I Includes supplementary material/ Only edition available/ Seule Mition disponible Pages wholly or partially obscured by errata slips, tissues, etc., have been refilmed to ensure the best possible image/ Les pages totalement ou partiailement obscurcies par un feuillet d'errata, une pelure, etc., ont M filmies A nouveau de faqon i obtenir la meilleure image possible. This item is filmed at the reduction ratio checked below/ Ce document est film* au taux de reduction indiquA ci-dessous. 10X 14X 18X 22X 26X 30X y 12X 16X 20X a4x 28X 32X -^nawa; Th« copy filmad h«r« has b««n reproduced thanka to tha ganaroaity of: Medical Library IMcGili University IVIontr'>al Tha Imagaa appaaring hara ara tha baat quality possibia eonaidaring tha condition and lagibiiity of tha original copy and in kaaping wKh tha filming contract spacificationa. L'axampiaira fiimi fut raproduit grica i la gAniroaiti da: lyfledical Library McGill University Montreal Laa imagaa auh^antaa ont 4t* raproduitaa avac la plua grand aoin, eompta tanu da la condition at da la nattat* da l'axampiaira film*, at an conformiti avac laa conditiona du contrat da filmaga. Original copiaa in printad papar eovars ara filmad beginning with tha front covar and ending on tha laat page with a printad or illuatratad imprea- •ion, or the back covar when appropriate. All other original copiaa ara filmed beginning on the f Irat page wKh a printad or iilcjatrated lmprea« aion, and ending on the laat page with a printad or illuatratad impreaaion. Lea axemplairea origlnaux dont la couvarture an pepier eat imprimte sont filmAa an commandant par la premier plat at en terminant solt par la darniire page qui comporta une ampreinte d'impraaaion ou d'iliuatration. soit par le second plat, salon la caa. Toua lea autrea axemplairea origlnaux sont filmis en commen^ant par la pramlAre pege qui comporte une emprelnte d'impreaaion ou d'iliuatration at en terminant par la darnlAre page qui comporte une telle emprelnte. The laat recorded frame on each microfleha shall contain the aymboi >-»• (meaning "CON- TINUED"), or the symbol ▼ (meaning "END"), whichever appllee. Un dee symbolaa suivanta apparattra sur la damlAre image de cheque microfiche, selon le caa: la symbole -^ signifie "A SUiVRE", le symbols V signifie "FIN". Mapa, plates, charta, etc.. may be filmed at different reduction ratloa. Thoae too large to be entirely included in one expoaura are filmed beginning in the upper left hand corner, left to righi and top to bottom, aa many framea aa required. The following diagrama illuatrate the method: Lea cartea. planchea. tableaux, etc., peuvent itre fiimia i dee taux de rMuction diff^rents. Lorsque le document est trop grand pour Atra raproduit en un seul cliche, il est film* A partir de I'angie aup^riaur gauche, de gauche k droite, et de haut an baa, an prenant le nombre d'imagea nteaaaaira. Lea diagrammea suivanta iiluatrant la mAthode. 1 2 3 j- 1 2 3 4 5 6 6 VK "^ ■^ ,^.NM. ^y/s^ Reprinted from Die Montreai Medical Journal, July, 1891 COMl'IJMKNTs (!)' J. W. Stirling- M.B. o.'iO i>(>i;c;ii i;sri';i; s MONT 1:1: A I.. OUR PROJECTION OF A SOUND SOURCE IN SPACE.* By J. W. Stirling, M.B., &c. Surgeon Oculist and Aurist to the Metropolitan Dispenaaryi The title of my paper would have better been " our present lack of knowledge," than our knowledge of the projection of sound in space by the human ear. It is very remarkable how hazy the knowledge of the scientific world is as yet in regard to the functions of the various parts of the inner ear. We know that it is the end organ of the auditory nerve, but when one proceeds to specialize further, the subject is to a ertain extent enveloped in doubt. This is the more striking when we compare it with our knowledge of the functions of the eye and its various elements, for in the latter we venture upon well trodden and thoroughly known ground. Before proceeding to the subject of this paper allow me very briefly to run over the anatomy and physiology of the inner ear, as it will assist in making the subject matter clearer later on. Hearing occurs in one of two ways, — either through the bones of the head, or per meatum- through the bones, as through all solid bodies, by waves of condensation. Let us glance at the latter and usual way. The acoustic waves enter the meatus and are transmitted from the membrana tympani by means of the ossicles of the middle ear to the fenestra ovalis. Here the repeated shocks of the stapes set up a series of waves in the lymph of the labyrinth. In order * Head before the Natural Hiatory Society of Montreal, April 27th, 1891. 