ON THE 
MECHANISM OF HEARING 
BY 
ALBERT H. BUCK, M. D., 
INSTRUCTOR IN OTOLOGY IN THE COLLEGE OF PHYSICIANS AND SURGEONS, NEW YORK; 
AURAL SURGEON OF THE NEW YORK EYE AND EAR INFIRMARY. 
L fIKPR IN TED FROM THE N. Y. MEDICAL JOURNAL, JUNE, 1874 ] 
NEWYORK: 
D. APPLETON AND COMPANY, 
549 & 551 BROADWAY. 
1874. TWELVE YEARS OF THE WORLD’S HISTORY! 
APPLETONS’ AMERICAN 
ANNUAL CYCLOPAEDIA 
FOE 1872. 
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The volume of the Annual Cyclopaedia for 187 a presents the satisfactory settlement of all disputed 
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The Annual Cyclopasdia for 1873 will be published in May, 1874. ON THE 
MECHANISM OF HEARING 
BY 
ALBERT 11. BUCK, M. D., 
INSTRUCTOR IN OTOLOGY IN THE COLLEGE OF PHYSICIANS AND SURGEONS, NEW YORK ; 
AURAL BURGEON OP THE NEW YORK EYE AND EAR INFIRMARY. 
[REPRINTED FROM THE N. T. MEDICAL JOURNAL, JUNE. 18fM] 
NEWYORK: 
D. APPLETON AND COMPANY, 
549 & 551 BROADWAY. 
1874. ON THE 
MECHANISM OF HEAEIAG.1 
It has often been said, and with entire justice, that no other 
part or organ of the human body is so beautiful in structure 
as the ear. Its tympanic membrane, its chain of ossicles, its 
wondrous labyrinth containing cavities within cavities and 
thousands of vibratile chords—all these together go to make 
up an organ of exquisite architectural beauty and one that is 
marvelously adapted to the purpose for which it is designed. 
This delicate organ, with its multitude of fragile elements, 
needs the most ample protection, and we find it, accordingly, 
buried deep in the bone whose name {petrous) tells the story of 
its hardness. Inaccessible, as it is, to the eye, the wondrous 
interior of the labyrinthine structures has long refused to give 
up its secrets. What we know to-day is the result of years 
spent in untiring energy, attended with repeated failures. 
One by one, however, the disputed anatomical points are be- 
ing settled, and it is safe to say that the day is near at hand 
1 Prize Essay of the Alumni Association of the College of Physicians 
and Surgeons, New York, March, 1874. 4 
ON THE MECHANISM OF HEARING. 
when our knowledge of the anatomy of the ear will be as ac- 
curate as that which we possess of the eye. In fact, the prog- 
ress which has been made, not only in the anatomy, but also 
in the physiology of the ear, has been so great during the 
past few years, that it will surely not appear a presumptuous 
thing to venture here upon an explanation—in some respects 
new—of how the act of hearing is performed. 
Fig. 1. 
Diagram showing the Relations op the Ossicles to the External Auditory 
Canal and Labyrinth. 
T, cavity of the tympanum; L, labyrinth; MA E, meatus auditorius externus; H, hammer; 
A, anvil; S, stirrup; MT, memhrana tympani; FR, fenestra rotunda. 
Before entering upon our topic, it will perhaps be best to 
state that for the sake of clearness we shall, as far as possible, 
avoid all discussion of disputed points. Bor the same reason 
we shall simply allude to or entirely omit all those points in 
anatomy which seem to have no important bearing upon the 
main questions at issue. Wherever diagrams will facilitate ON THE MECHANISM OF HEAKINO. 
5 
the description, they will be used, though in most instances in 
such a form as rather to throw light upon the theory' than to 
accurately demonstrate the anatomy. At the same time no 
essential anatomical point will be omitted or treated with un- 
due attention. 
The ear in mammals consists of three different cavities— 
the external auditory canal, the tympanum, and the labyrinth 
(Fig. 1). The first cavity is nearly cylindrical in shape; at 
one end it communicates with the outer world by a broad, 
free opening, while at the other it is entirely shut oft* from 
the cavity beyond (tympanum) by a thin, inelastic membrane 
—the membrana tympani {M T). The second cavity, so far 
as the naked eye can see, is a closed cavity, filled with air, 
which, through the medium of a minute canal (the safety-tube 
of the Eustachian canal) is kept at the same degree of density 
as the air on the outer side of the membrana tympani (Rii- 
dinger). The length and height of this cavity are very nearly 
equal, but the breadth is much less than either. The outer 
side of this peculiarly-shaped cavity is composed chiefly of 
the membrana tympani, while the inner is a somewhat dome- 
shaped surface of bone, covered of course with mucous mem- 
brane. In this inner wall of the cavity are two openings— 
the one oval in shape, the other round—which lead by separate 
passages into the last cavity or system of cavities of the ear— 
the labyrinth. In the natural state these openings are closed, 
the upper one by the foot-plate of the stirrup, the lower by a 
delicate membrane (membrana tympani secundaria). The 
connecting link between the vibrations of air in the first cavity 
(or external auditory canal) and the auditory nerve, which is 
spread out in a portion of the third cavity (labyrinth), is formed 
by a chain of three ossicles, which arch the space separating the 
inner from the outer wall of the second cavity (tympanum). 
