The mental trigone (trigonum mentale) is located in the region of the symphysis as a triangular bony support that strengthens the symphysis and safeguards against transverse stresses. The inferior corners of the mental trigone lying on one side are formed by the mental tubercles (tubercu-lum mentale).
The mandible (mandibula; Latin, mandere = to chew) is a paired bone that is fused at the symphysis before birth. Symphysis means the bony site where the two halves of the mandible are fused, which lies on the midline of the symmetric bony plate. The parabolically shaped bony plate, as the largest of the facial bones, determines the shape and size of the lower part of the face. The main parts of the mandible are the body and the rami of the mandible.
The bony palate (palatum osseum)forms the roof of the oral cavity and the floor of the nasal cavity (Fig 6-19). It is made up of the palatine processes of the maxillary body; the incisive bone, which forms a bony fusion with the maxilla in adults; and the horizontal plates of the palatine bones, the backward projection of the palatine processes (Fig 6-20).The bony palate is held together by the following sutures:
The frontal process (processus frontalis) rises vertically between the lacrimal bone and the nasal bone up to the frontal bone. It forms a closed bony connection with the cranial area so that masticatory forces can be transferred to the cranium while bypassing the nasal and orbital cavities.
The maxilla is a paired bone; ie, there are two maxillary bones that are joined by a bone suture. However, the individual maxillary bones also have a paired arrangement. Thus, it is still possible to detect the development from one maxillary process and the frontal process, giving rise to the unified maxillary bone for each half of the face.
The ethmoid bone is a sievelike, perforated, air-filled bone with a number of cavities known as ethmoidal cells (cellulae ethmoidales), which feed into the nasal cavity and are part of the paranasal sinuses. A plate of the ethmoid bone projecting downward, known as the perpendicular or vertical plate (lamina perpendicularis), forms the superior part of the nasal septum.The superior edge of the perpendicular plate forms a cockscomblike crest (crista galli) extending into the cranial cavity. A narrow bony plate forms the roof of the ethmoid bone (cribriform plate; lamina cribrosa), which is perforated with numerous holes like a sieve. The lateral walls of the ethmoid bone are smooth, paper-thin bony plates (orbital lamina; lamina orbitalis) that form one part of the medial orbital wall. The ethmoidal labyrinth is located on either side of the nasal septum and, with its curved, twisted lamellae of bone (superior and middle nasal conchae), forms the branched passages of the nasal cavities and the paranasal sinuses.
The frontal bone (os frontale) arises from a paired arrangement where two parts are fused into one bone in the adult (Fig 6-9).The frontal bone forms the anterior area of the cranium and the roof of the orbits. Behind the prominent orbital swellings are the frontal sinuses, which are air-filled and lined with mucous membrane. These sinuses are connected to the actual nasal cavity as paranasal sinuses. The nasal part (pars nasalis) of the frontal bone, on which the nasal skeleton grows, lies between the two roofs of the orbits.
The separation of the skull into the facial part and the cranial vault can be seen from the outer surface of the base of the skull (Fig 6-3). Removing the mandible to view the base of the skull (basis cranii) reveals the very highly differentiated anterior facial part and the clearly defined neurocra-nial part. The anterior area is formed by the two maxillary bones, to which the palatine bones are attached at the rear. To either side of that are the zygomatic bones (zygomata), which join the temporal bones to form the zygomatic arch (arcus zy-gomaticus). The middle connection between the roof of the palate (palatine vault) and the body of the sphenoid is formed by the vomer; the lateral connections between the sphenoid and the maxilla are formed by the pterygoid processes of the sphenoid bone.
The bony skull (cranium) forms the basic bony structure (skeleton) of the head. It is balanced on the first cervical vertebra or the spinal column. It is the point of origin and attachment of many muscles, which in turn partly defines its shape. The shape of the skull is mainly determined by its contents. For instance, there is a connection between the brain and the structure that houses the brain; if the brain grows excessively, the cranium surrounding it becomes enlarged. Equally, premature ossification of the cranium leads to malformations of the brain. A similar relationship exists in the facial part of the skull, where special functions such as that of the masticatory system enforce a specific shape on the jaws, the temporomandibular joints (TMJs), and the teeth.
As the teeth erupt, they have prominent cusps and ridges, by which they align themselves with each other in occlusal contact until optimal contact is achieved. The prominent functional surfaces are abraded as a result of masticatory function. The degree of wear depends on: