The human skull is a complex structure made up of 22 bones that provide protection for the brain and give shape to the face. While most of these bones are fixed and fused together, one prominent bone is uniquely capable of independent movement, forming the lower framework of the face.
The Mandible: The Lower Jaw Bone
The bone that constitutes the lower jaw is formally known as the mandible. It is the largest and strongest bone in the entire facial skeleton, and the only bone of the skull, besides the tiny ear ossicles, that can move freely. It is positioned directly beneath the maxilla, the fixed bone of the upper jaw that holds the upper teeth. The mandible presents a characteristic horseshoe or U-shape, housing the full set of lower teeth within its alveolar border.
Structural Components and Movement
The mandible is composed of two primary sections: a horizontal body and two vertical extensions called rami. The curved body holds the teeth and forms the chin, while the two rami project upward from the back of the body, creating the angle of the jaw.
The movement of the lower jaw originates at the paired temporomandibular joints (TMJ), located just in front of each ear. This joint is formed where the rounded projection, called the condyle, at the top of each ramus articulates with a depression in the temporal bone of the skull.
This articulation is not a simple hinge, but a complex joint that allows for both rotation and sliding motions. The jaw can move in a vertical direction (opening and closing), a forward and backward translation (protrusion and retraction), and a side-to-side motion. This extensive mobility is facilitated by an articular disc within the TMJ, which acts as a cushion and divides the joint space.
Primary Roles in Human Biology
The mobility provided by the mandible’s structure is indispensable for two fundamental human activities: mastication and phonation.
Mastication
Mastication, or chewing, is the process of physically breaking down food to prepare it for digestion. The powerful muscles of mastication, such as the masseter and temporalis, work to elevate the mandible, driving the lower teeth against the upper teeth. The side-to-side and forward movements allow for a grinding action, which effectively crushes and pulverizes food against the fixed upper jaw.
Phonation
The precise control over this bone’s position is necessary for producing articulate speech. Phonation requires subtle and rapid changes in the size and shape of the oral cavity to modulate the sounds produced by the larynx. The mandible’s movement alters the relative position of the tongue and teeth, which is essential for forming consonants and vowels.