Upper Jaw

The upper jaw, or the maxilla with the upper teeth, is the structure fixed to the skull, forming the palatal dome on the arch of the alveolar process with the teeth. It forms a fixed wall of the vocal tract and does not belong to the articulatory organs: yet it is a critical structure for speech articulation because it provides the frame of reference for many articulatory gestures. The structures of the upper jaw offer the location for contact or approximation by many parts of the tongue such as the apex, blade, and dorsum. The phonetics literature describes the place of articulation as classified according to the locations of lingual approximation along the upper jaw for dental, alveolar, and palatal consonants. The hard palate is covered by the thick mucoperiosteum, which has several transverse lines of mucosal folds called the palatine rugae.

The lower jaw, or the mandible with the lower teeth, is the largest rigid motor organ among the speech production apparatus. Its volume is about 100 cm3. As well as playing the major role in opening and closing the mouth, it provides attachments for many speech muscles and supports the tongue, lips, and hyoid bone. 

Figure 10 shows the action of the jaw and the muscles used in speech articulation. The mandible articulates with the temporal bone at the temporomandibular joint (TMJ) and brings about jaw opening–closing actions by rotation and translation. The muscles that control jaw movements are generally called the masticatory muscles. The jaw opening muscles are the digastric and lateral pterygoid muscles. The strap muscles, such as the geniohyoid and sternohyoid, also assist jaw opening. The jaw closing muscles include the masseter, temporalis, and medial pterygoid muscles. While the larger muscles play major roles in biting and chewing, comparatively small muscles are used for speech articulation.

The medial pterygoid is mainly used for jaw closing in articulation, and the elastic recoil force of the connective tissues surrounding the mandible is another factor for closing the jaw from its open position.

Figure 10a,b: Actions of the temporomandibular joint and muscles for jaw opening and closing. (a) The lower jaw opens by rotation and translation of the mandible at the temporomandibular joint. Jaw translation is needed for wide opening of the jaw because jaw rotation is limited by the narrow space between the condyle and tympanic bone. (b) Jaw opening in speech depends on the actions of the digastric and medial pterygoid muscles with support of the strap muscles. Jaw closing is carried out by the contraction of the lateral pterygoid muscle and elastic recoil forces of the tissues surrounding the jaw