These clips of a model ear were used in the National Geographic film "Inside the Living Body" to show the working of hearing and balance.

The hearing apparatus is divided into three connected sections. The outer ear, the part we see, is a sound trumpet to collect and focus the pressure waves created in the air by sound onto the eardrum or Tympanic membrane. This is a very thin flexible membrane about 810mm in diameter. It is prone to infection and so the external ear canal has sticky hairs to trap anything entering the ear. The stickiness is due to cells in the lining of the external canal which secrete a substance that coats the canal lining and hairs. Combined with trapped particles and dead skin cells, this creates the waxy material found in our ears.

The bones of the middle ear control the amount of vibration transmitted to the sensitive vibration detector, the Cochlea. These bones are connected to muscles that can change the sensitivity of the system attempting to keep the vibrations within acceptable levels.

The Cochlea itself is a complex system of fluid filled tubes and membranes coiled together to make a snailshell shaped structure. Vibration is channelled along these tubes from the Oval Window which receives the vibrations from the Stapes bone. Cells which are covered in fine processes or hair like projections from their surface are activated by the sound causing the fine hairs to move. The cells stimulate fibres of the Auditory Nerve which takes the sound information to the brain's auditory centre. Different frequencies are separated out along the length of the tube with lower (base) sounds travelling further to stimulate distal hair cells and the higher (treble) frequencies being picked up nearer the oval window. The way the Cochlea works allows it to amplify sound however the hair cells can be damaged by repeated exposure to excessively loud noise. The higher frequency sensitivity usually declines first. The normal range in humans is from around 30HZ at the base end up to 20KHz at the highest frequencies. This usually reduces as we get older. Many animals can detect far wider ranges. Bats and some whales can detect sounds up to over 100KHz.

Connected to the organs of hearing is the Vestibular System which is located within the inner ear. They supply the vital information which enables us to know where we are in 3dimensional space. The Utricle and Saccule detect which way up we are and linear acceleration, whilst the fluid filled Semicircular canals detect rotational movements as the fluid is sloshed around within them. The Semicircular canals are oriented in three planes to give information about any direction of rotation. The Utricle and Saccule contain small stones called Otoliths which stimulate hair cells as they move around. It is over stimulation of the vestibular organs that gives rise to the feeling of motion sickness. Orientation and balance also requires the brain to know where all the limbs are and visual cues can be used as well.


These clips can be found in the film archive at:

http://www.davidbarlowarchive.com/cat...