154 



OEIGINAL AETICLES. 



See also "Owen, On the Homologies of the Vertebrate Skeleton/' page 

 93. Eathke 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 trans- 

 verse 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. 



Fig. 1 . Atlas of a young Chelonia virgata — after Eathke. a. The 

 arch. b. Osseous centre of the tubercle. 



Fig. 2. Dorsal vertebra of a young seal, for comparison with the fol- 

 lowing figures : a. Oblique process. b. Articular surface for head of 

 rib. 



Fig. 3. Superior aspect of the axis of a human foetus, a. The cen- 

 trum, b. Odontoid process, c. Superior articular surface, d. Bulg- 

 ing 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 pro- 

 cess, b. Part of the arch entering into the composition of the body of 

 the vertebra. 



