25 Days of Skeletal Facts: Day 18 – The Mandible

Mandible. Source: Encyclopaedia Britannica

The mandible is your lower jaw bone and is also the largest, and strongest, bone of your facial skeleton or your viscerocranium. The upper jaw bone, the maxilla, is articulated with the rest of your viscerocranium, while the mandible articulates with the temporal bone via the condylar process (see image to the right) at the TMJ (temporomandibular joint), a hinge joint. Your four muscles of mastication attach to different places along the mandible in order to depress, elevate, protrude, and retract your jaw. It is the alternating contractions of these muscles that produces chewing.

Right view of primate skulls. Source: Nature

The anatomical term for the chin is mental protuberance or eminence and Homo sapiens are actually unique among primates in that we are the only species to have a true chin. You will notice in the image above that the chimpanzee, baboon, and gelada all display quite pronounced prognathism, or protrusion of the jaw/face below the nose. Comparatively, we modern humans have a very flat, tall face, a reorganization of our facial bones partly to accommodate what large brains we have. Researchers have wondered whether the chin emerged from this reorganization as a possible adaptation for the mechanical stressors of chewing or perhaps for speech, with little evidence to support either claim (Pampush & Daegling 2016). Studies performed to test how having or not having a chin impacts the mechanical stress of chewing have yielded inconclusive results though some suggest that the chin evolved independently of chewing mechanics (Holton et al. 2015). And the great debate over whether Neanderthals have chins (some specimens really look like they do), rages on.

Homo sapiens skull (left) with a Neanderthal skull (right). Source: Smithsonian

That’s all for your lower jaw – tomorrow we will have a brief discussion on the tiniest bones in the human body, the ear ossicles.

REFERENCES & MORE TO EXPLORE

The Mandible. TeachMe Anatomy Series. 2020

Why we have chins: Our chin comes from evolution, not mechanical forces. ScienceDaily. 13 April 2015

The enduring puzzle of the human chin. Pampush & Daegling. Evolutionary Anthropology. 2016

The ontogeny of the chin: an analysis of allometric and biomechanical scaling. Holton et al. Journal of Anatomy. 2015

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