Masseter, Temporalis and Infratemporal Fossa
Masseter (Fig. 7.29) attaches along the length of the zygomatic arch and its fibres slope downwards and backwards to the lateral surface of the ramus of the mandible adjacent to the angle (Fig. 7.31). This muscle is a powerful elevator of the mandible and is easily palpated when the teeth are clenched. It is supplied by the masseteric branch of the mandibular (V3) division of the trigeminal nerve.
Temporalis (Fig. 7.30) is a large fan-shaped muscle occupying the temporal fossa and taking attachment from the area of bone bounded by the inferior temporal line. The more superficial fibres arise from the temporal fascia that covers the muscle and is attached to the superior temporal line. All the fibres descend deep to the zygomatic arch to attach to the coronoid process and anteromedial aspect of the ramus of the mandible (Fig. 7.31). Temporalis elevates the mandible, as in closing the mouth, and its posterior fibres retract the mandible. The deep temporal branches of the mandibular (V3) division of the trigeminal nerve supply the muscle from its deep surface.
This fossa lies deep to the ramus of the mandible and is limited on its medial aspect by the lateral wall of the pharynx and the medial pterygoid plate of the sphenoid bone. The fossa is bounded by the posterior surface of the maxilla in front and by the styloid process and its attached muscles behind. The roof is provided by the temporal and sphenoid bones in the base of the skull while inferiorly the fossa is continuous with the neck.
Within the fossa are the two pterygoid muscles, the mandibular (V3) division of the trigeminal nerve and its branches, and the maxillary vessels and their branches. Adjacent to the fossa is the temporomandibular joint.
Each of the lateral and medial pterygoid muscles (Figs 7.32–7.34) has two attachments to the skull. The upper head of the lateral pterygoid attaches to the inferior surface of the greater wing of the sphenoid. The lower head attaches to the lateral surface of the lateral pterygoid plate. Both heads converge on the neck of the mandible and the capsule of the temporomandibular joint. The lateral pterygoid pulls forward both the neck of the mandible and the articular disc, thus depressing the mandible and opening the mouth.
The lower head of the lateral pterygoid is clasped by the two heads of the medial pterygoid. The deep head of the latter is larger and attaches to the medial surface of the lateral pterygoid plate. The superficial head is attached to the tuberosity of the maxilla. The fibres of both heads incline obliquely downwards, backwards and laterally to attach to the medial surface of the angle of the mandible. The muscle is a powerful elevator of the mandible.
The temporomandibular joint (Fig. 7.32) is a synovial joint. The head of the mandible articulates with the articular fossa and eminence of the temporal bone. Fibrocartilage covers the articular surfaces and also forms an articular disc, which divides the joint into two separate cavities. Within these cavities, the non-cartilaginous surfaces are lined with synovial membrane.
The fibrous capsule surrounding the joint is attached to the margin of the articular cartilage and to the neck of the mandible. Anteriorly, it receives the attachment of the lateral pterygoid while its deep surface is firmly adherent to the periphery of the articular disc.
Laterally, the capsule (Fig. 7.30) is thickened to form the lateral ligament, which inclines posteroinferiorly from the root of the zygomatic arch to the neck of the mandible. Two accessory ligaments lie medial to the joint, although not in contact with the capsule. The sphenomandibular ligament extends from the spine of the sphenoid to the lingula adjacent to the mandibular foramen. The stylomandibular ligament, a thickening of the parotid fascia, passes from the styloid process to the angle of the mandible.
The joint receives its nerve supply from the auriculotemporal and masseteric branches of the mandibular (V3) division of the trigeminal nerve.
Movements at the joint include elevation, depression, protraction and retraction of the mandible. The head of the mandible does not merely rotate in the articular fossa but also moves forwards onto the articular eminence of the temporal bone, taking the articular disc with it. The alternate protraction and retraction of right and left sides produces the grinding movements used in chewing. The muscles responsible for these movements are known collectively as the muscles of mastication. The mouth is closed by contraction of masseter, temporalis and medial pterygoid. The lateral pterygoid protracts the mandible and, assisted by digastric and mylohyoid (p. 348), also opens the mouth. Retraction is produced by the posterior fibres of temporalis. When the mandible is fully depressed, the joint is relatively unstable and dislocation may occur, the head of the mandible moving in front of the articular eminence and resulting in an inability to close the mouth.
