420 MR. J. "W. HULKE ON THE SKELETAL [NoV. 20, 



surface of the pars are truncated by the abutments of the most 

 anterior part of the neurapophysis of the epistropheus. Upon the 

 anterior or cranial surface of the pars are discernible: — (1) a smooth 

 upper tract of rhomboidal outline, of which the upper angle is 

 truncated by the neural canal ; this area contributes the upper, 

 central, and deepest part of the occipital condylar cup ; and (2) 

 a lower, larger tract lookinj; downwards and forwards, stamped by 

 wrinkling denoting synchondrosial union with the " basilar piece." 



Epistropheus (Axis). — This bone (Plate XVIII. fig. 1) differs 

 from all the vertebrae posterior to it, (1) in the great antero-posterior 

 extent of its spinous process and of its neurapophysis, which latter, 

 prolonged in advance of its proper centrum, abuts slightly upon tlie 

 pars ; and (2) in the flatness of the anterior terminal surface of its 

 centrum, which in immature individuals bears the stamp of synchon- 

 drosis, and in aged individuals is often synostosed with the pars. 

 The posterior terminal surface of the centrum is concave. In the 

 level of the neuroceutral suture, not quite equidistant from the two 

 ends of the centrum, but rather nearer to the cranial, is a stout, 

 upper, downward slanting, transverse process (diapophysis) ; its cross 

 section is oval in outline, the major axis horizontal ; and at the lower, 

 anterior angle of the lateral surface, where this joins the under sur- 

 face of the centrum, is an inconspicuous facet (parapophysis) for the 

 capitulum costce. Below the neural suture the middle of the 

 centrum is compressed, and its sides here inclining inwards meet 

 ventrally in a narrow edge or keel. 



The morphology of some of the component parts of the atlas has 

 been much discussed, nor have the la'st words been spoken. The 

 correspondence of the pars odontoidea to the odontoid process of 

 the epistropheus in higher Vertebrates was recognized by Cuvier 

 (15). If the body of a vertebra be defined as that part of it which 

 is traversed by the notochord, then, beyond doubt, embryology 

 demonstrates that the pars odontoidea is a vertebral centrum, and 

 also that it belongs to the atlas, since in an early embryonic stage 

 the notochord may be seen piercing it, and it evidently, together 

 with the pair of "lateral pieces" and the basilar piece, forms one 

 undifferentiated "continuum." These views of the morphology of 

 the pars have been held by nearly all writers. E. Deslongchamps 

 alone, I think, regarded the pars as representing the centrum, not 

 of the atlas, but of a vertebra once ancestrally present between the 

 atlas and the epistropheus, but now reduced to a rudiment. He 

 appears to have been led to form this opinion by the occurrence of 

 a notch m the free border of the spinous process of the epistropheus, 

 and by the great antero-posterior extent of the neurapophysis of 

 this latter. These facts appeared to him to hint that the neural 

 arch of the epistropheus comprises two parts originally distinct, — 

 one posterior, the proper arch of the epistropheus, the other anterior, 

 the neural arch of a vertebra immediately anterior to the epistro- 

 pheus which, its own centrum being reduced to a rudiment, has 

 coalesced with that of the epistropheus (16). This conception of 

 the pars is untenable. 



