OCR Text |
Show 1870.] AXIAL SKELETON OF THE URODELA. 263 category of paraxial parts. Were they really haemal arches (i. e. of the membranous connexions between the distal ends of the two series of ribs), we should have to adopt the somewhat violent conception that the lower parts of the haemal arches of the tail are detached from their proximal portions, and fixed directly to the under surface of each supporting centrum. For if the caudal vertebrae from before backwards are examined, it will be seen that the first hypapophysial arch is not formed by the bending down of the transverse processes, or by an extension of ossification along the membrane, connecting the distal ends of such transverse processes, but that it arises suddenly beneath the centrum. This is well shown in the sixty-fourth vertebra of Amphiuma (fig. 1). In this vertebra Under view of last trunk- and anterior caudal vertebra? oi Amphiuma (No. 5 7 0 A in College of Surgeons' Museum*). Hy. Hypapophysis. the caudal hypapophysis first makes its appearance as a long process on each side, the two processes being medianly disunited below, and distinct from the transverse process, which exists as completely in this vertebra as in the one preceding it. The three systems of parts, epaxial, paraxial, and hypaxial, are variously united together respectively. The epaxial parts, or neural arches, are serially connected together by means of special articular processes, zygapophyses. Each vertebra is provided with two of these in front and two behind, the posterior pair of which have their articular surfaces directed downwards, and somewhat obliquely outwards (fig. 16), reposing on the upturned and somewhat inwardly directed surfaces of the anterior zygapophyses of the vertebra next behind. The cervical vertebra forms an exception, inasmuch (fig. 19) as it has no anterior zygapophyses, but has special modifications for articulation with the skull. The paraxial parts are never serially connected together, but by muscular or connective tissue, except the occasional coalescence of such parts in the sacral region. The hypaxial parts in the tail form with each other serial connexions more or less simulating those existing between the successive neural arches. * The figures of specimens in the Museum of the College of Surgeons have been drawn by the kind permission of the authorities of that institution. |