Occipital angle of Broca ; horizontalization of the foramen magnum in the skull
Fig. 2
Orbito-occipital angle in humans: 10°; monkeys: 30–69°, carnivorous: 63–90°
Fig. 3
The lateral (or horizontal) semicircular canal is parallel to the nasion-opisthion plane (Beauvieux)
Fig. 4
Reference position; the nasion-opisthion line is horizontal and the axis of the orbit is 30° to this line
Finally, François Clarac (CNRS, Timone, Marseille) has shown that the foramen magnum has become more and more anterior in the evolution of primates, which tends to increase the leverage of the extensors on the occiput, an essential phenomenon for keeping the head and the eyes horizontal (Fig. 5).
Fig. 5
Evolution of the foramen magnum position in primates (Clarac)
Centre of Gravity of the Head
This centre of gravity was the subject of an anatomical study published in Surgical Radiology Anatomy in 1986 [1]. The so-called suspension method was applied to six cadaveric skulls from three women and three men weighing between 3.7 kg and 5.2 kg, with cranial indices ranging from 72 (dolichocephalic) to 85 (brachycephalic). The penetration points of Gardner’s tongs that allowed the centre of gravity of the head to be recognized were all projected over an area of one square centimetre above the tragus. Radiologically, the centre of gravity projected in the middle of the nasion-inion (external occipital protuberance) line. Slightly behind the sella turcica and directly above the external auditory canal (Table 1 and Figs. 6, 7, and 8). It is interesting to note in Fig. 9 that in the reference position, antagonizing anterior and posterior muscles maintain the head above the cervical spine. The lever arm of the weight of the head applied to the centre of gravity, previously described, is strictly equal to the lever arm of the extensor muscles inserted on the occipital aspect (Fig. 9); on the other hand, when the gaze is horizontal, the leverage of the extensors is greater than that of gravity (Fig. 10).
Table 1
Centre of gravity of the head; characteristics of studied skulls
Specimen number
Sex
Head weight (kgs)
Cranial index
Type
1
F
3.67
85
Brachy
2
F
3.73
76
Dolicho
3
F
4.03
72
Dolicho
4
M
4.33
80
Brachy
5
M
5.21
83
Brachy
6
M
4.82
73
Dolicho
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