- Scientists have discovered a 'new' muscle layer in the lower human jaw.
- It plays an essential role in chewing, according to the team.
- They believe its discovery could help doctors better perform surgery in the area.
A part of the human body has gone unnoticed by scientists – until now. The newly discovered, deep muscle layer is found in the masseter muscle, which raises the lower jaw and is critical for chewing.
According to modern anatomy, though, the masseter muscle consists of only two layers: one deep and one superficial.
The researchers of the new study, published in the Annals of Anatomy, wrote: "However, a few historical texts mention the possible existence of a third layer as well, but they are extremely inconsistent as to its position.”
And upon investigation, the scientists from the University of Basel, Switzerland, succeeded in finding this deep layer in the jaw muscle. They say that their study is the first detailed description of this part of the masseter muscle.
What they had to do
Their investigation was no easy task – they had to dissect 12 formaldehyde-fixed human cadaver heads. Formaldehyde fixation, which has been used for many years, is a standard method used to preserve tissues from clinical samples.
They also analysed CT scans of 16 fresh cadavers, looked at MRI data from a living 40-year-old female, and examined histological sections (microscopic study of tissue) of one formaldehyde-preserved head.
It was through these detailed efforts that they identified an "anatomically distinct", deep third layer of the masseter muscle.
They wrote: “The coronoid part of the masseter was present in each case studied, indicating that it is a constant architectural element of the masseteric muscle…”
The researchers would like discussion of this newly described part of the masseter to continue and named it “Musculus masseter pars coronidea" (the coronoid part of the masseter).
Discovery may help with surgery, treating TMJ
"This deep section of the masseter muscle is clearly distinguishable from the two other layers in terms of its course and function," co-author, Szilvia Mezey, a senior lecturer in the Department of Biomedicine at the University of Basel in Switzerland, said in a statement.
This M. masseter, Mezey said, is the only part of the masseter that can pull the jawbone backwards.
Co-author Dr Jens Christoph Türp, a professor and clinician at the University Center for Dental Medicine Basel, also commented: "Although it's generally assumed that anatomical research in the last 100 years has left no stone unturned, our finding is a bit like zoologists discovering a new species of vertebrate."
The team noted that additional knowledge of the architecture of the masseter muscle may help doctors in managing patients with temporomandibular disorders (TMJ) or help doctors better perform surgery in patients in that region of the jaw.
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