atlas there are no articular processes like the posterior pair; and there is presented for articulation with the condyle of the occipital bone, a single surface, exactly corresponding in extent with that which articulates with the body of the axis. As regards the occipital condyle, its constitution will be best understood by looking at the quite similar condyle of the occipital of the turtle. In it the middle and lower portions are formed by the basi-occipital, in precisely the same manner as the body of a vertebra is formed principally by the centrum, but has its superior angles derived from the arch. Thus, there can be no doubt that the atlo-occipital articulation in birds, as well as the inferior atlo-axoid articulation, belongs to the same series as those between the bodies of the succeeding vertebræ.
It remains for us to show that they also correspond to the atlo-occipital and atlo-axoid articulations in mammals: and that they do so will readily appear, on making a more careful examination of the anterior articular surface of the atlas of the bird in the recent condition. It presents the form of a cup perforated by a small foramen, through which a ligament passes from the tip of the odontoid process to the occipital condyle, and the part of the cup which lies above the foramen is formed by a transverse ligament. This transverse ligament corresponds to those which pass from side to side of the bodies of other vertebræ and are attached to the superior angles of their anterior aspects—those angles which are derived from the arches.[1] Now, in mammalia, not only is the function of the tranverse ligament of the atlas the same as in birds; but in many of them the heads of the ribs of opposite sides are united above the intervertebral discs by transverse ligaments (ligamenta conjugalia costarum), which very obviously correspond to the ligaments just mentioned on the vertebrae of the bird; for, though they do not, like them, pass from angle to angle of the bodies of the vertebræ, they are attached to structures interpolated between these angles. It appears, therefore, that the transverse ligaments of the atlas and other vertebræ in birds, and the ligamentum conjugale costarum, and transverse ligament of the atlas in mammals, are all homologous structures: and, in that case, the only difference between the atlo-occipital articulation in the mammal and in the bird is, that while in the latter it is single, in the former it is divided into two lateral parts. But this is not an important distinction; for in the atlo-axoid articulation, we find the arrangement in many mammals, as in the human subject, similar to that of the atlo-occipital; while in others, as in the sheep, a single joint extends across the middle line exactly as in the bird.
The serial correspondences of the vertebral articulations are very well illustrated in the human fœtus. The articular surfaces of the oblique processes are situated immediately behind the transverse processes, and
- ↑ I have described and figured the ligament here referred to in a paper "On the Structure, Actions, and Morphological Relations of the Ligamentum Conjugale Costarum," in the Edinburgh New Philosophical Journal, April, 1859.
