Scientists identify whole new muscle layer in human jaw

This muscle is prominently felt at the back of the cheeks when one presses the teeth together

A newly discovered muscle layer (denoted in the photo by ‘C’) runs from the back of the cheekbone to the anterior muscular process of the lower jaw (Jens. C. Türp, University of Basel / UZB)

Scientists have identified a new layer of muscles behind the cheeks that stabilise the lower jaw, a new finding that could rewrite anatomy textbooks.

The masseter muscle at the back of the cheeks – considered the most prominent of the jaw muscles – was until now thought to be made of one superficial and one deep layer, said scientists led by Szilvia Mezey from the University of Basel in Switzerland.

The action of this muscle is prominently felt at the back of the cheeks when one presses the teeth together, they said.

The study, published in the journal Annals of Anatomy on 2 December, describes the structure of the masseter muscle as consisting of an additional third, even deeper layer.

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

She said this layer is involved in the stabilisation of the lower jaw and could be the only part of the masseter that can pull the lower jaw backwards toward the ear.

Earlier studies had also suggested there could be three layers to the masseter muscle, but researchers said these divided the superficial section of the masseter into two layers.

They had agreed with standard works in their description of the deeper section.

While some anatomy texts in the past had also mentioned the possible existence of a third layer, scientists said these early descriptions were “extremely inconsistent” as to its position.

The newly discovered muscle layer runs from the back of the cheekbone to the anterior muscular process of the lower jaw. ‘S’ stands for superficial layer, ‘D’ for deep layer and ‘C’ for coronoid layer (Jens. C. Türp, University of Basel / UZB)

In the new research, scientists assessed jaw muscles preserved in formalin and computer tomographic scans of stained tissue sections of the muscles from deceased individuals who had donated their bodies to science.

They proposed that this layer be given the name Musculus masseter pars coronidea – the coronoid section of the masseter in other words – indicating it is attached to the muscular or “coronoid” process of the lower jaw.

“In view of these contradictory descriptions, we wanted to examine the structure of the masseter muscle again comprehensively,” said Jens Christoph Türp from the University of Basel.

“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,” Dr Türp added.