152 ORIGINAL ARTICLES. 



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 articu- 

 lates with the body of the axis. As regards the occipital condyle, its 

 constitution will be best understood by looking at the quite similar con- 

 dyle 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 vertebrae. 



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 rea- 

 dily appear, on making a more careful examination of the anterior arti- 

 cular 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 liga- 

 ment passes from the tip of the odontoid process to the occipital con- 

 dyle, 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 at- 

 tached to the superior angles of their anterior aspects — those angles 

 which are derived from the arches.* ]STow, 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 con- 

 jugalia 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 vertebrae, they 

 are attached to structures interpolated between these angles. It appears, 

 therefore, that the transverse ligaments of the atlas and other vertebrae 

 in birds, and the ligamentum conjugale costarum, and transverse liga- 

 ment 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 im- 

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

 rangement 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 ex- 

 tends 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 



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

 Structure, Actions, and Morphological Relations of the Ligamentum Conjugale Costa- 

 rum," in the Edinburgh New Philosophical Journal, April, 1 859. 



