222 Proceedings of the Royal Physical Society. 



circumstances which have led to this comparison being 

 made, are merely the rapid diminution in size of the inter- 

 vertebral discs from the thoracic region up to the axis, and 

 a general similarity of appearance between the articular 

 surfaces of the atlas and axis and those of succeeding ver- 

 tebras ; and though the impropriety of this comparison has 

 been exposed in very explicit terms by Professor Henle,* 

 there is still room for a few remarks as to the precise parts of 

 other vertebras to which the surfaces in question correspond. 



In order to arrive at a just conclusion upon this subject, 

 we shall find it advantageous to examine the atlas in the 

 bird. In it we find on the posterior aspect a pair of true 

 oblique processes passing backwards, to articulate above the 

 intervertebral foramina with a corresponding pair of pro- 

 cesses of the axis, similar to those of succeeding vertebras ; 

 while inferiorly there is a cartilaginous surface which forms, 

 with the body of the axis and its odontoid process, a joint 

 similar to those between the succeeding bodies of vertebras. 

 On the anterior aspect of the 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 occi- 

 pital 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 

 vertebras. 



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 care- 

 ful examination of the anterior articular surface of the atlas 

 of the bird in the recent condition. It presents the form of 



* Henle, Handbuch der Syst. Anat. des Menschen, i. p. 42. 



