The mandible is also acted upon by forces not experienced in the maxillae, and its outer layers reflect this. Muscle attachments in the maxillae are weak; in the mandible they are unusually strong. The powerful muscles that open and close the mouth exert bending forces. Lateral thrusts of the mandible exert compressing forces. In response to these distorting influences, the mandible has a "shell" of thick cortical compact bone (4) around its entire surface. This reinforcement alone makes the lower alveolar process far stronger than its maxillary counterpart (5) in most areas.

Muscle pull has encouraged the formation of distinct, bulky ridges, named masticatory muscle stress trajectories. There is in-creased deposition of bone on the mandibular angle, on its external surface by the action of the masseter (elevator) muscle (6), and on its internal by the internal pterygoid (elevator) muscle (7).

The deep tendon of the temporal (elevator) muscle, which is generally longer and more prominent than the superficial tendon, attaches at the coronoid process (8), and its tendonous insertion descends along the anterior edge of the ramus (9) to the temporal crest toward the alveolar process. The retromolar triangle is a continuation of the temporal crest that bifurcates around the alveolar process. Its buccal continuation, the external oblique ridge (10), is a particularly noteworthy bulwark against de-formation.

The mylohyoid (depressor) (11) muscle, which forms the floor of the oral cavity, attaches along the body of the mandible from near the midline to the third molar area. In most individuals,

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