Introduction

If a medium is set into vibration within certain frequency limits (average between 25 cycles per second and 18,000 cycles per second), we have what is called a sound stimulus (Figure 13-5). The sensation of sound, of course, occurs only when these vibrations are interpreted by the cerebral cortex of the brain at the conscious level.

a. The human ear is the special sensory receptor for the sound stimulus. As the stimulus passes from the external medium (air, water, or a solid conductor of sound) to the actual receptor cells in the head, the vibrations are in the form of (1) airborne waves, (2) mechanical oscillations, and (3) fluid-borne pulses.

FREQUENCY (PITCH)

FREQUENCY (PITCH)

1 SECOND

VOLUME (LOUDNESS)

VOLUME (LOUDNESS)

Characteristics Sound
Figure 13-5. Characteristics of sound.

b. The ear (Figure 13-6) is organized in three major parts: external ear, middle ear, and internal (inner) ear. Each part aids in the transmission of the stimulus to the receptor cells.

EXTERNAL EAR MIDDLE EAR INNER EAR

MIDDLE EAR CAVITY MEMBRANOUS &

EXTERNAL EAR MIDDLE EAR INNER EAR

MIDDLE EAR CAVITY MEMBRANOUS &

Middle Ear Cavity
Figure 13-6. A frontal section of the human ear.

13-11. THE EXTERNAL EAR

The external ear begins with a funnel-like auricle. This auricle serves as a collector of the airborne waves and directs them into the external auditory meatus. At the inner end of this passage, the waves act upon the tympanic membrane (eardrum). The external auditory meatus is protected by a special substance called earwax (cerumen).

13-12. THE MIDDLE EAR

a. Tympanic Membrane. The tympanic membrane separates the middle and external ears. It is set into mechanical oscillation by the airborne waves from the outside.

b. Middle Ear Cavity. Within the petrous bone of the skull is the air-filled middle ear cavity.

(1) Function of the auditory tube. Due to the auditory tube, the air of the middle ear cavity is continuous with the air of the surrounding environment. The auditory tube opens into the lateral wall of the nasopharynx. Thus, the auditory tube serves to equalize the air pressures on the two sides of the tympanic membrane. If these two pressures become moderately unequal, there is greater tension upon the tympanic membrane; this reduces (dampens) mechanical oscillations of the membrane. Extreme pressure differences cause severe pain. The passage of the auditory tube into the nasopharynx opens when one swallows; therefore, the pressure differences are controlled somewhat by the swallowing reflex.

(2) Associated spaces. The middle ear cavity extends into the mastoid bone as the mastoid air cells. The relatively thin roof of the middle ear cavity separates the middle ear cavity from the middle cranial fossa.

c. Auditory Ossicles. There is a series of three small bones, the auditory ossicles, which traverse the space of the middle ear cavity from the external ear to the internal ear. The auditory ossicles function as a unit.

(1) The first ossicle, the malleus, has a long arm embedded in the tympanic membrane. Therefore, when the tympanic membrane is set into mechanical oscillation, the malleus is also set into mechanical oscillation.

(2) The second ossicle is the incus. Its relationship to the malleus produces a leverage system which amplifies the mechanical oscillations received through the malleus.

(3) The third ossicle, the stapes, articulates with the end of the arm of the incus. The foot plate of the stapes fills the oval (vestibular) window.

d. Auditory Muscles. The auditory muscles are a pair of muscles associated with the auditory ossicles. They are named the tensor tympani muscle and the stapedius muscle. The auditory muscles help to control the intensity of the mechanical oscillations within the ossicles.

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Essentials of Human Physiology

Essentials of Human Physiology

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