It is therefore essential to clearly understand the mechanism 
of these ossicles before we can discuss intelligently the more 
complicated mechanism of the labyrinth, and especially of 
that part of it called the cochlea. 
Let us first take the sea-turtle and examine the simpler 
auditory apparatus (see Fig. 2) which we find in that animal. 
Here the substitute for the membrana tympani is situated al- 6 
ON THE MECHANISM OF HEARING. 
most immediately beneath the surface of the hard shin or shell 
on the side of the head, this animal having no external audi- 
tory canal. This substitute is nothing more nor less than the 
flaring end of a slender bone, about two inches in length, 
whose other end, likewise somewhat flaring, terminates in the 
labyrinth. This slender rod (columella;) lies, throughout almost 
its entire length, in a bony canal, and fits it so accurately that 
Fig. 
Diagram oe Sea-Turtle’s Ear. 
T, tympanum; L, labyrinth; c, columella. 
the only possible motion which can be communicated to the rod 
must be in the direction of its length. Pressure made at the 
outer end causes the inner end of the rod to project to a corre- 
sponding degree into the labyrinth; and, the moment the 
pressure is withdrawn, the rod, by reason of the highly-elastic 
ligaments which hold it in place, instantly returns to its ori- 
ginal position. 
The sea-turtle, then, furnishes us with a good example of 
the simplest substitute for the chain of ossicles, while at the 
same time the piston-like action of the rod throws out a hint 
as to the possible mechanism of the more complicated appa- 
ratus in the human ear. 
Instead of a single rod, we find in the mammalian ear 
three distinct ossicles, called respectively the hammer, the 
anvil, and the stirrup. These three are connected together in 
a peculiar manner. The hammer, the first of the series, oc- 
cupies such a position on the outer side of the tympanum that 
its handle (s pto £7, in Fig. 3) projects from above downward 
as far as to the centre (£7) of the membrana tympani, with ON THE MECHANISM OF HEAEING. 
7 
which it is intimately united {see also Fig. 1). In the next place 
the upper limit of this membrane consists of a band of fibres, 
which holds the hammer very firmly at a point midwTay be- 
tween the end of the handle and the top of the head, and 
allows it to perform only limited rotary motions, inward and 
outward. The second ossicle, the 
anvil, grasps as it were the head of 
the hammer by means of a peculiar 
interlocking of their respective ar- 
ticular surfaces.1 From the body 
of the anvil two processes project, 
the one backward to articulate with 
the posterior wall of the cavity, the 
other downward to articulate with 
the head of the third ossicle, the 
stirrup. While the articulation be- 
tween the hammer and the anvil 
is of such a nature as to admit of almost no play of the articu- 
lar surfaces upon each other, that between the long process of 
the anvil (see Fig. 1) and the head of the stirrup,* on the 
contrary, admits of full play between the two surfaces. 
Finally, the articulation between the foot-plate of the stir- 
rup and the margin of the oval opening, which leads to the 
labyrinth, consists simply of a band of elastic fibres that span 
the narrow space which separates the border of the foot-plate 
from the margin of the window in which it fits. 
Fig. 3. 
Diagram showing the Relations 
of the Hammer to the Mem- 
BHANA TtMPANI. 
//, head of hammer; TJ (umbo), the 
tip of ttie handle of hammer, cor- 
responding to the centre of the 
membrane; sp, the short pro- 
cess of the hammer. 
What part, then, does this apparatus play in the function 
of hearing, and what are the mechanical effects of its action 
upon the fluid of the labyrinth ? Obviously, to transmit accu- 
rately to this fluid body the same impulses which the mem- 
hrana tympani receives. The mechanism may be thus de- 
scribed : 
1 Helmholtz, who was the first to describe the real nature of the malleo- 
incudal joint, compared it to “ the joint used in certain watch-keys, where 
the handle cannot be turned in one direction without carrying the steel 
shell with it, while in the opposite direction it meets with only slight re- 
sistance.” 
