Figure 932

A superior view of the muscles that move the eyes to direct the gaze and give convergence.

Superior oblique

Lateral rectus

Medial rectus

Superior rectus

Superior levator removed from both sides medium. When there are no molecules, as in a vacuum, there can be no sound. Anything capable of creating a disturbance of molecules—for example, vibrating objects—can serve as a sound source. Figure 9-33 demonstrates the basic principles using a tuning fork. The disturbance of air molecules that makes up the sound wave consists of zones of compression, in which the molecules are close together and the pressure is increased, alternating with zones of rarefaction, where the molecules are farther apart and the pressure is lower (Figure 9-33a through d).

A sound wave measured over time (Figure 9-33e) consists of rapidly alternating pressures that vary continuously from a high during compression of molecules, to a low during rarefaction, and back again. The difference between the pressure of molecules in zones of compression and rarefaction determines the wave's amplitude, which is related to the loudness of the sound; the greater the amplitude, the louder the sound. The frequency of vibration of the sound source (that is, the number of zones of compression or rarefaction in a given time) determines the pitch we hear; the faster the vibration, the higher the pitch. The sounds heard most keenly by human ears are those from sources vibrating at frequencies between 1000 and 4000 Hz (hertz, or cycles per second), but the entire range of frequencies audible to human beings extends from 20 to 20,000 Hz. Sound waves with sequences of pitches are generally perceived as musical, the complexity of the individual waves giving the sound its characteristic quality, or timbre.

We can distinguish about 400,000 different sounds. For example, we can distinguish the note A played on a piano from the same note on a violin. We can also selectively not hear sounds, tuning out the babble of a party to concentrate on a single voice.

Sound Transmission in the Ear

The first step in hearing is the entrance of sound waves into the external auditory canal (Figure 9-34). The shapes of the outer ear (the pinna, or auricle) and the external auditory canal help to amplify and direct the sound. The sound waves reverberate from the sides and end of the external auditory canal, filling it with the continuous vibrations of pressure waves.

The tympanic membrane (eardrum) is stretched across the end of the external auditory canal, and air molecules push against the membrane, causing it to vibrate at the same frequency as the sound wave. Under higher pressure during a zone of compression, the tympanic membrane bows inward. The distance the membrane moves, although always very small, is a

Vander et al.: Human I II. Biological Control I 9. The Sensory Systems I I © The McGraw-Hill

Physiology: The Systems Companies, 2001 Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems

Picture Vibrating Tympanic Membrane

FIGURE 9-33

Formation of sound waves from a vibrating tuning fork.

function of the force with which the air molecules hit it and is related to the sound pressure and therefore its loudness. During the subsequent zone of rarefaction, the membrane returns to its original position. The exquisitely sensitive tympanic membrane responds to all the varying pressures of the sound waves, vibrating slowly in response to low-frequency sounds and rapidly in response to high-frequency ones.

The tympanic membrane separates the external auditory canal from the middle ear cavity, an air-filled cavity in the temporal bone of the skull. The pressures in the external auditory canal and middle ear cavity are normally equal to atmospheric pressure. The middle ear cavity is exposed to atmospheric pressure through the auditory (eustachian) tube, which connects the middle ear to the pharynx. The slitlike ending of this tube in the pharynx is normally closed, but muscle movements open the tube during yawning, swallowing, or sneezing, and the pressure in the middle ear equilibrates with atmospheric pressure. A difference in pressure can be produced with sudden changes in altitude (as in an ascending or descending elevator or airplane), when the pressure outside the ear and in the ear canal changes while the pressure in the middle ear remains constant because the auditory tube is closed. This pressure difference can stretch the tympanic membrane and cause pain.

Time

The second step in hearing is the transmission of sound energy from the tympanic membrane through the middle-ear cavity to the inner ear. The inner ear, called the cochlea, is a fluid-filled, spiral-shaped passage in the temporal bone. The temporal bone also houses other passages, including the semicircular canals, which contain the sensory organs for balance and movement. These passages are connected to the cochlea but will be discussed later.

Because liquid is more difficult to move than air, the sound pressure transmitted to the inner ear must be amplified. This is achieved by a movable chain of three small bones, the malleus, incus, and stapes (Figure 9-35); these bones act as a piston and couple the motions of the tympanic membrane to the oval window, a membrane covered opening separating the middle and inner ear (Figure 9-36).

The total force of a sound wave applied to the tympanic membrane is transferred to the oval window, but because the oval window is much smaller than the tympanic membrane, the force per unit area (that is, the pressure) is increased 15 to 20 times. Additional advantage is gained through the lever action of the middle-ear bones. The amount of energy transmitted to the inner ear can be lessened by the contraction of two small skeletal muscles in the middle ear that alter the tension of the tympanic membrane and the

FIGURE 9-33

Formation of sound waves from a vibrating tuning fork.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

The Sensory Systems CHAPTER NINE

The Sensory Systems CHAPTER NINE

Malleus Incus

Semicircular canal

External auditory canal

Malleus Incus

Semicircular canal

Semicircular Canal

Cochlear nerve

Cochlea

\

Stapes

Middle

(in oval

ear

window)

Cochlea

Auditory

(eustachian)

tube

FIGURE 9-34

The human ear. In this and the following drawing, violet indicates the outer ear, green the middle ear, and blue the inner ear. The malleus, incus, and stapes are bones even though they are colored green in this figure to indicate that they are components of the middle ear compartment. Actually, the auditory tube is closed except during movements of the pharynx, such as swallowing or yawning. % KQ]

Malleus Helicotrema Incus

Malleus Helicotrema Incus

Congenital Bone Fixation The Malleus

Cochlea

Cochlear duct

Scala vestibuli

Scala tympani

Round window Middle ear cavity

FIGURE 9-35

Relationship between the middle ear bones and the cochlea. Movement of the stapes against the membrane covering the oval window sets up pressure waves in the fluid-filled scala vestibuli. These waves cause vibration of the cochlear duct and the basilar membrane. Some of the pressure is transmitted around the helicotrema directly into the scala tympani.

Redrawn from Kandel and Schwartz. %

Cochlea

External auditory canal Tympanic membrane

Round window Middle ear cavity

Cochlear duct

Scala vestibuli

Scala tympani

FIGURE 9-35

Relationship between the middle ear bones and the cochlea. Movement of the stapes against the membrane covering the oval window sets up pressure waves in the fluid-filled scala vestibuli. These waves cause vibration of the cochlear duct and the basilar membrane. Some of the pressure is transmitted around the helicotrema directly into the scala tympani.

Redrawn from Kandel and Schwartz. %

Membrane over oval window

Membrane over oval window

Basilar Membrane Inner Ear

Inner ear (fluid)

FIGURE 9-36

Diagrammatic representation showing that the middle ear bones act as a piston against the fluid of the inner ear.

Redrawn from von Bekesy. %

Inner ear (fluid)

FIGURE 9-36

Diagrammatic representation showing that the middle ear bones act as a piston against the fluid of the inner ear.

Redrawn from von Bekesy. %

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems

-\ y j i

(3) Membrane in oval window

V f\ A \\ t \ \ *

moves

(1) Tympanic \\\xS membrane deflects

-

(4) Basilar i membrane „W X \ moves

(5) Membrane in / round window

moves

\ \

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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