Morphological disparity, conservatism, and integration in the canine lower cervical spine: Insights into mammalian neck function and regionalization

The neck skeleton is constituted as a highly mobile, multi-element and multi-joint kinematic chain. This construction leads to a kinematic redundancy on several levels. The problem of coordinating a large number of joints and muscles is solved by reducing the degrees of freedom to only few preferred motor axes. This is achieved by the regionalization of the cervical spine into three functional compartments. However, the neck skeleton traditionally is only partitioned into two units basing on the derived morphology of the two uppermost vertebrae (atlas and axis). To broaden the understanding of the functional morphology and regionalization of the mammalian neck skeleton, the morphology of the lower cervical vertebrae of a variety of different sized breeds of domestic dogs is compared using 3D geometric morphometrics. The shape analysis reveals significant differences among all lower cervical vertebrae. However, the specific shape of each vertebral level is independent of the actual breed and breed size. The individual vertebrae as well as the whole vertebral proportions are highly integrated. Nevertheless, the mid-cervical vertebrae (C3–C5) are more uniform in their shape whereas the lower ones (C6, C7) have a unique morphology. The tripartition of the cervical spine into functional compartments is confirmed for the neck of dogs. The conserved pattern of level specific vertebral shapes suggests that the morphological and functional tripartition of the cervical spine could be a general trait of all mammalians.