2 to allow of tlio waves beinj^ traiismtted through the lymph there must he yielding points, and these occur at the i'enestra rotunda, the two openings of the aqueduct of the vestibule, the membranes of the aqueduct of the cochlea, and the pores of the blood-vessels ifi the bone. Impulses started in the fluid of the labyrinth would thus result in its movements back and forth so as to produce a friction against the end apparatus of the auditory nerve. This friction would be increased by the action of the otoliths found in the fluid. Thus the waves started at the fenestra ovalis would be diffused over the vestibule and into the scala vestibuli of the cochlea, where they would flow to its head, being prevented by the separating membrane from entering the scala tympani. It is not known certainly to what extent these waves flow through the helicotrema into the scala tympani, nor what are the exact relations between the waves in the scala tympani and those in the scala vestibuli. , The basilar membrane is thrown into vibration through the unequal pressure of the moving fluid, and by its vibrations stimulates the nervous structures, ending in it, giving rise to the perception of sounds. In the same manner the lymph of the semi-circular canals, their ampullse and vestibuli, is thrown into movements. ,, ^, The structure of the end apparatus of the vestibule and semi- circular canals is evidently not adapted to the analysis of musical tones like the basilar membrane. The otoliths found in the ampullae and also the hairs are not capable of regular sympathetic vibrations ; moreover, they f jrm no scale of structures corresponding to the scale of sensations of tone. This fact led Helmholz to consider that tones — i.e., sounds of regular periodic vibrations — are heard only by the cochlea, while noises — i.e., sounds of short, temporary, irregular vibra- tions — are procured by the saccule and ampulla. Helmholz has, however, abandoned this theory since Exner has shown that a tone pitch is also perceived in noises. The function oi the semi-circular canals has mainly to do with the space sense, acting in the preservation of our equilibrium, which, despite emphatic contradictions, Breuer of Vienna has 8 lately proved very successfully to be the case. Breuer stimu- lated the ampullae of the various canals by the galvano-cautery and electricity. He found that on stimulating the utricular end of the ampulla there was a movement of the head in the plane of the canal so stimulated and to the same side as that to which the canal appertained ; while if the canal end of the ampulla were stimulated, the movement was in the plane of the canal, but to the opposite side. However, there is little doubt but that the ampulla and canals have something to do with sound perception, and that likely in the way of perception of the source of a sound. I will dilate on this theory later on. It is well at the outset to bear in mind that hearing is a mental act. Every auditory sensation is an affection of the mind, recognized as connected with an extra mental reality through the activity of the auditory nerve. All mental sensations arising from stimulation of the auditory nerve are called sensations of sound. We know nothing directly through sensations either of the structure of the ear or of vibrat- ing strings, or particles of air or of the physics of music. That there is a certain sense of space or localization attendant on all sense perceptions there can be no doubt, but it is still a question what determines this power of localization. It is un- doubtedly a matter of memory, experience and education. Preyer of Jena, in 1884, made a series of very interesting investigations on infants and very young animals, in order to determine, if possible, the nature and development of this space sense. As a result, he found that at a very early stage of psychical development, animals and children regard the various parts of their body as entirely distinct from themselves. The chick plucks at its own nails, just as it would at the corn thrown out to feed it. The child tried to tear off its finger or divide its foot from its leg. Only later on do they begin to grasp the idea that limbs, etc., appertain to themselves and are distinct from the surrounding world, and then begin to locate each sensory impression correctly in a certain spot or direction. It would be quite impossible, after this, to have any conscious sensory impression which will not have a certain sense of space or position in regard to the body, once that this latter is per- ceived to be distinct and separate from the rest of the universe. Children are born deaf and only begin to hear about the fourth day after birth. About the eleventh week they begin to move the head in the direction from which a sound comes. A week later the turning of the head in the direction of the source of a sound is sudden and rapid, even when the glance o( th'^ eye does not take at once the correct direction, but once the direc- tion was discovered the child hearkens with the closest attention. After the lapse of two or three weeks, the turning of the head toward the source of sound occurs with the certainty of a reflex movement. Now for the first time distant sound impressions are perceived. The perception of sound th: ough the bones of the head occurs somewhat earlier than fhe twentieth month. Darwin, Vierordt and Demme give varying periods as to the first perception of the direction of sound by an infant, the extremes being twelve weeks for the earliest and seventeen weeks for the latest. Guinea-pigs noted sound and its direction the very day of birth, although deaf immediately after birth. The little auricles contracted and moved with great rapidity, comparable to the action of the pupil under light stimulus. Cats and dogs I have found to give similar results, although they have a tendency to project a sound on their own level even though coming from above them. We thus see animals are in advance of infants in regard to the early age at which they appreciate sound and its direction. As Preyer remarks, the child hears nothing at