Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 2.djvu/39

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Fig. 4, b. Oblique triangular facet on the lower margin of the front of the Atlas; this facet articulated with the first sub-vertebral wedge, placed between the Atlas and Occiput.

Between the Atlas and Axis, the two sub-vertebral facets formed a triangular cavity for the reception of a second wedge (Fig. 4. c) and a similar, but smaller cavity received another wedge of the same kind, between the Axis and third Vertebra. This third wedge gave less support to the head, and admitted of more extensive motion than the second. All these three wedge-shaped bones are seen nearly in their natural position in a specimen from Lyme Regis, in the Collection of Sir P. G. Egerton.

Fig. 4'. First sub-vertebral wedge, auxiliary to the anterior cavity of the Atlas, in completing the articulating socket for the basilar process of the Occiput (3. a.)

4. a. Crescent-shaped front of the first sub-vertebral wedge.

4'. b. Head of the same Wedge.

4'. c. Obtuse apex of the same, articulating with the triangular frontal facet of the Atlas (4. b.) In young animals this frontal facet is nearly smooth and flat; in older animals (3. b'.) it is rugged and furrowed. This articulation must have given to the first sub-vertebral wedge great power as a stay or prop, to resist the downward pressure of the head, at the same time facilitating the rotatory movements of the Occipital bone.

Fig. 4. c. Second sub-vertebral wedge articulating with the triangular cavity formed by the marginal facets of the Atlas and Axis. This second Wedge acted as a strong prop supporting firmly the lower portion of the Atlas, and at the same time admitting the small amount of motion here required.

c'. Head of the sub-vertebral wedge (c) strengthened by a projecting boss of solid bone.