Hearing.

When sound waves cause the eardrum to vibrate, the chain of bones in the middle ear ( the ossicles) transmits the vibrations to the oval window of the vestibule in the inner ear.

The vibrations brought to the oval window by the footplate of the stapes then move as fluid-pressure waves through the vestibular canal to the tympanic canal and across the cochlear canal.

Because of its mechanically tuned structure, the basilar membrane vibrates in patterns that depend on the incoming sound frequencies.

Thus, the activity induced in the various nerve fibers by the organ of Corti encodes information that the brain uses to discriminate different tones.

Balance.

Balance in Motion. When the head is rotated in any of the three planes of semicircular canals, the fluid in the semicircular canal corresponding to the plane moves toward the ampulla while the fluid in the opposing canal moves away from the ampulla.

The effect is that the change in the fluid pressure in each ampulla stimulates a sensitive group of cells connected with a nerve which, in turn, carries the sensation of a change of position to the brain.

The vertical canals are stimulated by such actions as falling or jumping and the horizontal canals by turning and spinning.

Static Balance.

While the semicircular canals are concerned with body balance when in motion, the utriculus and the sacculus sense the stationary orientation of the head in relation to gravity.

Within both the utriculus and the sacculus there is a small group of cells with short, protruding hairs that support a gelatinous layer containing crystals of calcium carbonate.

Since the gelatinous layer, called the otolithic membrane, is relatively heavy and is supported only by hairs, whereas in another position it will bend others.

The hair cells are connected through the vestibular nerve to the brain.

Watch the latest videos on YouTube.com

Automatic Reflexes of Balance.

Everyday experience shows that it is not necessary for a person to think about his balance in motion or a position in relation to gravity.

This is because such adjustments are automatic.

The semicircular canals and the utriculus are connected to a set of complicated reflexes governing the tension of all supporting muscles of the body, but these reflexes act through the brain stem and the spinal cord without participation of the thought areas of the brain.

Another set of reflexes connects the semicircular canals to the muscles of the eyes, so that when a person moves, his eyes automatically are adjusted to retain the field of vision