first, then hears some sounds indistinctly, later hears many sounds indistinctly. Now very gradually he distinguishes an individual sound dis- distinctly from among many indistinctly, and finally hears much distinctly and distinguishes stiong high tones earlier than deep ones. Only when this latter stage is being reached does the infant seem to obtain some certain idea as to the direction from which a sound comes. This perception of sound and its direction, in common with all perceptions, is the result of an extremely complex activity of the mind. It is carious to note how, as it were irrationally, the child undertakes the synthesis of the number of sense data which it involves. The exactness, however, of this space perception varies greatly in the different senses. The eye is very exact, thanks to its great motility and manifold innervation. The skin is not nearly so exact, and varies in different parts of the body. The hearing perceives not only distant sources of sound, but locates sound in the interior of the head, and perceives very slight differences in the pitch of sound by the vibration of the air in the external meatus. Only thus far has the entire tone perception by means of the ear any sense of space, for although these sound sources are perceived, they are rarely proJ3cted correctly. Hence this highly developed function is rarely used for the perception of space sense or orientation. The oldest theory in regard to our perception of the direction of a sound is what is known in Germany as the recht-links, or right-left localization. The perception of direction according to this theory depends on the relation oT either meatus auditorius externus to the source of the sound, and we obtain our informa- tion, especially, if we turn our head in the supposed direction of the sound. If both ears are stimulated to the same extent, we project the sound source in the median plane f criorly ; but when one ear is stimulated more strongly than the other, the sound source is always projected to the side of the most powerful sensation. In testing this by moving a sounding object in a horizontal circle round the head on the level of the meatus auditorius externus, Rayleigh found that a variation of 1 per cent, in the strength of stimulus between the two ears is perceived. This seems slightly overdrawn, as 10 to 20 per cent, have been the figures asserted by all other authorities, and indeed the marked uncertainty of aural projection would favour the acceptance of the latter figures. Rayleigh further asserts that we perceive the direction of noises mixed with musical tones more easily than tl^at of pure tones. . ! f 6 The position and shape of the auricle is an imf'uriuit Im. tn in this right-left localization, for the projection ol n hmiii,.i h much more faulty in those cases where the ears mo clnsi-ly applied to the side of the head ; at the same time, in those cases in which the auricles project from the side of the head, the pos- terior projection is generally false. Simple proof of this effect of the auricle may be obtained by placing the hollow of our hands in front of the ears, when any sound perceived will in- variably be projected backwards, no matter from what direction it may come. That the power of orientating ourselves in space with reference to external sounds varies in different individuals cannot be doubted It is no (loul)t an acquired art, and depends on atten- tion and experience, as well as the accurate interpretation of the smallest details. Just as in the matter of the vision of savages, which is proverbially so acute, it is a question of attention and practice in the interpretation of minute indications, and the per- petuation of this typo through the requirements of their life. Weber thought we could tell the direction of a sound by means of the perception of the varying swing of the membrana tympani. He instanced, in support of this theory, that eccentric projection is hindered if the meatus is filled with water. It hinders, but does not prevent the perception of the direction ; about this I will speak later on. These facts of the perception and projection of a sound to the side of the greatest intensity gave rise to the theory of acoustic shadows or of the amount of covering power which the sound produced by waves of a given intensity entering one ear would have upon the sound produced by waves of a different intensity entering the other ear. According to Kessel, the best binaural audition is produced when the sound proceeds from the mesial plane anteriorly ; the best monaural when the projection is exactly in a line with the meatus, at right angles with its opening. Kessel further advances the function of the pinna as being the main factor in determin- ing the perception of the direction of a sound. He divides the pinna into five auditory districts which are sharply defined from one another, and are characterized hy the fact that they convey sound to the ear with a differonr intensity, according to the direction of the head at rest or during movement These dis- tricts for the perception of sound are : anterior, posterior, supe- rior, inferior and central or direct. The different auditory dis- tricts are hrougfit into use by means of suitable movements of the head ; and in association with the sense of 8i horizontal canal, were more poworfuilj stimulated than those of the other two, that the sound came directly from tlw left, and in like manner f )r the other two ampullae, the superior and pos- terior, the sound must come from above or behind. The specific energy of the ampullae is a sense of space in connection with sound, and more precisely a sense of direction, the perception of a particular direction depending on the par- ticular one or pair of canals which may be stimulated most powerfully as follows : 1. The horizontal canal is most powerfully stimulated by sound from a direction on the same side in the horizontal plane. 