Homologies of the Mammalian Axis, &fc. 223 



a cup perforated by a small foramen, through which a liga- 

 ment 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 vertebrae, and are attached 

 to the superior angles of their anterior aspects — those angles 

 which are derived from the arches.* Now, in mammalia, 

 not only is the function of the transverse 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 interver- 

 tebral discs by transverse ligaments (ligamenta conjugalia 

 coslarum), 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 vertebrae, they are attached to structures inter- 

 polated between these angles. It appears, therefore, that 

 the transverse ligaments of the atlas and other vertebrae in 

 birds, and the ligamentum conjugale costarum, and trans- 

 verse 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 impor- 

 tant distinction ; for in the atlo-axoid articulation, we find 

 the arrangement in many mammals, as in the human sub- 

 ject, 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 foetus. The articular 

 surfaces of the oblique processes are situated immediately 

 behind the transverse processes, and 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 



* I have described and figured the ligament here referred to in a paper 

 " On the Structure, Actions, and Morphological Eelations of the Ligamentum 

 Conjugale Costarum/' in the Edinburgh New Philosophical Journal, April 

 1869. 



