Scientists Find Body Part You Didn't Know You Had

Coronoid layer on jaw's messeter muscle described in humans for first time
By Arden Dier,  Newser Staff
Posted Dec 21, 2021 10:00 AM CST
Updated Dec 26, 2021 8:45 AM CST
Here's a Body Part You Didn't Know You Had
The newly discovered muscle layer is highlighted as "C," for coronoid layer ("S" = superficial layer, "D" = deep layer).   (Jens. C. Turp, University of Basel/UZB)

Press your fingers behind your cheeks and clench your teeth. What you're feeling is the masseter muscle, a key muscle allowing humans to chew and eat, which is usually described as being made up of a deep layer and a superficial one, per the Hill. Except researchers say that's not quite right. Indeed, they say they've discovered a third, even deeper layer of the masseter, which is attached to the muscular or "coronoid" process of the lower jaw, according to a release. Hence the proposed name of Musculus masseter pars coronidea, or the coronoid section of the masseter, as revealed in the scientific journal Annals of Anatomy.

Dr. Szilvia Mezey of the University of Basel—part of a team that discovered the layer in fresh and preserved human cadaver heads as well as in MRI data from a living person—says it's "clearly distinguishable from the two other layers in terms of its course and function." The deepest layer appears to stabilize the lower jaw and may be the only part of the masseter that can pull the lower jaw backward toward the ear, according to the study. A third layer of masseter has been referenced before, including in a 1995 edition of Gray's Anatomy. But that book cited non-human studies of jaw musculature, which conflicted with each other, per the release. More recent studies describing three masseter layers have instead concluded that the superficial layer is two layers in itself.

"In view of these contradictory descriptions, we wanted to examine the structure of the masseter muscle again comprehensively," says study co-author Jens Christoph Türp of the University Center for Dental Medicine 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." The researchers say naming the muscle layer, running from the lower part of the skull to "the root and posterior margin of the coronoid process," will "facilitate discussion" of this newly-described body part moving forward. (More discoveries stories.)

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