The third joint of the canine jaw

Abstract

The dog mandible has three jointstwo temporomandibular and one symphyseal joint. In studies of jaws and their mechanics usually only the temporomandibular joints are considered. The symphysis is consistently neglected and because of this, remarkable features of the temporomandibular joints have never been noted.The symphysis has special connective tissue arrangements and its own vascular and nerve supply. Along the anterosuperior part of the symphysis, a strip of fibrocartilage intervenes between the bony articulating plates. Below and posterior to the fibrocartilage, and running more or less parallel to it, strong cruciate ligaments bind the symphyseal plates together. Small arteries run into the symphyseal joint and it is drained by an anastornosing venous plexus. The posterior, ligamentous, portion is laced with a profuse network of nerves.The temporomandibular joint is unusual in that its joint disc is powerfully anchored to the skull medially and to the outer pole of the mandibular condyle laterally.The temporomandibular joint disc thus acts as a ligament that halts lateral movement of the condyle just as the carnassial teeth are aligned for cutting function. As the mandible moves laterally, the space between the posterior ends of the symphyseal plates widens: at the same time i t narrows anteriorly. The cruciate ligaments control this movement and stabilize the symphyseal joint by preventing direct lateral displacement of the symphyseal plates. The fibrocartilage cushions this movement and also mediates rotational movements about the longitudinal axis of the mandible when crushing molar teeth come into function.J. MORPH., 116: 23-50.

Nectarivorous feeding mechanisms in bats

“…comm.). Scapino (1965) found mobile, unfused symphy-ses in carnivorans that need precise registration. Registration of teeth may not be as precise or may be non-existent in megachiropterans and in microchiropteran nectarivores and frugivores, and all of these bats have fused mandibulae.…”

Ecology and Biology of the Pacific Walrus,Odobenus rosmarus divergensIlliger

“…The cheek teeth are incapable of chewing or grinding, other than by vertical action, for the mandible is hinged in such a way as to prevent side-to-side or forward-and-back motion. Furthermore, the mandibular symphysis, the "third joint" of the jaw (Scapino 1965), becomes solidly ankylosed at an early age. Thus, the occlusive stresses on the teeth are exclusively vertical-percussive.…”

Long-Legged Pursuit Carnivorans (Amphicyonidae, Daphoeninae) from the Early Miocene of North America

“…Except for the presence of M3, the teeth of amphicyonids are similar in number and form to those of canids, so much so that they were for many years classified as Canidae (e.g., Simpson, 1945;Romer, 1966). Despite the similarity of the dentition, the morphology of the mandibular symphysis of daphoenine amphicyonids differs in some respects from Scapino's (1965) description of living Canis. However, as in canids, the symphysis of amphicyonids remains unfused, with the exception of some very old adults.…”

“…Here, as in the wolf, the upper and lower toothrows are in contact in central occlusion yet the carnassials are not tightly registered. As the jaws are brought together in the wolf (Canis lupus), neuromuscular mechanisms first approximate the carnassial blades, followed by interlocking of the canines and I3 (Scapino, 1965;Mellet, 1981). Because canine contact is retarded relative to that of the carnassial pair, the wolf does not utilize the autocclusal bite mechanism described by Mellett (1984) in which the canines establish contact prior to the carnassials.…”

“…The anatomy of the mandibular symphysis was analyzed in depth by Scapino (1965Scapino ( , 1981. Carnivoran symphyses were classified according to the way soft tissues in the symphyseal space related to the form of the mandibular symphyseal plates in histologic sections.…”