2. Superior vertical canal, which has a direction outwards and forwards, is most markedly stimulated by a sound from above, in front, and to the same side. 3. Posterior canal, which has a direction downwards, back- wards and outwards, perceives most acutely sounds from below, behind and the same side. If two sounds of different intensity come from opposite sides of the head, the sound is projected in the direction of that of the greatest intensity. If both sounds are of equal intensity and pitch, they are projected in the median plane. Preyer applies this theory to all the high vertebrata. Preyer further says that in any case all three canals are stimulated, of course those of the side nearest the sound source more powerfully than those of the opposite side, and again of those canals of the more powerfully stimulated side, one or two more powerfully than the remaining, depending on the direction from which the sound came. This hypothesis being correct, then must all those sound directions be frequently confused with one another, in which a nearly equally strong stimulation of the different ampullae occurs— e.^., Anterior — Ant.-sup. ' Post. — Post.-inf. Post.-inf. — Inf. Ant.-sup. left — Sup. left. Ant. left — Sup. left. Ant. left— Ant.-8up. left. Post, left— Post.-inf. left. This Preyer found to be really the case. The decisions were very uncertain and varying in re antero- n I . inferior, left antero-inferior, and right antero-inferior directions. Prayer conside'rs this due to the stimulation of all the ampullae being of equal intensity. By normal hearing, a sound from the left is never projected to the right, and vice versdj on account of the much greater strength of the stimulation of the ampulla of the side nearest the sound. For the same reason, no sound from the left or right is normally projected in the median plane, nor one in the median plane projected to the right or left. Preyer next experimented with both ears closed, first by press- ing in the tragus, or by filling the meatus with a solution of sodium chloride. The results arrived at were as follows : 1. The correct right and left projections which had been so good before were now uncertain. This being clearly to be ex- pected since the horizontal canals are more easily affected by the movement of the stapes produced by serial sound vibrations from the right or left than are the other two canals, the stapes lying in an almost parallel plane to the horizontal semicircular canal. Hence the cutting off of the serial transmission by the meatus must annul the effects of this factor. 2. The absolute transposition of right to left or of either side to the median plane, and vice versd, only occur as with open ears, for the first few trials. This holds even in a large series of e;xperimcnts ; hence one cannot discover a difference in audi- tion by head conduction as compared to the ordinary audition. 3. The recognition of the direction is greatly increased in difficulty, likely from the absolute decrease in intensity of the sounds, and correspondingly relative decrease of the difference of intensity. 4. The number of correct decisions is greatly lessened. In regard to the perception of sound direction with one ear closed, Ghladni found, in 1802, that sound is always projected to the side of the open ear. (If bone conduction is equal nor- mally to serial, then sound would appear to come from the side of the closed ear.) The idea is false that in usual audition the sound is entirely conducted by the labyrinthine fluid. Ghladni showed, in 1802, 14 that much is heard even after both ears are stopped. (Ewald has just demonstrated on pigeons that after removal of the labyrinth they perceive sounds acutely ; hence that the trunk of the auditory nerve possesses power of sound perception. Only after destruction of the trunk were the pigeons completely deaf.) Kessel says that with open ears the head conduction plays an important r61c, and without it the localizing of a sound source would be far more uncertain than it is. Diminishing the intensity of the sound until it was no longer perceived with closed ears, then opening the ears, was the plan adopted by Preyer. The decisions were very uncertain and incorrect. He considered the bone conduction from the meatus to be better than from other parts of the head, and hence this method only partli^ shut off factor of bone conduction. Preyer wants more experiments in this series. It is likely that repeated practice would render better results. This is Preyer's theory, and there is no doubt something in it. The chief fault I find is, that it is hard to believe that with errors of 180^ in the median plane there can exist a special physiological mechanism for the appreciation of the direction of sound. This