The ears and hearing (1949)

through our ears enter the multitude of sounds from all about us ah without these sounds our existence would be much different but what is involved in hearing and what is sound to begin with let's consider a very common sound as the hammer strikes the gong molecules of air are set into waves or patterns of motion these waves shown here schematically act upon the ear which then transmits nerve impulses to the brain this is what happens whenever we hear a sound sound waves enter the ear through an external opening here we show the path taken by sound waves as they enter the ear first the sound waves pass through an outer part of the ear called the outer ear as the eardrum vibrates it moves the chain of three tiny bones connected with it these bones are in the middle part of the ear called the middle ear the largest of the three bones is about one quarter of an inch long the bone attached to the eardrum is the hammer the middle bone is the anvil and the bone at the right is the stirrup in this view we're looking into the middle ear the semi-transparent membrane vibrating at the lower left is the eardrum the bones connected with it are the middle ear bone this is the sound that is causing the eardrum to vibrate here is a closer view of the middle ear bones in motion at the lower left we again see the eardrum and attached to it the hammer just to the right of center is the anvil and connected with it the spirit because of the angle not all of the stirrup can be seen we can see how small the middle ear bones are by comparing them with the head of an ordinary pin here is the hammer the anvil and the syrup these three tiny bones transmit sound through the middle ear thus in effect the eardrum acts as a diaphragm the hammer and the anvil act as levers and the stirrup transmits sound wave energy through its footplate to the inner ear here is a close view of the inner ear or labyrinth as it is often called the upper part shown in a lighter shade is concerned with our sense of balance the spiral lower part the cochlea is involved in hearing there are two openings or windows into the cochlea the stirrup fits into the uppermost opening called the oval window the stirrup transmits sound waves through the oval window to the fluid in the cochlea if we could uncoil the cochlea and open it we would see three chambers inside filled with fluids the central chamber separates the two outer chambers except at the tip the oval window shown at the top connects into the uppermost chamber the lower chamber also has an opening through the bone called the round window this opening is covered by an elastic membrane when the syrup moves it sets in motion the fluid causing the round window to bulge when the round window returns the fluid moves in the opposite direction high pitched sounds mainly affect sections near the oval window low pitched sounds mainly affect sections toward the tip of the cochlea whenever a vibration occurs nerve impulses are set up in the cochlea that pass along the auditory nerve from the auditory nerve the nerve impulses go on to the brain this is how we hear most of the impulses reaching either ear go to the opposite side of the brain next we will consider some of the diseases that may affect our hearing sometimes wax a normal secretion of glands in the outer ear accumulates and blocks off part of the canal probing for the wax may pack it more tightly or even break the eardrum when wax interferes with hearing a doctor should be consulted he can remove the wax without danger to the ear some people have middle ear trouble as we have seen the middle ear is essentially a cavity filled with air it connects with the outside air through the eustachian tube which leads to a space between the throat and the back of the nose here at the mouth the tube comes together and forms a flexible seal yawning or swallowing opens this seal permitting air to go in or out this movement of air is especially important when flying to balance sudden changes in outside air pressure if the tube becomes inflamed it may not open at all inflammation can be caused by infections that reach the eustachian tube from diseased adenoids tonsils and sinuses blowing the nose too hard may force infectious germs back into the eustachian tube if the tube fails to open properly air indicated that the large arrow becomes trapped in the middle ear gradually oxygen in the trapped air gets absorbed and this creates a partial vacuum in the middle ear as this vacuum forms the outside air pressure forces the eardrum inward fluid forms in the part of the middle ear shown by the black arrow this causes impairment of hearing sometimes infections in the middle ear may cause destruction of the eardrum the hammer and the anvil in severe cases almost all of the transmitting parts are destroyed an electrical hearing aid may help restore partial hearing with the use of a hearing aid the handicap of deafness can often be overcome a hearing aid is a miniature public address system that amplifies the intensity of sounds and transmits them to the ear the receiver fits into the outer ear the listener can adjust the volume by means of a tuning device in this film we have seen how the ear changes vibrations in air molecules into nerve impulses that we call sound since next to sight hearing is our most important sense it is fortunate that for most of us our ears will serve well throughout life helping us to understand and enjoy our environment you