The temporo-mandibular joint of shrews
- 1The mandibular condyle of shrews has two distinct, bearing surfaces or facets separated by a non-articular region. The superior facet articulates with the upper part of the glenoid or superior glenoid shelf, the inferior facet with an inferior shelf-like extension of the glenoid.
- 2During movement of the jaw the inferior condylar facet rocks on the inferior glenoid shelf, while the superior facet glides backwards and forwards beneath the superior glenoid shelf.
- 3The post-glenoid part of the skull is long, approaching the length of the pre-glenoid part. The fibres of the very large temporalis muscle are also elongated and are mainly aligned in the horizontal plane.
- 4The zygomatic arch is absent and the deep layers of the masseter muscle complex have disappeared or become blended with the temporalis.
- 5Analysis of the stresses set up by contraction of the jaw muscles when biting against a resistance provides an explanation for the condition of the joint surfaces. The regions of the inferior and superior condylar facets are subjected to pressure as a result of the contraction of the temporalis and superficial masseter muscles respectively. The reduction in the number of more vertically disposed fibres, associated with lengthening of the skull and loss of the zygomatic arch, has relieved the back of the joint from pressure and allowed its surfaces to become non-articular.