218 Prof. Owen on the Atlas, Axis, and Subvertebral 



occipital or centrum of the occipital vertebra ; t n its internal or 

 medullary structure ; e x its cortical compact portion : the arrest 

 of ossification in the posterior part of its medullary portion has left 

 the deep concavity turned towards the atlas, and which was filled 

 by the liquified remains of the gelatinous part of the notochord ; 

 but continuous ossification in the notochordal capsule has anchy- 

 losed the cortical part of the occipital centrum (ex) with that (ca,ex) 

 of the centrum of the atlas, c a is the ossified medullary part of 

 the centrum of the atlas ; n o is the neural arch of the occipital 

 vertebra ; n a the neural arch of the atlas, separated from its 

 centrum ; c x, central part of the body of the axis ; ex, ex, cortical 

 part of ditto ; n x, neural arch of the axis perforated by the 

 motory and sensory roots of the nerves separately ; c 3, central 

 part of body of third vertebra ; c 3, e x, cortical part of ditto ; 

 n 3, neural arch ; c 4, central part of body of fourth vertebra ; 

 c 4, ex, cortical part of ditto ; n 4, neural arch of ditto ; c 5, cen- 

 tral part of body of fifth vertebra ; c 5, ex, cortical part of ditto ; 

 n 5, neural arch of ditto. Here the vertebrae begin to exchange 

 their elongated figure for the ordinary short one, which is exem- 

 plified in c 6, where they begin to be free. 



In the fish-like batrachians, the Menopome for example, the 

 body and neural arch of the atlas have coalesced : the anterior 

 zygapophyses descend from the fore-part of the neural arch upon 

 the sides of the fore-part of the centrum which projects forwards 

 between them, like an odontoid process : the articular surfaces of 

 the zygapophyses (oblique or articular processes of human ana- 

 tomy) are subcircular, slightly concave, directed forwards and a 

 little upwards : they receive the convex zygapophyses or condyles 

 of the coalesced exoccipitals. The posterior zygapophyses of the 

 atlas have also large subcircular articulations directed downwards. 

 The body of the atlas appears to have been developed at the ex- 

 pense of the central part of the notochord, which forms the an- 

 terior convex part which articulates with the basi-occipital plate 

 at the bottom of the foramen magnum. A deep concavity at the 

 back part of the atlas contains the unossified remnant of the 

 central or gelatinous part of the notochord. 



In the extent, however, to which the centrum of the atlas is 

 ossified, the Menopome and other perennibranchians resemble 

 the fishes. If, indeed, the persistent portion of the notochord 

 which fills the anterior concavity of the atlas and the posterior 

 concavity of the basi-occipital in the Siluroid or other osseous fish 

 were ossified and anchylosed to the atlas, that vertebra would 

 closely resemble the atlas of the Menopome, and I regard the 

 singular modification of form which the atlas of the Menopome 

 presents, as compared with that of the osseous fish, to be due to 



