in the cervical region the arches are bulged outwards at the points where they are placed (Fig. 5). The axis is shaped altogether like one of the succeeding vertebræ, except only that the odontoid process is super-added to the centrum: and the bulging of the arch on each side behind the transverse process is well marked, and bears the inferior articular surface on its under side. On the other hand, the superior articular surface is placed partly on the odontoid process, but principally on the most anterior part of the arch, viz. that part which, in all the succeeding vertebræ, forms the posterior angle of the body (Fig. 3). So also on the anterior extremity of the arch are placed the articular surfaces (both superior and inferior) of the atlas (Fig. 4); and also, in the dorsal region, the surfaces for the heads of the ribs. The occipital condyles are placed upon the most anterior parts of the arch of the occipital bone, and to a small extent upon the centrum.
The foregoing examination of vertebral articulations leads us to observe, that, when surfaces for a synovial joint are present upon the body of a vertebra, however little of the body they may cover, they are never absent from those angles which are formed by the arches.
The synovial articulations between the bodies of vertebræ in mammals are arranged in the following manner: In the dorsal region are the synovial capsules for the heads of the ribs, which always occupy the angles of the bodies, but are also, in many animals, united across the middle line between the intervertebral disc and the conjugal ligament; while in some cases, as in the horse and the sheep, a small line of cartilage is stretched along the superior margin of the posterior of the two vertebræ concerned in each joint. In the cervical region in the human subject, the minute joints described by Luschka,[1] are situated between those parts of the bodies which are formed by the arches. Lastly, in the atlo-axoid and atlo-occipital articulations, the principal parts of the articular surfaces are placed upon those parts of the arches which correspond to the angles of the bodies of succeeding vertebræ, while the intervertebral discs have disappeared.
I may here remark that, if the odontoid process be regarded as the centrum of the atlas,—a view which seems to be supported by its very large comparative size in the young condition, long before the anterior tubercle of the atlas makes its appearance—then we must recognise in the odontoid ligaments the terminal member of the series to which the transverse ligament of the atlas and the ligamenta conjugalia belong: and indeed the arrangement of their fibres, some of which are continuous from side to side, is favourable to this supposition, and reminds one of the ligamentum conjugate in the sheep.
Note.—Since writing the above, my attention has been called to Rathke's work "Ueber die Entwickelung der Schildkröten," in which (page 77), the view that the odontoid process is the centrum of the atlas is strenuously urged, and strong evidence brought forward in its favour.
- ↑ Luschka, Die Halbgelenke des Menschlichen Körpers, 1858, p. 71, and Tab. I., fig. 1.
