Talk:Paper - A note on the comparative size of the cochlear canal in mammals (1939)

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A NOTE ON THE COMPARATIVE SIZE OF THE COCHLEAR CANAL IN MAMMALS

By J. A. KEEN Department of Anatomy, St Thomas’s Hospital Medical School

"Tue late Dr Albert Gray’s Labyrinth of Animals published in 1907 remains the standard work on the labyrinth of mammals, birds, reptiles and amphibians. The two volumes contain stereoscopic photographs of the labyrinths of seventy-five species of animals with tables of measurements. The main

Fig. 1. Section of a typical mammalian cochlea with three turns. (Diagrammatic.) S =scala media.

impression which one obtains from the descriptions and dimensions given is one of great diversity of both the cochlear and vestibular parts.

The fully developed cochlea is only found in mammals, a spirally coiled tube being characteristic of all orders. The number of turns in the spiral varies from 14, e.g. in the mouse, to 43, e.g. in the guinea-pig, the human cochlea occupying an intermediate position with 2} turns. The spirally coiled scala media or ductus cochlearis (s) together with the spiral ligament lies against the outer endosteum of the spiral bony canal (Fig. 1). In the preparations of The Comparative Size of Cochlear Canal in Mammals 593

the membranous labyrinth, made by Gray, the coiled scala media was separated out, together with a bridge of tissue, composed mainly of nerve fibres, which passes across the osseous spiral lamina and the modiolus; the bony parts were removed by decalcification. Gray’s measurements, which are set out in Table II, show the distances from points 4—B, C-D and E-F (Fig. 1). It will be obvious that these measurements actually represent dimensions of the bony cochlea.

SCALE

Fig. 2. Appearance of basal coil of cochlea of various mammals in cross-section. Tracings from microphotographs taken at the same magnification.

When the dimensions of the scala media are studied in microscopic sections of the cochlea, there is found to be far closer similarity in different mammals than might be expected from Gray’s measurements. The whole bony cochlea may indeed be relatively large or small. It may be a flattened out structure with wide separation of the scala media in each whorl, or a sharp-pointed organ with a steeply ascending spiral and close approximation of the whorls. The bony canals in cross sections may show as wider or as smaller spaces. Yet, 594 J. A. Keen

in spite of all these differences, there are found very close resemblances in the scala media, and more particularly in the width of the basilar membrane. The following mammalian cochleae were available for study: man, cat, sheep, calf, dog, guinea-pig and mouse. In order to make measurements which are comparable it is necessary to section the cochleae in corresponding planes. The most useful plane is one in which the whole of the modiolus and every coil is seen. As is well known, the width of the basilar membrane gradually increases from the base to the apical turn, and there was no exception to this rule in the seven types of cochleae which were examined. In each section one of the basal coils was chosen for comparison. It was decided to select the one further away from the vestibule, i.e. coil H in Fig. 1, for the reason that the coil on the vestibular side often showed some feature peculiar tq the species, e.g. a bulging out of the floor of the scala tympani. The coils chosen were photographed at the same magnification. Tracings were then made from the microphotographs, the scala media being shown as a shaded area (Fig. 2).

Table I. Measurements of the width of the basilar membrane (in millimetres)

Base Intermediate coils Apex Human 0-20 0-23 0-28 0-37 0-48 Cat 0-20 0-22 0:25 0-28 0-33 0:37 Sheep 0-20 0:25 0-32 0-35 Calf 0-22 0-23 0-28 0-32 0-35 Dog 0-27 0:28 0-32 0:33 0-33 0-40 Guinea-pig 0-15 0-20 0-21 0-22 0-23 0-25 0-25 0-25 Mouse 0-13 0-15 0-17

The shaded parts in the illustrations are the areas enclosed by the basilar membrane below, the vestibular (Reissner’s) membrane above, and the spiral ligament on the outside. The sulcus spiralis internus, the organ of Corti, hair cells, supporting cells and the membrana tectoria are enclosed in the darkened area. The limbus laminae spiralis and the stria vascularis have not been included since there is some doubt whether these latter structures are developed from the original epithelial lining of the otic vesicle or not.

Measurements of the basilar membrane in the coils from base to apex are found in Table I. It has been recommended that the basilar membrane should be measured from the tip of the spiral ligament to a point corresponding to the base of the inner rod of Corti (Bethe, 1926) and this method has been followed. Determination of the width of the basilar membrane in this way has the advantage that the free part of the membrane is measured. The base of the inner rod of Corti usually rests on the extreme edge of the osseous spiral lamina or very near it, and this rod may be looked upon as the “hinge” around which the basilar membrane moves (Wilkinson & Gray, 1924).

