See also "Owen, On the Homologies of the Vertebrate Skeleton," page 93. Rathke points out that the ligamentum suspensorium, which, in the birds and higher reptilia unites the odontoid process to the occipital condyle, is the serial representative of the intervertebral discs behind. He found that in most chelonians it consisted of true cartilage, and that in certain birds it was composed of fibro-cartilage. This view of the ligamentum suspensorium is quite consistent with the suggestion which I have offered, that the ligamenta alaria are homologous with the transverse ligament.
The large portion of the cup on the anterior aspect of the chelonian atlas, which is formed by the expanded inferior extremities of the arch, illustrates very well the unity of plan upon which the articular surfaces of the atlas are formed in animals having one occipital condyle, and those which have two. This will be seen by comparing the woodcuts below.
When, in the human subject, a process of bone passes up from the arch of the atlas, to meet the superior articular surface and convert the groove for the nerve and vertebral artery into a foramen, the process in question is a true oblique process. If, in addition, we were to imagine the tip of the transverse process thickened and projecting upwards to meet the superior articular surface, we should then have presented to us the condition of parts found in the pig and the sheep.
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Fig. 1 . Atlas of a young Chelonia virgata—after Rathke. a. The arch. b. Osseous centre of the tubercle.
Fig. 2. Dorsal vertebra of a young seal, for comparison with the following figures: a. Oblique process. b. Articular surface for head of rib.
Fig. 3. Superior aspect of the axis of a human fœtus, a. The centrum, b. Odontoid process, c. Superior articular surface, d. Bulging of the arch in the situation of the inferior articular surface.
Fig. 4. Inferior aspect of the atlas of the same subject, a. Articular surface.
Fig. 5. Cervical vertebra from the same subject, a. Oblique process, b. Part of the arch entering into the composition of the body of the vertebra.
