Place your fingers on the back of your cheek and clench your teeth. You will notice a tense muscle that slowly relaxes.
It is the masseter muscle, the most important muscle part of the jaw.
Anatomy textbooks traditionally describe the masseteric muscle as a part of the body consisting of two layers: superficial and deep.
But a group of scientists in Switzerland say they have discovered a part of the jaw muscle that had gone unnoticed until now.
The results of their research show that there is another and deeper, third layer of the masseter muscle, otherwise very important in the chewing process.
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"Although it is assumed that anatomical research in the past 100 years has discovered almost everything, what we have done is the same as when zoologists find a new species of vertebrate," said Professor Jens Kristof from the University Center for Dental Medicine in Basel.
Deep layer
To reach this discovery, the researchers studied the structure of the jaw muscles, tomographic images, tissue sections of dead people who donated their bodies to science, and data from magnetic resonance imaging of the living.
In this way, they found a deep third layer, connected to the muscular process in the lower jaw.
That muscular process is called the coronoid, which is why the authors of the study propose to name this new layer Musculus masseter pars coronidea.
Silvija Mezei, co-author of the study, explains that although the masseteric muscle with three layers has been described before, the existing literature is unclear and sometimes contradictory.
Sometimes it is stated that the masseteric muscle has one layer, sometimes two, and when it is stated that it has three, it is perceived as a special variation, says the expert.
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"We wanted to see what caused this inconsistency," Mezei, who works at the Department of Biomedicine at the University of Basel, told BBC Mundo.
"Thanks to our research we realized that it wasn't just a variation, it was consistent, it was there all the time," she added.
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What function does it have?
This new layer has a completely different function than the other two parts of the masseter muscle, says Mezei.
The first layer serves to lift the jaw, the second helps to prevent it from going backwards or forwards, and the third helps to retract, stabilize and close the jaw.
This discovery is important for dentists and surgeons, as it indicates that the masseter muscle is formed and behaves in a different way than previously thought.
In addition, the specialist says, the research shows that scientists have not yet fully studied the human body.
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