3 This articulation is not unlike the knee-joint, where the articular sur- 
faces are separated by an interarticular cartilage (called meniscus in the 
stapedo-incudal joint). 8 
ON THE MECHANISM OF HEAPING. 
Suppose a purely simple tone of one hundred vibrations 
per second to be sounded within hearing distance. The air 
in the immediate vicinity of the membrana tympani will be 
alternately condensed and rarefied one hundred times during 
the period of a second ; or, in other words, the membrane it- 
self will vibrate inward and outward the same number of 
times. But inasmuch as the handle of the hammer is, so to 
speak, an integral portion of the membrana tympani, it, too, 
will perform the same number of excursions inward and out- 
ward. Now, in the next place, the firm union between the 
head of the hammer and the body of the anvil will oblige the 
latter to follow the direction communicated to the head of the 
hammer by the excursion of its handle. A glance at the fig- 
ure (Fig. 1) will show that on the anvil the direction of the 
motion imparted to it will vary according to the locality: 
thus, if the handle of the hammer be in the act of making an 
excursion inward, the top of the body of the anvil will be 
found moving in an outward and somewhat upward direction, 
while at the extremity of its long process (near the head of the 
stirrup) the motion will be chiefly upward, with somewhat of 
an inclination inward. The stirrup, whose head plays freely 
within the articular capsular ligament upon the end of this 
long process, will convert the impulse thus received into a 
motion, partly hinge-like and partly piston-like in its char- 
acter. 
It is, therefore, the function of the chain of ossicles to 
transmit to the fluid of the labyrinth whatever impulses may 
be communicated to the membrana tympani by sonorous vi- 
brations in the air of the external auditory canal.1 
Now, whatever may be the nature of these vibrations, it 
is evident, from the character of the attachments which hold 
the hammer in situ, that the only impulses which this ossicle 
is capable of transmitting are such as may be communicated 
to a single point in the handle, as, for instance, the tip. In 
the case, therefore, of a purely simple tone—by which is 
meant one that is perfectly free from all admixture of over- 
1 This doctrine, which was first put forth by Edward Weber, in 1851, 
was confirmed experimentally byPolitzer, in 1868, and afterward by other 
observers. 01ST THE MECHANISM OE HEARING. 
9 
tones—the point referred to would be subjected to a simple, 
pendulum-like motion. In the case of a complex tone, or of 
two or more tones produced simultaneously, it is not so easy 
to represent clearly to the mind’s eye the movements which 
the point in question must execute. Prof. Alfred Mayer, of 
the Stevens Institute of Technology (Hoboken, Hew Jersey), 
has devised an experiment which almost renders these move- 
ments visible to the eye. It was executed in the following 
manner: 
Placing a stretched membrane in a vertical position, and 
attaching fine threads of silk to its centre, Prof. Mayer put 
them moderately upon the stretch, and fastened them, each 
one to a tuning-fork (to one branch only) of different pitch. 
Upon sounding an organ-pipe (an instrument rich in over- 
tones) on the other side of the membrane, he distinctly heard 
the tones of some of the forks. The moment the threads were 
cut, the tones ceased. After restoring the threads, the forks 
again vibrated audibly. When the organ-pipe was changed 
for one of a different pitch, it was observed that a different 
set of forks were excited to vibration. 
While this experiment, then, furnishes us with a most val- 
uable means of analyzing sound, it also clearly demonstrates 
that a given point in a solid body can be made to perform, at 
one and the same instant of time, the vibrations belonging to 
a number of tones of different pitch ; in the present instance 
these were the fundamental note of the organ-pipe, together 
with its overtones. This complex vibration has often been 
compared to the changes which take place on the surface of a 
body of water. Thus, for instance, in looking at the ocean, 
we often observe that the surface is thrown into undulations 
by long and heavy swells; while upon them are numerous 
waves and smaller wavelets. Here, then, we see an illustra- 
tion of the manner in which several kinds of waves may set in 
motion the same body of water at the same moment of time, 
without interfering with each other in any way. 
How, if we admit that the ossicles can vibrate in this man- 
ner in response to four or five tones, sounded simultaneously, 
there remains no serious obstacle to the belief that they can 
also vibrate in response to an almost indefinite number of 10 
ON THE MECHANISM OF HEAEING. 
tones. In listening to conversation, to music, and to simple 
noises, our ossicula auditus are simply called upon to transmit 
at one and the same moment of time a varying number of im- 
pulses—for music, noises, and conversation, are all reducible to 
combinations of purely simple tones. 