Gray’s measurements of the cochlea are the following (see Fig. 1): diameter of lowest whorl of.cochlea (A—B); diameter of second whorl of cochlea (C-D); The Comparative Size of Cochlear Canal in Mammals 595

slant height of cochlea (E-F). In addition, the number of turns of the cochlea in each species is given. The “slant height of cochlea” is defined as the distance from the upper margin of the oval window to the apex of the organ. There is no difficulty in making corresponding measurements on the sectioned cochleae, and it appeared to be of some value to compare the two sets of figures. Gray’s figures are given in Table II, my own corresponding ones in Table ITI.

Table II. Measurements from Gray’s Labyrinth of Animals (in millimetres)

Diameter of Diameter of Slant Number lowest whorl second whorl height of turns of cochlea of cochlea of cochlea of cochlea Human 8-25 4-62 7-0 23 Cat 6-0 3-5 4:25 3+ Sheep 7-0 3-5 5-0 2} Calf / Not given Dog 6-25 3-5 4-75 3} Guinea-pig Not given Mouse 15 0-75 1-5 2

Table III. Corresponding measurements obtained from sections of cochleae (in millimetres)

Diameter of Diameter of . Slant Number lowest whorl _ second whorl height of turns of cochlea of cochlea of cochlea of cochlea Human 6-0 40 6-0 23 Cat 4:5 2-5 4-25 3 Sheep 75 40 6-0 2 Calf 5-5 40 6-0 2 Dog 5-0 3-5 5-0 3 Guinea-pig 3-5 2-3 5:0 44 Mouse 1-4 _ 1-65 1} DISCUSSION

It is impossible not to feel surprise at the extraordinary resemblance in the dimensions of the scala media in man and. in the six other mammalian species which were examined (Fig. 2). The resemblances in the width of the basilar membranes are equally striking (Table I). The size of this inner ear duct appears to have very little relation to body size as a whole. Organs such as the eye or related parts of the central nervous system show differences which bear a much closer relation to absolute body size.

The part of the inner ear which contains the sense organ of hearing is nearly as large in cross section in the guinea-pig as in man or calf. The mouse appears to be an exception. Even so, the cross section of the scala media in the mouse is approximately two-thirds of that of the guinea-pig, and the width of the basilar membrane in the basal coil of the mouse is 0-18 mm. as against 0-20 mm. in the corresponding position in the human cochlea. 596 J. A. Keen

In the resonance theory of hearing attention is generally directed to two factors: (a) the width of the basilar membrane, i.e. the length of the fibres making up the basilar membrane; (b) the state of tension of these fibres, which is said to be dependent on the width of the spiral ligament. This ligament is the structure seen at the outer edge of the basilar membrane, stretching between the basilar membrane and the outer endosteum of the bony canal. It is wide at the base and thins away towards the apex (Fig. 1). By taking into account the differentiation of the basilar membrane, both as to the length of the fibres, and their state of tension, it is possible to explain on physical grounds the tuning of the fibres for the whole range of tones comprised in the audible scale, some 10-11 octaves (Wilkinson & Gray, 1924).

We have very little knowledge about the range of hearing in animals. Pavlov’s experiments with conditioned reflexes suggest that dogs have an extensive range of hearing, perhaps more extensive than ours (Pavlov, 1927; Beatty, 1932). On the present findings one is tempted to speculate that the mouse has a high-pitched range of hearing; it may well be that its cochleae in the upper registers are sensitive to pitches so high as to be inaudible to the human ear. But such a speculation involves the acceptance of some form of resonance theory. The high-pitched squeaks of mice, almost inaudible to our own ears, would lend support to such a view.

No animal can escape from sound. An elaborately coiled cochlea became evolved in mammals, and evidently the fundamental plan tends to be the same whatever the size of the animal. Once the organ was evolved, nature allowed the minimum of variation. The remarkable correlation in actual size in different species suggests that the cochlea is set apart not only for the perception, but also for the analysis of sound vibrations; a strong point in favour of the resonance theory. It is hoped to extend this study in order to present a more complete survey of the inner ear in the mammalian series.

I wish to thank Prof. A. B. Appleton for his many helpful suggestions and advice in the preparations of this paper, and for the facilities he has afforded me in the Anatomy Department of St Thomas’s Hospital Medical School. I also have to thank Mr F. J. Cleminson, the Director of the Ferens Institute of Oto-Laryngology for his kind permission to examine the many beautiful preparations of human and animal cochleae at that Institute, and Mr W. Pilgrim, the senior technician, for his help with the microphotographs.

REFERENCES

Bzarty, R. T. (1932). Hearing in Man and Animals. London: G. Bell and Sons.

Bertue, A. (1926). Handbuch der normalen und pathologischen Physiologie, Bd. 11, 1, S. 483. Gray, A. A. (1907). The Labyrinth of Animals. London: J. and A. Churchill.

Pavuov, I. P. (1927). Conditioned Reflexes. Oxford Univ. Press.

WiLxrnson, G. & Gray, A. A. (1924). The Mechanism of the Cochlea. London: Macmillan and Co.