The jaw provides the foundational bony structure for the mouth, supporting the teeth and anchoring the muscles necessary for movement. This arrangement allows for crucial actions like chewing food during the initial stages of digestion and producing speech sounds. The jaw is composed of two main bony elements: the fixed upper jaw, known as the maxilla, and the movable lower jaw, called the mandible.
The Primary Bony Structures
The mandible is the largest and only movable bone of the facial skeleton, forming a U-shape that houses the lower teeth. The horizontal portion is the body, a dense curve where the teeth are embedded within the alveolar process. Rising vertically from the back of the body is the ramus, which provides a broad surface for muscle attachment.
The ramus extends upward to form two processes at its superior end: the coronoid process and the condylar process. The coronoid process is a sharp, triangular projection that serves as an attachment point for the temporalis muscle. The condylar process is a rounded structure that forms the “ball” component of the temporomandibular joint.
The maxilla is a fixed bone that makes up the upper jaw and contributes to several facial structures. It forms the roof of the mouth, the floor of the eye orbits, and the sides and floor of the nasal cavity. Like the mandible, the maxilla possesses an alveolar process, which securely holds the upper set of teeth. Its primary function is to provide a stable upper dental arch against which the mandible moves during chewing.
The Temporomandibular Joint (TMJ)
The temporomandibular joint (TMJ) links the mandible to the temporal bone of the skull. This paired joint is responsible for the wide range of motions the lower jaw executes. It is classified as a synovial joint, meaning it is enclosed by a capsule and contains lubricating fluid.
The joint is formed by the mandibular condyle fitting into a concave area on the temporal bone called the mandibular fossa. The articular disc, a fibrous pad of cartilage positioned between the condyle and the fossa, is a distinguishing feature of the TMJ. This disc acts as a shock absorber and allows for smooth, coordinated movement between the bony surfaces.
The TMJ performs a dual action combining a simple hinge motion with a gliding movement. The hinge action, or rotation, occurs in the lower compartment and is responsible for simple mouth opening and closing. The gliding action, or translation, takes place in the upper compartment, enabling the jaw to move forward, backward, and side-to-side for chewing.
Muscles Controlling Jaw Movement
Jaw movement is powered by the muscles of mastication, which originate on the skull and insert onto the mandible. The Masseter is a thick, powerful muscle running from the cheekbone down to the angle of the jaw. Its contraction provides the greatest force for elevation, or closing, the jaw during biting.
The Temporalis is a fan-shaped muscle covering the side of the head, attaching to the coronoid process of the mandible. This muscle also serves as a strong elevator, working with the Masseter to close the jaw. Its posterior fibers assist in pulling the jaw backward.
Deeper within the skull are the two Pterygoid muscles, which are necessary for the grinding movements of chewing. The Medial Pterygoid muscle runs vertically, forming a sling with the Masseter, assisting forcefully in closing the jaw and moving it forward. The Lateral Pterygoid muscle assists in depressing the jaw to open the mouth. It is also responsible for protracting the jaw forward and moving it side-to-side.