3D-HuBEd No.9 -EARS-

The ear has two important functions. One is the perception of sounds and another is the sense of balance. For example, the external ear, the middle ear and the cochlea in inner ear play a roll of the auditory sense, and three semicircular canals, the utricle and the saccule play a roll of sense of balance.

Refer to chapter 0 for more information on how to use this textbook.

　　9.1 The structure of the ear
　　9.2 The way of listening sound
　　9.3 The sense of balance

9.1.1 The structure of the ear

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The ear is an organ not only to hear sounds but to balance ourselves. We can divide the ear into three parts, the external ear, the middle ear and the inner ear.

The external ear consists of an auricle and an auditory canal. The middle ear and the inner ear, which are main parts of the ear, are in the temporal bone. A middle ear consists of ossicles, and an eustachian tube that connects the ear to the nose. An inner ear consists of a cochlea, a semicircular canal and a vestibule. The semicircular canal and the vestibule are necessary for balancing.

Waves of sounds from outside the ear go through the auditory canal and make the tympanic membrane vibrate, and the vibration goes to the cochlea through ossicles. The vibration transmitted to the cochlea is changed into electrical nerve signals, and next it is transmitted to brain. Finally the brain recognizes the signal as sounds.

The structure of the inner ear

There are three semicircular canals, an utricle and a saccule beside the cochlea in the inner ear.

The cochlea is an important organ concerned with the auditory sense. The organ of corti is in a median canal and it is on the basement membrane of strong connective tissue. This membrane reaches from the lamina spiralis ossea to the wall of the other side.

A median canal is filled with the endolymph. On and under a median canal, there is the perilymph of scala vestibuli and the scala tympani. There are about 16,000 auditory cells with sensory hair in the organ of corti.

Semicircular canals are important organs concerned with rotation, and the sensory epithelium recognizes the slant and the linear motion. The membranous portion, the utricle and the saccule are organs filled with lymph. The shapes of them are like a tube, and the organs are in the vessel called "labyrinth osseus" that is made of bone.

9.2 The way of listening sounds

The vibrations of air from the auditory canals vibrate the tympanic membrane between the external ear and the middle ear. The tympanic membrane is a thin conic membrane. The handle of the malleus is on the center of it. The malleus, the incus and stapes are called three ossicles. The malleus and the incus enlarge the vibration of the tympanic membrane and transmit it to stapes. After that, the vibration is transmitted through the membrane of an oval window in the inner ear, the lymph in cochlea and the basement membrane on the center of the cochlea. Then the vibration is changed into electrical signals by hair cells of crista of the organ of corti on the basement membrane. Finally, the signal is transmitted to the central auditory sense in a cerebrum and then we can perceive sounds.

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It has been proved that the constant frequent vibration occurred in the endolymph moves a specific part of basement board. These phenomena are called theory of place. In other words, the difference between heigh and low sounds changes the point where is stimulated in the basement board. The strength of sounds appears in the difference of frequency of impulse.

9.3.0 The sense of balance.

Semicircular canals, the utricle and the saccule are beside the cochlea in the inner ear. Semicircular canals consist of three half-sphered tubes that make a right angle with each other. When we rotate our head the lymph in the semicircular canal flows and moves. The flow and movement move the sensory hair of hair cells of crista in ampulla and cause signals of acceleration of rotation. Then the signals reaches the vestibular nerve. As the effect of these actions, we can recognize rotation. A sensory epithelium is in the utricle and the saccule, which are parts of the utricle and the saccule. It is covered with membrane of gelatins with statoconium, and sensory hair of hair cells of the crista is in it. We can sense the slant and linear motions from the change of the pressure at this sensory hair.

The sense of rotation

The cupula is on the sensory cell of ampulla of the semicircular canal. The sensory hair is in it. When the rotate speed acceralates, the endolymph starts to flow and the cupula comes to slant to a side. Then sensory hair moves. The hair cells of crista come to excite in repsponce to the movement. The lines of sensory hair make a proper nerve impulse. Because three semicircular canals make a right angle with each other, we can distinguish rotating motion for any direction.

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9.3.2 The sense of equilibrium

The sensory field of view in the utricle and the saccule react to the change of speed moving linearly in the three planes of space. Each sensory epitheliums consist of sensory cells and these cells have long pappus and operculum in sensory hair. This operculum is called the statolith membrane, and there is small the statolith in it. When the speed changes, sometimes the statolith membrane pulls sensory hair and sometimes pushes it, then nerve impulse is created. If the vestibulum was broken, the animal cannot move linearly but rotate like drawing a circle.

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