It’s time to update the anatomy textbooks – a brand new body part has been discovered! A muscle layer in the lower jaw that plays an essential role in chewing has been described by scientists for the very first time in the journal Annals of Anatomy.

Nestled deep in the masseter muscle – the most prominent of the jaw muscles – the novel muscle is found between the back of the cheekbones and lower jaw.

You can feel the masseter muscle by placing your fingers on the back of your cheeks while you chew. It was generally considered to consist of just two layers: one deep and one superficial. However, some texts have alluded to a mysterious super-deep third layer.

“In view of these contradictory descriptions, we wanted to examine the structure of the masseter muscle again comprehensively,” Professor Jens Christoph Türp, from the University Center for Dental Medicine Basel, said in a statement.

It’s this elusive third layer that the team, led by Türp and Dr Szilvia Mezey from the Department of Biomedicine at the University of Basel, have finally discovered.

“This deep section of the masseter muscle is clearly distinguishable from the two other layers in terms of its course and function,” Mezey said. 

The new layer is involved in stabilizing the lower jaw, Mezey added, having studied its arrangement of muscle fibers. She also believes it to be the only part of the masseter that pulls the lower jaw backward.

The team dissected 12 heads preserved in formaldehyde and examined CT scans of 16 “fresh” cadavers, plus an MRI scan from a living subject to identify the position and probable function of the new muscle layer.

Of course, a shiny new body part needs a shiny new name. In their paper, the team suggests it be called Musculus masseter pars coronidea – meaning coronoid part of the masseter – as it attaches to the muscular (coronoid) part of the lower jaw.

The finding is not only anatomically significant, the authors conclude, but it could be clinically relevant too. Precise knowledge of the masseter muscle will improve surgery and therapies of the lower jaw.

“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,” said Türp.