In the last place, how is it possible for the terminal fila- 
ments of the auditory nerve to analyze these impulses of such 
varying rates of vibration? Before answering this question, 
it will be necessary to take a glance at the anatomy of the 
last cavity mentioned above—the labyrinth—and to inquire 
into the mechanical effects produced upon its contents by the 
piston-like motion which we have attributed to the stirrup. 
If we take a temporal bone, either in its natural condition 
or in the dried state, and endeavor by our eye to get some idea 
of the form, size, and relations of the labyrinth, we shall fail 
completely. There are only two points where we can get a 
glimpse of this portion of the ear, and these are at the two 
openings mentioned above as being situated in the inner wall 
of the middle cavity or tympanum. At all other points the 
various channels and cavities of the labyrinth are deeply em- 
bedded in the substance of the petrous portion of the bone. 
It is only by the aid of the hammer, the chisel, and the knife, 
that we can obtain the thin shell of bone which represents, as 
it were, the mould of the contained cavities. In this way we 
find that the labyrinth—we are speaking now only of its osse- 
ous boundaries—consists of a central cavity, not more than 
two lines in diameter, from one side of which spring, like 
arches, the three semicircular canals, while from the other side 
a canal leads into the snail-shaped body called the cochlea. In 
their natural state these bony cavities are filled with membra- 
nous structures and fluid. Thus, for example, the central cav- 
ity—or vestibule, as it is technically called—contains two dis- 
tinct membranous sacs (see Fig. 4), which together do not 
quite take up the entire space of the cavity, but leave room, 
in the immediate vicinity of the foot-plate of the stirrup, for 
a certain amount of free fluid (Henle). The smaller of the 
two sacs, the saccule (A, Fig. 4), communicates with one of 
the membranous tubes of the cochlea, the ductus cochlearis. 
The larger one, the utricle (U, Fig. 4), is continuous with the ON THE MECHANISM OE HEARING. 
11 
membranous tubes, which partially till the semicircularcanals ; 
in fact, the two constitute an intercommunicating system of 
cavities, containing fluid, and bounded by membranous walls, 
so that pressure made upon the utricle will result in an increase 
of tension in the walls of the membranous semicircular canals. 
If we uncoil the cochlea, and so convert it into a single 
straight bony tube, we shall find that its shape resembles that 
of an elongated truncated cone, whose larger base corresponds 
to the vestibular end of the cochlea, while the smaller would 
Fig. 4. 
Diagram of the Labtkinth. 
V, the vestibule, or central cavity, which is nearly filled by the utricle (U) and the saccule 
(S'). Near the foot-plate of the stirrup, there is free fluid, which also extends up into 
the scala vestibuli (8 V) of the cochlea. I) C, the ductus cochlearis, which communi- 
cates by a slender membranous channel (canalis reunions) with the saccule. Between 
the cochleal duct and the scala tympani (S T) is a narrow white band, representing 
the membrana b isilaris. At the extreme tympanic end of the scala tympani, a faint 
white line indicates the position of the membrana tympani secundaria. A, A, A, am- 
pull® of the membranous semicircular canals, which, in these regions, fit pretty closely 
their surrounding bony walls. 
represent the cupola, or highest point of the cochlea in its 
natural position. ISTow, there are certain membranous 1 septa 
which divide this single conical tube into three distinct and 
entirely separate channels, one communicating directly with 
the vestibule, while the other two are entirely shut otf from 
any communication with it. In the first place, a sort of dia- 
1 In one instance partly membranous and partly osseous. 12 
ON THE MECHANISM OF HEARING. 
phragm divides the tube horizontally into an upper and a lower 
passage, each of nearly equal size. The upper one communi- 
cates by a broad mouth with the cavity of 
the vestibule, and for this reason has been 
named the vestibular channel or scala ves- 
tibuli • the lower follows its course toward 
the tympanum, but at the round window* 
is shut off from it by a thin membrane— 
the membrana tympani secundaria. This 
lower channel is called the scala tympani. 
(Follow Fig. 4 for all these points.) A 
second diaphragm, which runs at an angle 
of about 45° with the first {see Fig. 6), 
divides the upper channel into the scala 
vestibuli proper and the ductus cochlearis. 
This last-named duct or channel is in reali- 
ty a closed sac; its shape being that of an 
elongated truncated cone, whose larger 
base occupies the cupola, while the smaller 
lies at the very entrance to the cochlea in 
the vestibule. The ductus cochlearis may 
therefore be described as a body of fluid 
imprisoned within membranous walls, and 
containing certain peculiar structures, 
which will be mentioned in detail here- 
after ; and it may also be described as 
playing the part of a diaphragm between 
the fluid of the scala vestibuli and that of 
the scala tympani. 