fault, however, may be simply due to lack of edu- cation of this special division of the organ of audition, this lack of education extending through generations. Gruber, in 1869, expressed the opinion that the semicircular canals were not exclusively an organ of the sense of space, and was induced to believe that they were concerned rather in a participation of the hearing function. There can be no doubt, after Breuer's experiments and the results therefrom attained, that they are at any rate partially organs for the perception of our position in space and maintenance of our equilibrium, might they not be also organs for the proper projection of sound in space — organs for space perception in a double sense ? Add to this the two factors of bone conduction and their existence as the main part of the hearing organ in fishes. That the eyes assist in the appreciation of the direction of the sound there can be no doubt, and that they have filled up or 16 possibly have caused this deficiency in function of auditory pro- jection is quite likely. Hogyes holds there is a close connection between the eyes and the semicircular canals. He says he has demonstrated that a peculiar bilateral reflex connection exists between the muscles of the eye and the ampuUary nerves, in conformity with which a reflex stimulus is transmitted by each labyrinth to certain muscles — from the left vestibular nerve to those turning the left eye upwards and outwards and rotating it inwards, and to those which turn the right eye downwards and inwards and rotate it outwards, the right vestibular nerve acting in exactly the opposite way. H. vonKreiss has, during the past year, pursued a series of investigations much on the same plan as Preyer's. Judging that possibly some modification in the quality or intensity of a certain sound, according as it came from in front of or behind the head, would assist in localizing the source, Kriess varied the sound and also the distance from the head. He held that this modification in the quality or intensity could be learnt, and hence the projection would be really per meatum and fall under the category of the right-left theory. This power of localization from some modification he called " mediate localization." However, his experiments after this manner showed the uncertainty of median localization, as also the precaution with which the results must be judged. Kriess conducted all his experiments in the median plane, ■ holding that these points would be sufficient to prove the possi- bility of a special sound space mechanism, whereas the number of points examined by Preyer were only confusing. He used castanets and the telephone at a distance of twenty centimetres from the head. The antero-posterior differentiation varied greatly, but the superior-inferior was more accurate. Practice - in some cases seemed to improve, in other cases to fail. Kriess and his assistant projected almost always the posterior in front. In the posterior and inferior, both in front and behind, one-third were false. Kriess, to completely eliminate the factor of mediate localization, used seven different instruments, when, instead of producing a diminution of correct decisions, it caused 16 an increase. A slightly prolonged sound was more accurately projected than a short one. Kriess considers the capability of a median projection cannot be doubted, and it depends on (1) practice, (2) the nature of the sound, and (3) some disposition. Among a number of per- sons he examined there was one who was absolutoly correct even at different examinations, and this one had no special talent for music, so that any variation in tone might have assisted. It was easier to distinguish change of direction if one sound was produced very shortly after the other, say half a second. Another peculiarity is the decided tendency which exists for certain errors — e.g.^ some people invariably project a sound from behind as coming from in front, and in front always correct, also vice versa. Again, in others the sound scurce is projected much higher than it really is, although this appeared by prefer- ence when the weak telephone click was used, and more rarely when the castanets were employed. Kriess gathered from his investigations that an almost certain median localization (at least in so far as a decision of posterior from anterior) can occur under certain conditions — i.e., if the sound stimulus from examination to examination is changed both in regard to its quality, intensity and distance. On the other hand, one cannot help but notice the extraordinary uncertainty which appears if the same localization and other conditions are imposed. The theory of a mediate localization (i.e., by the slight modi- fications of a sound arising frem its position) receives a check, for it is the very opposite to find that the certainty of localization is favoured by altering the sound from time to time. There are here two facts worthy of mention. 