Anatomy of the external and middle ear

Recently I began studying my current university degree which is a Masters in Clinical Audiology. To be honest, I hadn’t really known much about the ear before I began, but since starting, I have learnt so much and thought about so many things that I hadn’t even considered before. How can we tell where sounds come from? What is a ‘sound’? How does the brain use the information a sound brings? I’ll probably end up writing more about the ear and concepts related to audiology more than anything else from now on so I apologise in advance if this is not of interest to any of our current readers. This post will be a bit of an introductory one to the anatomy of the ear. Hopefully, what I write makes sense even if you don’t have any background knowledge!

Starting off with the part of the ear that is visible from the outside, this is called the pinna. It is made of cartilage which means that is is fairly bendy and elastic and is a little different for everyone, however the majority of ears have the same main features.

ear

Looking at the ear above, the curved edge around the top and side, is called the helix. The bump following that same line, one step inwards, is called the antihelix. The lobule is the meaty bit at the bottom where earrings typically go. The tragus is like a little flap at the opening of the ear canal (on the left in this picture), and if you go to the right, there is another little outcrop, and that is the antitragus. The concha is the indented bowl where the opening of the ear canal is, and between the tragus and antitragus. These are the main features of the pinna, and however it looks, its main function is to collect sound and direct it into the ear canal, so it can be heard.

Moving inside the ear canal, this can be divided into two parts. The first is a cartilaginous part which is the outer third of the canal. It has sebaceous (oil) and cerumen (wax) glands as well as little hairs that help to trap foreign things before they get too deep into the ear and degrade microbes or anything that happened to get in. The inner two-thirds of the canal are set in bone, and don’t have any wax glands. The entire canal in an adult is approximately 2.5 cm long.

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Once sound has made its way through the ear canal, it will hit the tympanic membrane, more commonly known as the ear drum. A soundwave is a vibration that can travel through air. So as the vibration reaches the tympanic membrane, it gets vibrated. The tympanic membrane is connected to a series of bones (see diagram above). The first is called the malleus, which then connects to the incus, which connects to a stirrup shaped bone called the stapes. These bones in the middle ear, are collectively called the ossicles, and are the smallest bones in the body. They can be fractured, which as you can imagine means that hearing loss is probable, and although they may heal, a prosthetic replacement may need to be inserted surgically. There are a number of other pathologies which could affect these bones which will be covered in a later post. The vibrations that caused the tympanic membrane to vibrate, also cause vibration of the ossicles. This vibration gets transferred to the cochlea where the vibration gets converted to a signal that gets sent to the brain (next post!) The bones sit in a cavity called the middle ear cavity, and this is normally filled with air. If you have a cold, or an ear infection, fluid may accumulate here, which contributes to the blocked up feeling often experienced. This is normally regulated by the Eustachian tube which connects the middle ear cavity to the nasopharynx which is behind the nose. Fluid is allowed to drain out of the ear via the Eustachian tube, and it will also open to help with pressure equalisation. It is normally held closed by the tensor veli palatini muscle, however is opened with swallowing, hence why chewing and swallowing are recommended when going up and down in aeroplanes.

These are the major features of the external and middle ear. The stapes which was the innermost of the bones in the middle ear, connects to the cochlea which is classified as the inner ear. In the next post, I will talk about how that sound vibration gets converted to an electrical signal that can be received by the brain. Just think about how much you rely on your auditory system. Whilst reading this, there is likely going to be some background noise in your environment. Maybe the air conditioner is running, people are talking in the background or maybe you are listening to music, and all this time, the tiny bones in your ear are vibrating and the brain is receiving all that information, even if you are not concentrating on those sounds.

I’m going to finish up here, but feel free to comment with any questions!

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