Fig. 5. 
We have now completed our hasty 
sketch of the labyrinth as a system of 
channels and closed sacs containing fluid, 
and it still remains for us to speak of the 
peculiar structures contained within the 
ductus cochlearis—structures which have 
been shown to stand in direct communi- 
cation with filaments of the auditory nerve, and which must 
therefore be considered as constituting the real terminal ap- 
paratus of hearing, in the same sense as the retina is the ter 
miual organ of sight. 
Diagram of the Mem- 
BBANA BASILAEIS. 
F, its vestibular end; C, 
that in the cupola. ON THE MECHANISM OF HEARING. 
13 
In the first place, the diaphragm, mentioned above as divid- 
ing the entire cochleal tube into an upper and a lower passage, 
will be found on microscopic examination to consist partly of 
bone and partly of membrane. The membranous portion of 
this diaphragm—if it were possible to dissect it out entire and 
spread it out before us—would be found to have a shape 
something like that given to it in Fig. 5. The small end V rep- 
resents the commencement of the membrane near the round 
window, and the large end c its termination in the cupola. 
If we remove all the superimposed structures and examine 
this membrane under the microscope, we shall find it to 
consist of an almost countless number of rods, separated 
one from another by a glue-substance of but slight adhesive 
power. If we leave the superimposed structures in situ, and 
make a transverse section through the membrane, we shall 
find that it carries upon its upper surface the following peculiar 
anatomical elements {see Fig. 6). 
1. A nearly central1 arch, composed of two pillars, called 
respectively the inner and outer pillars of Corti. The 
shafts of these pillars are comparatively slender, but their 
bases and their heads are broad. The points to which atten- 
tion should be called in the anatomy of these pillars are, their 
great strength and stiffness, compared with the other struct- 
ures to be found in the ductus cochlearis ; the constantly-in- 
creasing distance between the bases of the inner and outer 
pillars, as you go from the vestibule to the cupola; and, final- 
ly, the rounded character of the articular surfaces, where the 
heads of the inner and outer pillars come together—a pecu- 
liarity of connection which would indicate that a certain 
amount of play will be possible between the opposite heads of 
the pillars, in case the vibrations of the entire apparatus take 
place in a vertical plane. 
2. A peculiar fenestrated membrane, the lamina reticu- 
laris, which extends in an horizontal direction some little dis- 
1 The arch occupies such a position that the bases of the outer pillars 
stand very nearly in the centre or median line of the membrane. It is a 
significant fact that the outer rows of hearing-cells also occupy the cen- 
tral portion of the membrane, or that part which performs the greatest 
amplitude of excursion. 14 
ON THE MECHANISM OF HEARING-. 
tance outward from the heads of the pillars of Corti, to which 
it seems to be in some way attached. 
3. Five rows of ciliated cells, four on the outer side of the 
arch, and one on the inner side, close to the inner pillars of 
Corti. The four outer rows insert their heads into the open- 
ings of the fenestrated lamina reticularis in such a manner 
Fig. 6. 
Transverse Section op a Cochleae Whorl. 
On the right, embedded in the substance of the bone, is a group of ganglion cells, through 
which the fibres of the auditory nerve pass before entering the lamina spiralis ossea. 
This latter is represented in the figure as a broad septum, separating the scala vesti- 
buli (8 V) from the scala fympani (8 T), and containing a canal for the passage of the 
auditory nerve-fibres. While in the canal these fibres still possess a medullary sheath, 
but, on emerging therefrom into the ductus cochlearis (D C), they break up into naked 
axis-cylinders and terminate (as white threads in the drawing) in the single iniier and 
four outer hearing-cells. The cilia of the four outer hearing-cells may be seen projecting 
through the lamina reticularis ; those of the single inner cell (invisible in the drawing) 
project above the head of the inner pillar of Corti. The lamina reticularis is drawn as 
an horizontal projection (outward) of the upper portion of the head of the outer pillar of 
Corti. Underneath, these pillars rest upon the membrana basilaris, which terminates 
on the outer side in the ligamentum spirals (represented in the figure as a pale, cres- 
cent-shaped pad of connective tissue, fit ting accurately to the surrounding shell of bone). 
