Anatomical bases of the study of the constraints to which the cervical spine is subject in the sagittal plane. A study of the center of gravity of the head.

The authors have determined the position of the center of gravity of six isolated formolized heads by the suspension method, based on the work of the Bordeaux anthropologist, Beauvieux; in his opinion, the nasion-opisthion line (root of nose to posterior edge of the foramen magnum) characterizes the horizontality of the head, irrespective of species. The center of gravity is situated at the middle of the nasion-inion line (root of nose to external occipital protuberance), behind the sella turcica, above and slightly in front of the external auditory meatus. In profile, the axis of gravity falls on leaving the center of gravity and passes in front of the cervical spine, which remains in balance thanks to a fulcrum lever whose two arms are equal when the gaze is directed 30 degrees downwards in relation to the horizontal. This is the true reference position for the cervical spine in the sagittal plane.