Homologies of the Mammalian Awis, ^c. 223 
a cup perforated by a small foramen, througli 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 vertebrse, 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 
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 vertebra, 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 
^re 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 Kelations of the Ligamentum 
Conjugale Costarum," in the Edinburgh New Philosophical Journal, April 
1869. 