The cilia of the hearing-cells come in contact above with the membrana tectoria or Cor- 
n's membrane, which springs from the upper side of the lamina spiralis ossea, near its 
free extremity, and terminates, by a thin extension, in the ligamentum spirale, Meiss- 
ner s membrane is represented as a straight band, which stretches from the bony wall 
above, down to the upper side of the lamina spiralis ossea, and separates the ductus 
cochlearis from the scala vestibuli. 
that the cilia stand up like bunches of hair at regular inter- 
vals along the surface of the membrane. Bundles of primi- 
tive nerve-fibrils from the cochleal branch of the auditory 
nerve have been traced to all five rows of ciliated cells, but to OUST THE MECHANISM OF HEARING. 
15 
no other structures in the ductus cochlearis. Hence the name 
of hearing-cells, which has very appropriately been awarded 
to these evidently the most important elements in the appa- 
ratus we are endeavoring to describe. 
4. Simple epithelial cells, destined apparently to serve as 
a support to the lamina reticularis and to the outer and inner 
rows of hearing-cells. 
5. A peculiar membranous structure, which starts from 
the upper side of the hook-shaped process of the lamina spiralis 
ossea and extends outward over the lamina reticularis. This 
membrane is mucoid (or doughy) in consistency, and, from 
the fact that it lies like a gelatinous veil over, and in contact 
with, the cilia of the hearing-cells, it is called the membrana 
tectoria1 or covering membrane. As far as to the outer limit 
of the hearing-cells, this membrane retains its thick, doughy 
consistency, but beyond this region it is continued in the form 
of a very thin lamina to its line of insertion in the ligamentum 
spirals. 
It is not necessary, to a clear understanding of the mechan- 
ism of the final act of hearing, that we should enter, any more 
fully than we have done, into the minute anatomy of the 
ductus cochlearis. What remains for us to do is to follow out, 
step by step, the mechanical effects upon the various labyrin- 
thine structures of the piston-like vibrations of the stirrup, for 
it was here that we left off in our efforts to trace the effects of 
sound upon the apparatus of hearing. 
The theory, which is universally accepted by physiologists 
in explanation of this point, is the one given by Prof. Helm- 
holtz, and called, in honor of that distinguished physicist, “the 
Helmholtz theory of hearing.”3 The essential points in it are 
these: The component fibres of the membrana basilaris (Fig. 
5) are to be considered in very much the same light as sepa- 
rate strings, which by a process of loading—that is, weigh- 
ing them down with the pillars of Corti, the hearing-cells, etc. 
—have been tuned so as to vibrate in sympathy with all the 
1 Also membrane of Corti. 
2 See his work, entitled “ Die Lehre von den Tonempfindungen,” 
Braunschweig, 1870. 16 
ON THE MECHANISM OF HEAEING. 
appreciable tones, from the highest to the lowest.1 In fact, 
for every half-tone of our present musical scale there must be 
in the cochlea at least thirty-three strings to represent all its 
possible shades. When the stirrup, for instance, vibrates one 
hundred times in the second, it must excite to action that par- 
ticular string, or group of strings, which is tuned to vibrate one 
hundred times per second. The vibration thus caused makes 
an impression upon the nerve supplying the vibrating part, 
and this in turn produces in the brain the sensation of sound. 
The damping of the vibrations, at the instant the irritation 
which produced them ceases, is attributed partly to the pres- 
ence of a fluid medium, and partly to the fact that the vibrat- 
ing structures proper (the strings of the membrana basilaris 
and the pillars of Corti) are pressed upon by cell-bodies which 
are not adapted for vibration. 
The vertical vibration of the basiliar membrane is distinctly 
recognized in this theory ; but, in the preliminary description 
of the anatomy of the cochlea, mention is made of the exist- 
ence of a small opening (helicotrema) in the cupola, through 
which a communication is established between the scala ves- 
tibuli and the scala tyrnpani. How, according to Kolliker and 
Beichert, the blind sac of the ductus cochlearis entirely fills 
the cupola (the vestibular half of it). This statement would 
seem, then, either to throw doubt upon the existence of such an 
opening, or to make it a channel of communication between 
the ductus cochlearis (instead of the scala vestibuli) and the 
scala tympani. The latter supposition, however, involving as 
it necessarily does a certain degree of longitudinal current in 
the fluid of the ductus cochlearis, cannot for a moment be en- 
tertained, as it would utterly overthrow any theory based upon 
the sympathetic vibration of the fibres of Corti’s organ. On 
the other hand, if the blind termination of the ductus coch- 
learis does not entirely fill the cupola—as some authorities 
maintain—but leaves room for the fluid of the scala vestibuli 
to pass through a small opening in the septum into the scala 
tympani, the theory of a vertical motion for the membrana 
basilaris will still hold good, as the existence of the helicotrema 
1 The highest tones corresponding to the short strings near the vestibule, 
the lowest to the long ones in the cupola. ON THE MECHANISM OF HEAPING. 
