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- 
tebree ; 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 vertebrge 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 vertebrae ; 
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 vertebrae. 
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 
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 readilj^ 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 
* lloulo, Handbucli der Syst. Anat. des Mensclien, i. p. 42. 
