Ernest Glen Wever

The Amphibian Ear

Professor Wever studies the structure of the ear and its functioning as a receptor of sounds in all amphibian species (139) for which living representatives could be obtainedOriginally published in 1985.The Princeton Legacy Library uses the latest print-on-demand technology to again make available previously out-of-print books from the distinguished backlist of Princeton University Press. These paperback editions preserve the original texts of these important books while presenting them in durable paperback editions. The goal of the Princeton Legacy Library is to vastly increase access to the rich scholarly heritage found in the thousands of books published by Princeton University Press since its founding in 1905.

Sound Conduction in the Dolphin Ear

By use of the electrical potentials of the cochlea, the pathway of sound conduction to the ear was investigated in two species of dolphins, Tursiops truncatus and Lagenorkynchus obliquidens. The observations show that the external auditory meatus is vestigial and that neither this meatus nor the tympanic membrane plays any part in sound reception. The ossicular chain has an essential function, though the outermost ossicle, the malleus, can be removed without seriously affecting the conductive process. It is concluded that auditory stimulation in the dolphin occurs by bone conduction. Acoustic vibrations in the water reach the ear through the tissues of the head, and set up differential motions between the stapes and the cochlear capsule. These differential motions involve the cochlear fluids, which are given mobility by the presence of gas in the region of the round window. The vibrations of the basilar membrane and stimulation of the hair cells of the organ of Corti then follow in the usual way.

The Evolution of Vertebrate Hearing

The origin of the vertebrate ear is shrouded in the mystery of the origin of the vertebrates themselves. As with any problem of evolution, a consideration of vertebrate ancestry follows two procedures: the tracing among animals now living of likely lines of relationship, and an examination of the fossil record for evidence to verify and extend the living history. Here, however, we are faced with special difficulties, for the ancestors of the vertebrates were softbodied marine creatures that have left insignificant fossil remains and that have had few survivors.

The Acoustic Pathways to the Cochlea

Measurements are made of the relative effectiveness of the round window as a route of entrance of sounds to the cochlea. When the ear is normal this route is of no importance, but when the middle ear apparatus is absent and its advantage is lost to the oval window route a sound will have nearly equal access by both windows. When the inner ear is reached by both pathways at once the cochlear potentials represent the vector sum of what would result from the two separate waves. As phase and intensity relations between the two pathways are altered the potentials pass through maximum and minimum values, which for equal intensities of the two waves vary from a 6 db gain to a complete loss.Evidence is produced to show that each pathway of stimulation excites the same sensory cells and in the same intensity pattern.Over the major portion of the frequency range a minimum of response results when the waves are in phase as they enter the oval and round windows. Departures from this relation, which are most prominent...

LXXVII. The Tensor Tympani Muscle and its Relation to Sound Conductions

The intensity-control theory holds that the tensor tympani muscle by its degree of contraction alters the efficiency of transmission of sound by the middle ear system. This theory in its most complete form supposes that under appropriate conditions the efficiency may be either increased or decreased.2 The theory in a more limited form, known as the protection theory, supposes that efficiency can be changed only in the negative direction. According to this theory, the muscle acts so as to safeguard the inner ear against stimuli of excessive magn i t~de .~

II The Width of the Basilar Membrane in Man

Practically all theories of hearing are based upon particular assumptions regarding the physical characteristics of the basilar membrane. These assumptions are most definite for the place theories, which account for pitch discrimination by the differential action of regions of the membrane. The resonance type of place theory, as maintained by Helmholtz and his followers, considers the physical properties of the membrane to vary systematically throughout its length, so that separate fibers or bands respond selectively to tones of different frequency. The physical features that have received most consideration in relation to the differential response of the membrane are those that determine the natural frequencies of stretched strings, namely, the width of the membrane, and the mass, tension and loading of its transverse fibers. A further dimension that obviously is involved is the length of the membrane from base to apex.

Sound Reception in the Porpoise as it Relates to Echolocation

Previous to the work of our group which was published in 1970 (McCormick, et al., 1970), all theories of hearing in Cetacea were based on dissections and experiments with dead specimens. Such experiments continue to be published in the literature to this day, and just as the earlier studies of dead material did, they only serve to confound the many students and investigators of Cetacean hearing, especially those who have little formal training in the science of physiological acoustics.

The transmission properties of the middle ear.

The basic function of the middle ear mechanism is well known: it serves as a mechanical transformer, providing for the ready transfer of acoustic energy from the outside air to the fluid of the cochlea. Without such transformer action the vibratory motions of the air particles would not easily be communicated to the heavier particles of the cochlear fluid, but to a large extent would be reflected back from the boundary. When given the proper mechanical advantage, which produces an increase in the pressure, the transfer of energy is made without loss.

The Patterns of Response in the Cochlea

The general form of the intensity function for the electrical responses of the cochlea is now well known. As we raise the intensity of a sound applied to the ear, the cochlear potentials rise at first in simple proportion to the intensity, and then at a high level, as the stimulus is elevated further, they cease to follow this simple relation and give a bending curve. From the first departure from linearity onward the responses rise more and more slowly in relation to the stimulus until at last they reach a maximum value, a value beyond which they actually diminish as the intensity is raised.So much of the form of the function is established on the basis of numerous experiments (see reference 2), yet there still are many details not yet worked out with exactness. The present study seeks in a measure to supply this lack. We have been concerned especially with the initial departure from linearity and the maximum value of the response for various frequencies over the tonal range. This information, as we shal...

Electrical Output of Lizard Ear: Relation to Hair-Cell Population

Cochlear potentials measured in several species of lizard show a close correlation between maximum electrical output and number of hair cells, whereas there is no uniform relation to sensitivity. These results are interpreted as indicating structural differentiation and frequency discrimination in spatial terms in the more advanced lizard ears.