17 
will simply serve to diminish the force of the vertical motion, 
but not to annul it. My own investigations, so far as they go, 
favor the view that no communication exists between the two 
scalse in the immediate vicinity of the cupola, unless the open- 
ing, spoken of so vaguely by the authors, be microscopic in size.1 
In the light of the histological discoveries, made since the 
time when Helmholtz published his last edition of the “ Ton- 
empfindungen ” (1870), we would, with all due reserve, ven- 
ture upon the following explanation of the mechanism of hear- 
ing in the cochlea: 
We have described the labyrinth as a closed cavity, hav- 
ing but two channels of approach—the round and the oval 
window—one of which is filled up tightly by the foot-plate of 
the stirrup. How, since, according to physical laws, the fluid 
contents of the labyrinth cannot be supposed to undergo dimi- 
nution in bulk, under the pressure made by the foot-plate of 
the stirrup, we must assume—what has now been proved by 
direct observation2—that the elastic membrane of the round 
window affords the required yielding-point for this displace- 
ment.3 
1 To satisfy myself, if possible, on this point, I removed the cochleae en- 
tire from the temporal bones of an infant, and softened them in a solution 
of chromic acid and hydrochloric acid (not more than five per cent, of 
each). When the earthy materials had been entirely removed from the 
bone, I placed the specimens in ordinary alcohol for a few hours, and with 
the razor divided them each into two unequal parts, the plane of section 
being at right angles to the base of the cochlea. In one of the cochleae, 
the section passed so close to the bottom of the cul-de-sac of the cupola, 
that it was difficult to determine any thing satisfactory regarding the open- 
ing in question. In the other it was a comparatively easy matter, with an 
ordinary magnifying-glass, and after blowing out the free alcohol, to survey 
both the vestibular and tympanic surfaces of the septum, which in the cu- 
pola divides the two channels from each other. No trace whatever of any 
communication between the two passages could be detected. 
The results of a single examination like this cannot, of course, be con- 
sidered as sufficient to overthrow the statements of the anatomists. Nev- 
ertheless, the lack of definiteness in their descriptions of this opening {see 
Henle’s “Anatomy” and Waldeyer’s article on the “ Labyrinth” in Striek- 
er’s “ Histology ”), taken in connection with its apparent absence in this 
one case, should lead to new investigations on this point. 
a Dr. Charles H. Burnett, of Philadelphia, and others. 
3 This statement will be slightly modified farther on. 18 
ON THE MECHANISM OF HEARING. 
The first body to receive the impulse of the stirrup is the 
free mass of fluid which fills the unoccupied portion of the 
vestibule and the scala vestibuli. Without stopping here to 
discuss the effects of the stapedial1 impulse upon the utriculus 
and semicircular canals, let us proceed at once to the consid- 
eration of the effects which this impulse will produce upon 
the cochlear structures. 
In the first place a moment’s thought will convince us 
that, in estimating the effects of the stapedial impulse upon 
the ductus cochlearis, we may totally disregard the saecu- 
lus and the canalis reuniens.2 The uniform pressure of the 
fluid upon all sides of these last-named cavities will effectu- 
ally prevent any thing like a current from the ductus coch- 
learis to the sacculus, or the reverse, through the canalis 
reuniens. The ductus cochlearis—that is to say, the upper 
and inner wall of it, or Reissner’s membrane—is therefore sub- 
jected to a uniformly distributed pressure from above down- 
ward by the centrifugal (if I may so use the term) force of 
the fluid of the scala vestibuli. As it yields beneath this force, 
the incompressible body of fluid which fills the ductus coch- 
learis must in turn find its yielding-point in the entire mem- 
brana basilaris; for this and Reissner’s membrane are the 
only two portions of the wall of the duct which are mem- 
branous, and therefore capable of yielding. Finally, the dis- 
placement caused in the contents of the scala tympani, by the 
depression of the entire membrana basilaris, is provided for 
by the presence of an elastic membrane (membrana tympani 
secundaria) at the larger end of this channel. 
This brings us, then, face to face with the fact that the 
entire membrana basilaris (including high and low notes alike) 
is obliged to perform all the excursions which may be com- 
municated to the stirrup or the membrana tympani by sono- 
rous vibrations; or, to speak more minutely, with every tone 
sounded, every “ organ of Corti ”3 must perform the excursions 
2 The little canal (Fig. 4) which unites the sacculus and the ductus 
cochlearis. 
