Ear Anatomy

THE OUTER EAR

The outer ear consists of the:

The PINNA (the part you can see on the side of the head)

The EAR CANAL

Details

The pinna is the part of the ear that everyone can see. It is made of cartilage covered with skin. In many animals the pinna is important in funnelling the sound into the ear canal and can be moved towards the direction of the sound. This function of the pinna is less developed in humans.

The ear canal is about 2.5 cm long and is not straight but curved. The outer third is a tube of cartilage continuous with the pinna and extend upwards and backwards. The inner two thirds of the ear canal is a bony tube facing downwards and forwards.

At the end of the ear canal is the ear drum. The ear drum is a thin membrane which separates the ear canal from the middle ear. The ear drum is fixed to part of the first hearing bone which is called the malleus.

The ear canal and the ear drum are covered with skin just like the skin on the outside of the body. The skin over the outer part of the ear canal is thick and has hairs and glands which produce secretions that mix to form wax. These secretions keep the ear canal acidic and protect the ear from infections.

The skin over the deep part of the ear canal is very thin and easily damaged by cotton buds, hairgrips, matchsticks or any of the other things people sometimes stick into their ears. You should never put anything into your ear canal.

The skin on your body is 5 layers thick. The skin over your body is continually growing and replacing itself. The new skin cells start growing in the deepest layer of the skin and move up to the surface layer where they are rubbed off by friction.

The skin in the ear canal is different. There is no friction in the ear canal so the continually growing skin has to clear itself in a different way. It does this by a process called ‘migration’. There is a continual movement of the top layer of the skin cells from the centre of the ear drum to the edge of the ear drum and then out along the ear canal. If you put an ink dot on the ear drum and take regular photographs you will see the ink dot move from the ear drum onto the ear canal and out towards the opening of the ear canal where it sheds off and becomes mixed with the wax.

THE MIDDLE EAR

The middle ear consists of the:

The EAR DRUM, 

The AIR FILLED SPACE behind the ear drum = THE MIDDLE EAR

The HEARING BONES (OSSICLES)

Details

The middle ear can be thought of like a box. The box is made out of bone. On the outside wall of the box has a large hole in it which is covered by the ear drum.

On the front wall of the box is an opening that leads to a tube which runs down to the back of the nose. This is called the Eustachian tube.

On the back wall of the box is another opening which leads into the mastoid bone. The mastoid bone is the bone behind the ear which is hollow.

On the inside wall of the box are two openings into the inner ear. One opening is covered with the stapes bone and is called the oval window; the other opening is covered with a thin membrane and is called the round window.
A very important nerve called the facial nerve runs across the inner wall from front to back then turns downwards and runs down the back wall. This nerve controls the muscles of the face. If it is damaged the face can not move.

The roof of the box separates the middle ear space from the brain.

In the middle ear space there are three hearing bones:
the malleus(hammer),
the incus(the anvil)
the stapes (the stirrup).

The malleus bone attaches to the ear drum and the incus bone. The incus bone attaches between the malleus bone and the stapes bone and the base of the stapes bone fills the opening into the inner ear.

Any movement of the ear drum is transmitted into the inner ear.

The middle ear is normally filled with air.

The floor of the middle ear is formed by the bulge of two big blood vessels usually covered by a plate of bone – these are the carotid artery and the jugular vein.

THE INNER EAR

The inner ear consists of the:

The COCHLEA – the hearing part

The VESTIBULAR LABYRINTH – the balance part

Details

The inner ear consists of two parts.

The hearing part called THE COCHLEA and the balance part called the VESTIBULE and THE SEMI-CIRCULAR CANALS.

The function of the cochlea is to convert sound energy into nerve impulses which then travel along the cochlea nerve up into the brain.

The cochlea is a fluid filled tube with bony walls and a shelf along its length dividing it into three compartments. One end of the tube is closed off. At the other end the upper and lower compartments of the tube open into the middle ear space. The tube is coiled up like a snail shell.

The open end of the upper compartment(the oval window) is covered by the stapes bone.

The open end of the lower compartment (the round window) is covered by a membrane.

The stapes bone is connected to the other hearing bones which are connected to the ear drum. When sound vibrates the ear drum it makes the hearing bones vibrate. The vibration of the stapes bone sends a wave of energy through the fluid of the upper compartment. Because fluid is not compressible that wave of energy has to escape somewhere. It does this by going through a small hole at the far end of the bony tube which joins the upper and lower compartments and travelling back along the lower compartment and causing the membrane at the end of the lower compartment (the round window membrane) to bulge outwards.

As the wave of sound energy travels along the upper compartment it bends the flexible partition of the middle compartment. Sound waves of different frequencies will make the flexible partition bend the most at a particular point along the partition depending on the frequency of the sound.

A high frequency sound will make the flexible partition bend most just inside the stapes. A low frequency sound will make the partition bend most at the far end of the tube.

The middle partition is full of little detectors (also called hair cells) which detects where the flexible partition has bent the most. These hair cells are attached to the nerve endings of the cochlear nerve. When a detector detects where the flexible partition has bent the most it will trigger the nerve endings situated at that particular point along the partition.