THE MUSCLES OF MASTICATION AND THE TEMPOROMANDIBULAR JOINT
The muscles of mastication are well developed. The masseter takes origin along the whole length of the facial crest and zygomatic arch and inserts on the mandible between the vascular notch and condyle (Figure 18-7/11).
It is a multipennate muscle constructed so that the fibers of the superficial strata run caudoventrally, while those more deeply placed are nearly vertical. Its cranial margin produces a very prominent surface contour that serves as a guide to the location of the facial vessels and parotid duct. Its caudodorsal part is overlain by the parotid gland but to a variable depth and extent, which affect the accessibility to palpation of the parotid lymph nodes. Laterally, the masseter is traversed by buccal branches of the facial nerve.The temporalis almost fills the temporal fossa, where it is easily palpated despite the partial covering of thin muscles concerned with the movement of the external ear (Figure 18-23/1). It arises from the wall of the fossa and from the sagittal crest that forms its median margin, and it envelops the coronoid process of the mandible. On contraction it raises the mandible.
The pterygoideus medialis and lateralis, deep to the mandible, broadly correspond to the masseter in position, orientation, and attachments (Figure 18-23/2,3). The medial muscle, always the larger, extends from the pterygoid process to the mandibular margin. The lateral
[*]It takes about 6 months for an erupted tooth to reach the height of its neighbors.
Di, Deciduous incisor; I, incisor; I's, incisors.
muscle runs more horizontally to insert close to the condyle. The masseter and contralateral pterygoid muscles act together to produce the horizontal shifts that supply the principal grinding movement.
The digastricus and Occipitomandibularis (strictly a part of the digastricus; Figure 18-23/4,4') are responsible for active opening of the mouth.
Despite its much greater bulk, the latter may be regarded as a detachment from the caudal belly of the digastricus. It extends between the paracondylar process of the occipital bone and the caudal border of the mandible. The much more slender digastricus has a similar origin. It presents an intermediate tendon that passes through a split in the insertion of the stylohyoideus. The rostral belly attaches to the ventromedial part of the molar region of the mandible. When the mouth is closed, contraction of the digastricus raises the hyoid apparatus (by virtue of its association with the stylohyoideus) and thus the root of the tongue (Figure 18-23, B).A thick intraarticular disk is interposed between the expanded and rather flat facets of the mandibular condyle and articular tubercle of the temporal bone (Figure 18-23, A/5). Hinge movements occur at the lower level, which is supported by a tight capsule; the lateral and slight protrusive movements occur at the upper level where the joint cavity is more capacious. The whole joint is supported by a fibrous lateral ligament and an elastic caudal one.