1 Stapes —a stirrup. 
8 The term “ organ of Corti ” is used to designate the entire group of 
structures built upon the membrana basilaris. ON THE MECHANISM OF HEAEING. 
19 
which belong to that particular tone. But if that be true, it 
will be asked, why should not all the nerve-filaments send 
communications to the brain; or, in other words, why should 
we not hear all the notes of the scale whenever any one of 
them happens to be sounded % This is our answer, and we 
might add that it has been suggested to us by the peculiar 
relations of the membrana tectoria to the hearing-cells, and 
by the passing remark of Waldeyer (in the article referred to 
above) that “no portion of the labyrinth could be better 
adapted, by reason of its anatomical construction and position, 
to perform this function [that of a damper] than the mem- 
brana tectoria.” 
Although it be true that, with every note sounded, all the 
organs of Corti must perform the excursions belonging to that 
note, yet it must be remembered that the semi-gelatinous mem- 
brana tectoria, which rests upon the cilia of the hearing-cells, 
will likewise be obliged to perform these same excursions. 
Over one particular region, however, of the membrana basilaris 
this will not be the case, namely, in that portion where the 
thirty or more basilaris fibres are tuned to vibrate in sympathy 
with the note sounded. At this point the vibrations will be of 
sufficient vigor to throw off the membrana tectoria. So long, 
then, as that particular note is sounded, the cilia of the hear- 
ing-cells, in the region referred to, will receive a succession of 
taps from the membrana tectoria, or, to speak more strictly, 
will strike against this membrane. These blows constitute the 
true irritation of the auditory nerve. Wherever the blows do 
not take place, although the auditory nerve-filaments may be 
agitated in a direction at right angles to their length, there no 
sensations of sound will be communicated to the brain. 
Before leaving the subject, we should like to venture a sug- 
gestion as to the function of the semicircular canals. 
As is well known, it has been the custom until recently to 
consider these structures as governing the function of equi- 
librium (Flourens’s theory). About three years ago Arthur 
Boettcher,1 of Dorpat, proved by experiments upon living 
animals that this theory is incorrect. He proposed no new 
theory, but opened the way for others to do so if they wished. 
1 “Dorpater Med. Zeitschrift,” vol. iii., part ii., 1872. 20 
ON THE MECHANISM OF HEAEING. 
The first thing that attracts our attention in. the anatomy 
of these organs is the peculiar relation of the membranous to 
the bony canals, the former occupying scarcely a third of the 
calibre of the latter. Then, in the next place, the space be- 
tween the membranous canal and the bony walls is filled, not 
with free fluid, as is the case, for example, in the cochlea, but 
Fig. 7. 
Transverse Section of Semicirctjlar Canal. (Copied from Rlidinger.) 
MS (7, membranous semicircular canal. (For further details, see the text.) 
with a reticulated connective tissue, rich in blood-vessels and 
made up of large meshes containing fluid. In the vicinity of 
the mouths of these canals, however, it should be noticed that 
the membranous tubes hug the bone more closely {see Fig. 4). 
If we examine the mode of construction of the membranous 
canals, we find that the outer elastic wall {see Fig. 7) is lined 
with a sort of mucous membrane whose free surface undulates 
or is thrown into mounds, and carries a lining of ordinary ON THE MECHANISM OF HEARING. 
21 
pavement epithelium. Another point worthy of notice is the 
absence of the undulations or mounds on that side of the canal 
which lies next to the bone—the side where dilatation cannot 
take place. These anatomical relations and the absence of 
nerves are certainly suggestive of the thought that the func- 
tion of these canals must be a mechanical one. Is it not their 
function, we would ask, to protect the ductus cochlearis and 
the organs of Oorti from injury in cases where the stirrup is 
driven too violently into the oral window ? Any pressure 
made upon the utriculus can only find a yielding-point in the 
elastic walls of the membranous semicircular canals; and, 
furthermore, if the latter were lined with a smooth instead of 
an undulating1 epithelial surface, their distention would be fol- 
lowed by a rupture of the epithelial lining. Any sudden and 
loud noise, like the report of a cannon or a peal of thunder, 
would be likely to injure the cochlear structures, were not 
some provision made in the labyrinth for emergencies of this 
kind. As we have shown, the semicircular canals are not 
only admirably fitted to serve in the capacity of safety-valves, 
but their entire mode of construction, especially the absence 
of all nervous structures, would seem to exclude them from 
any higher office in the mechanism of hearing. 
The suggestion of a recent writer (Hensen), that their 
function is that of secreting the labyrinthine fluid, has against 
it the fact that the epithelial cells lining the canals are of the 
pavement and not of the cylindrical or glandular variety. 
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