This joint (Articulatio mandibularis) is the only diarthrosis formed
between bones of the skull.
The =articular surfaces= are dissimilar in form and size. That on the
squamous temporal bone is concavo-convex, and the long axis is directed
outward and somewhat forward; it consists of a glenoid cavity, which is
continued upon the postglenoid process behind, and a condyle in front.
The mandible presents a transversely elongated condyle.
The =articular disc= is placed between the joint surfaces, which it
renders congruent. Its upper and lower surfaces are molded upon the
temporal and mandibular surfaces respectively, and its circumference is
attached to the joint capsule; thus it divides the joint cavity into
upper and lower compartments, the former being the more roomy.
The =joint capsule= is strong and tight. It is reinforced by two
ligaments. The =external ligament= (Ligamentum laterale) extends
obliquely across the anterior part of the outer surface of the capsule,
from which it is not distinctly separable. The =posterior ligament=
(Ligamentum posterius) is an elastic band which is attached above to the
postglenoid process, and below to a line on the posterior face of the
neck of the mandible.
=Movements.=—The chief movements take place around a transverse axis
passing through both joints. Associated with this hinge-like action is
slight gliding movement, as in opening and shutting the mouth. When the
mouth is shut, the condyle lies under the glenoid cavity. When the
mandible is depressed, the condyle moves forward under the articular
eminence of the temporal bone, carrying the disc with it. In protrusion
and retraction of the lower jaw the gliding movement just described
occurs without the hinge-like rotation of the condyle. These movements
are similar in both joints. In the lateral movements (as usually
performed in mastication) the action consists of rotation of the
condyles around a vertical axis, while the disc glides forward on one
side and backward on the other.