Charles W. Bray

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 .~

LX The Nature of Bone Conduction as Shown in the Electrical Response of the Cochlea

In these experiments the fundamental nature of bone conduction has been investigated. The electrical response of the cochlea produced by bone conducted sound is compared with the response produced by air conducted sounds. This comparison is made, first of all, in terms of the functional relations between the magnitude of the cochlear response and the intensity of sounds conducted to the ear by the two methods. A further comparison is made in terms of the relative effects of middle ear lesions. The experiments were performed on eleven guinea pigs. Under avertin anesthesia the bulla was opened, and an active electrode was placed in contact with the round window. An inactive electrode was sewn into the muscles of the neck. The electrical impulses picked up by these electrodes were conducted through an amplifier to a cathode ray oscillograph. Air conducted stimuli were produced by an electric system consisting of an oscillator, filters, attenuators and a loud speaker. A tube from the loud speaker conducted the sounds to the ear of the animal. The tones were of a high degree of purity and of known intensity.* The apparatus for producing bone conducted stimuli was the same as the above except that a Western Electric bone conduction receiver (Type D-80904) was substituted for the loud speaker. One end of a bakelite rod, 3/16 inch in diameter and I1 inches long was fitted to the plunger of the bone conduction receiver, and the other end was bolted firmly to the exposed parietal bone of the animal. The body of the bone conduction receiver was thoroughly shielded to eliminate electromagnetic radiation. In order to obtain a measure of intensity of the bone conducted stimuli a vibration microphone was so clamped to the bakelite rod

III. Discussion from the Point of View of Animal Experimentation. LXXV

In two previous papers, Horton! has reported the results of a series of experiments on stimulation deafness in guinea pigs. In these experiments three groups of animals were subjected to an intense tone for a considerable number of hours, at the rate of ten hours daily. Group I, containing seven animals, was exposed to a tone of 1000 cycles at an intensity of 125 decibels (db) above the human threshold for eleven days, a total exposure of 110 hours. Group II, containing eight animals, was exposed to 1500 cycles at an intensity of 100 db. for 200 days, a total of 2000 hours. Group III, containing four animals, was exposed to 3000 cycles at an intensity of 100 db. for 125 days, or 1250 hours.

Temporary Deafness in Birds

It has been reported by hunters that certain birds become temporarily deaf. Ewald suggested that the deafness is caused by a change of intralabyrinthine pressure resulting indirectly from opening the mouth. In order to test Ewalds hypothesis, the electrical responses of the cochlea to sound stimulation were observed in the pigeon with mouth open and with mouth closed. The results show that the cochlear responses are reduced when the mouth is opened. Even slight movements of the lower jaw, such as those sometimes associated with breathing, produce a noticeable effect. The maximum effect, occurring when the mouth is wide open, varies from 8 to more than 10 db. The effect occurs for all frequencies used, from 100 to 10,000 cycles. It is suggested that the primary source of the effect is tension on the ear drum rather than a change in intralabyrinthine pressure. It may be concluded that movements of the mouth of the pigeon, even such movements as those of eating, drinking and singing, impair the hearing of t...

Combination Tones: Their Nature and Origin in the Auditory Mechanism

The electrical responses of the cochlea were used to study combination tones as produced through distortion in the ear. Guinea pigs were stimulated with two pure tones under two conditions, (1) with a loudspeaker when the ear was intact, and (2) with a mechanical vibrator applied to the head of the stapes after removal of all the more peripheral portions of the middle ear. The parts removed included drum, malleus, incus, and tensor tympani muscle, and in some of the experiments the stapedius muscle. Combination tones obtained from normal and operated ears were similar, and it is concluded that the peripheral mechanism of the ear transmits vibrations with high fidelity. The results thus do not support the assumptions made by Helmholtz and others that there is a significant degree of nonlinearity and asymmetry in the movements of the drum and ossicles. Distortion in these structures is not wholly excluded, but it is shown to be unimportant in relation to the distortion that appears beyond the stapes. We con...

The Stapedius Muscle in Relation to the Conduction of Sound to the Inner Ear

The action of the stapedius muscle was simulated by applying tension to a thread attached to the stapedius tendon. The experiments were performed on cats under Dial anesthesia and curare, and the electrical responses of the cochlea were used to measure the effects. The application of tension to the stapedius tendon causes, in general, a reduction of transmission. The change is greatest for low tones, and grows less as the frequency is raised. Exceptional effects are shown for a limited range of tones in the region of 2000∼; for these tones slight tension may cause an increase in response. However, this increase is limited, and as the tension is further raised the response passes through a maximum and rapidly falls. For all tones, therefore, transmission is reduced by large tensions. The results thus show that the function of the stapedius muscle is fundamentally that of protecting the ear against over‐stimulation. Observations were made also on the effects of tension upon the patterns of distortion in the...

The Effects of Pressure in the Middle Ear Upon the Electrical Responses of the Cochlea

The effects of increased air pressure in the middle ear cavity upon the cochlear responses to sound stimuli have been studied in the cat. The general result of increased pressure is a reduction in the magnitude of the response. The effects are independent of the middle ear muscles since they occur in curarized animals. The amount of reduction in the responses varies with the amount of pressure and the frequency of the stimulus tone. The role of the eardrum and other parts of the ear in the effects will be discussed.