Located in the head, it appears to be a "fifth muscle of mastication," according to Dr Gary Hack of the University of Maryland's Dental School. In other words, it helps us move our jaws to chew food.
The new muscle was identified by Dr Hack and his colleague Dr Gwendolyn Dunn, an orthodontist and volunteer dissector with the anatomy department of the uni- versity's School of Medicine. They announced their findings just as the annual meeting of the American Association for the Advancement of Science in Baltimore was coming to a close yesterday.
The researchers have dissected dozens of human cadavers and found the muscle is present in every one of them.
However, there is no reference to it in medical literature. "We have been studying the muscles of mastication for several years, but have found no description of this muscle in any anatomy textbook," Dr Hack said.
It was present in every specimen examined and while "there was variability in the size of the muscle" this would be expected, he concluded.
The new muscle extends from just behind the eye socket to the lower jaw. It is approximately one and a half inches long, three-quarters of an inch wide, and half an inch deep. There are four known muscles of mastication, those that move the jaw, according to the textbooks. The fifth one showed up because Dr Dunn was using different dissection methods from the norm.
The two dentists dissect from unusual angles thus obtaining different views of anatomical structures. "We do not follow everyone's preconceptions. We aren't duplicating surgical procedures," Dr Dunn said.
Non-invasive body scans of living people, using magnetic resonance imaging (MRI), also revealed the muscle. "The muscle shows up quite clearly in MRI scans - once you know where to look," according to Dr Michael Rothman, director of the university's MRI centre.
Anatomists will have to know where to look to find a complete description of the new muscle. For when they have completed their work, Dr Hack and Dr Dunn will inform the scientific world in detail through the pages of a journal whose title should be on everyone's lips (or jaws at least): Cranio: The Journal of Craniomandibular Practice.Reuse content