The mandibular division of the trigeminal nerve (Figs 7.33 & 7.34) contains sensory and motor fibres and enters the infratemporal fossa through the foramen ovale in the sphenoid. Two small branches arise from the short main trunk of the nerve. The first branch ascends through the foramen spinosum to receive sensation from the meninges of the middle cranial fossa. The other branch is motor, supplying the medial pterygoid and giving a small branch that passes through the otic ganglion (lying just medial to the main trunk of the mandibular division) to supply tensor tympani and tensor veli palatini.
The main trunk descends between the lateral pterygoid and tensor veli palatini muscles, dividing into anterior and posterior divisions. The anterior division is mainly motor and gives masseteric, deep temporal, lateral pterygoid and buccal branches. The masseteric nerve (Fig. 7.32) curves laterally above the lateral pterygoid to enter the deep surface of masseter. Two or three deep temporal nerves (Fig. 7.33) ascend deep to temporalis, which they supply, and further branches enter the deep surface of the lateral pterygoid. The buccal nerve (Figs 7.33 & 7.34) is a sensory branch that passes forwards between the two heads of the lateral pterygoid to supply the skin over the cheek and the mucosa lining the cheek, which it reaches by piercing, but not supplying, buccinator.
The posterior division of the main trunk is mainly sensory and has three branches, the auriculotemporal, lingual and inferior alveolar nerves. The auriculotemporal nerve (Fig. 7.34) arises by two roots, which clasp the origin of the middle meningeal artery. The nerve passes backwards before turning superiorly behind the temporomandibular joint to ascend in company with the superfi- cial temporal vessels. It gives secretomotor branches to the parotid gland (p. 339) and conveys sensation from the temporal region, the upper half of the pinna and most of the external acoustic meatus.
The lingual nerve (Figs 7.33 & 7.34) inclines downwards and forwards between the pterygoids, deviating medially to pass below the superior constrictor of the pharynx. In the floor of the mouth it runs forwards lateral to the hyoglossus muscle, at whose anterior border it again turns medially to pass inferior to the submandibular duct and enter the base of the tongue. It conveys general sensation from the anterior two-thirds of the tongue. Near the lower border of the lateral pterygoid the lingual nerve is joined by the chorda tympani (a branch of the facial nerve). Arising within the temporal bone, the chorda tympani emerges from the petrotympanic fissure. It carries taste fibres, which have travelled in the lingual nerve from the anterior two-thirds of the tongue and preganglionic parasympathetic fibres destined for the submandibular ganglion (p. 352).
The inferior alveolar nerve (Figs 7.33 & 7.34) descends medial to the lateral pterygoid and gives rise to a motor branch that curves downwards to supply mylohyoid and the anterior belly of digastric. The inferior alveolar nerve then enters the mandibular foramen in the ramus of the mandible and runs forwards in the mandibular canal, supplying the lower teeth and alveolar ridge. Its mental branch emerges from the mental foramen to supply skin overlying the chin. Local anaesthetic injected near the inferior alveolar nerve as it enters the mandibular foramen will block sensation from the lower teeth and gums on that side of the mouth. Often there is loss of sensation in the same side of the tongue because of the proximity of the lingual nerve
This artery (Figs 7.33 & 7.34) arises in the parotid gland (p. 339) as a terminal branch of the external carotid artery, passes antero- superiorly across the infratemporal fossa, usually lateral to the lateral pterygoid, and traverses the pterygomaxillary fissure to enter the pterygopalatine fossa where terminal branches arise. These correspond to branches of the maxillary nerve (p. 353).
In the infratemporal fossa, the maxillary artery gives branches to supply masseter, temporalis and the pterygoid muscles. In addition, the middle meningeal artery arises deep to the lateral pterygoid, embraced by the two roots of the auriculotemporal nerve. It traverses the foramen spinosum and within the cranium supplies the meninges of the middle cranial fossa and the cranial vault.
The maxillary artery also gives rise to the inferior alveolar artery, which accompanies the nerve into the mandibular canal. Further small branches supply the middle ear and the lining of the external acoustic meatus.
Veins within the pterygopalatine fossa form a plexus that extends through the pterygomaxillary fissure into the infratemporal fossa, where the plexus is related to the pterygoid muscles. This pterygoid plexus has important connections to the cavernous sinus in the skull and infraorbital and ophthalmic veins. The plexus drains by the maxillary vein into the retromandibular vein (p. 340).