OCR Text |
Show 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 tbe 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 looking 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 the pars; and (2) in the flatness of the anterior terminal surface of its centrum, which in immature individuals bears the stamp of synchondrosis, and in aged individuals is often synostosed with the pars. The posterior terminal surface of the centrum is concave. In the level of the neurocentral 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 surface 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 last 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 epistropheus which, its own centrum being reduced to a rudiment, has coalesced with that of the epistropheus (16). This conception of the pars is untenable. |