1st, One could decidedly speak of the indirect nature of median localization if the distinction of the location were positively decided by the nature of the chosen sound stimulus — i.e., a weak sound pro- jected by preference behind and a loud one in front. This did not occur in any of Kriess' cases. Only in one case were nearly all projections accurate, even after repeated investigations. From this, one might conclude that if a physiological mechanism existed 17 for the ajjpreciation of sound direction, with it also the (juality and intensity of the sound would come into consideration, and would be of influence in forming our decisions. Just a few words now about the double direction perception. Using two sounds of different quality, n fife note and a hiss- ing sound, Kriess found both in front or both behind were cor- rectly localized, but so soon as one was in front and the other behind, a degree of uncertainty appeared. The fife note behind and the noise in front were correctly, as a >ule, distinguished ; but with the fife in front and the noise behind, both were pro- jected behind. There was no doubt but that the noise from behind interfered with the localization of the tones in front. By the opposite arrangement the sounds were much more certainly projected, thus clejirly proving the power of a double localization. The results varied greatly, but clearly showed, nevertheless, the existence of this power. Of course all the foiegoing e.xperiments were performed on individuals with their eyes shut. The great fault I find with both Kriess' and Prever's investi;'ations is, that in their endea- vours to develo[) a new theory, they seem entirely to have lost sight of the 8ha{)e and position of auricle, which there is no denying greatly influences the projection of a sound source. In my own investigations it became very evident, insomuch that before beginning ray work I could pretty clearly see where the greatest tendenc;y to enor would exist. As a test in my own work I used Politzer'a acoumeter, which gives a short, sharp sound. My experiments on cats and dogs I gave up as fruitless in one sense. One idea struck uie that in animals with deficient accommodative power of the eye we find the auricle highly de- veloped and very mobile, as in the horse, cat, dog, guinea-pig ; in these animals the eye is markedly hypeiopic, and for near objects the nose, as we continually see, is used as almost a third eye. On the other hand, in birds, where the accommodative power is more highly developed than in any of the vertebrata, being necessitated by their rapid flight, in these the auricles are absent, as also in aquatic animals. 18 In mj own investigations the results were very variable, and only differed from those recounted above by the fact that I ;^ot a lar;^er |)crceiita:j;o of correct results. There wltc quite u number who, at any rate, at first pr(»jecied the posterior sources most correctly, but these were generally endowed with auricles which were rather flatly apj)lied to the head. Again, there was no doubt that the intellectunl status had a good deal to do with the decisions. In those ol rather dull mental condition, there was a hesitation generally terminating in a false statement. Thinking and studying the matter well ov.er, one can detect in all the theories some pretty big gaps But just as the projection of an object to its correct position in space by the human eye brings into action many different factors, so I think the same holds good for the ear. For as in the eye an object makes itself perceived by stimulating the optic nerve, so in the ear the auditory nerve perceives the sound. The size and dis- tance of an object is partly judged by the strain on accommoda- tion by the projection of the two eyes and by experience. In the ear the accommodation act is performed by the tensor tympani muscle, as Strieker, Hensen, Backendahl and Pollak have shown. In the eye, we judge of the direction of an object by move- ment of the eye and the portion of the retina stimulated ; in the ear we have not this, but we should reasonably look for a sound projection apparatus, and judging by the comparative anatomy of the fish, for instance, as already noted, it would lie in the semicircular canals or ampullae, assisted also by the meatal audi- tion, as factors in the latter are the auricles, which I dilated on in treating of Kessel's theory, and also the walls of the meatus and membrana tympani, if one were to judge by the case of Charcot's already noticed, where, in anaesthesia of the walls of the meatus and tympanic membrane, the perception of the direc- tion of sound was lost. As a brief resum^, let me say — 1. The binaural audition plays the main part in our apprecia- tion of the direction of a sound source, as evidenced in the elaboration of the right-left theory, with its modifications by , Id Kessol in rci^ard to tlie runetioii of the auricle, also possibly by the sensory state of the meatus wall. In this connection may be mentioned Miinaterberg's oculoauial theory. The failure of these theories is in connection with the projection of any source in the median plane. 2. To fill this gap, Preyer and Kriess brought out and elabo- rated the theory of projection by the ampuUsB of the semicircular canals, which, judging by comparative anatomy, appears correct. The fault here is that it is, as I have already said, hard to believe an error of 180° j)088ible with the existence of a physio- logical mechanism for the appreciation of direction ; but against this we have to put those cases where the projection was accu- rate, also some possible weakness of discrimination or lack of attention. It is likely that through lack of use or education the organ has never assumed its full function.