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Show 260 MR. ST. GEORGE MIVART ON THE [Apf- 23, 7. On the Axial Skeleton of the Urodela. By ST. GEORGE MIVART, F.R.S. In this communication I shall confine my observations to the spinal column, neglecting for the present the skull, together with the mandibular, hyoidean, and branchial arches*. * In a communication read before tbe Linnean Society on the 21st of April of the present year, I stated m y views as to the general and serial homologies of the vertebrate skeleton. I said that the axial system in its most developed form might be considered as consisting of three longitudinal series of parts, continued for a greater or less extent along each side of the vertebral axis. The upper longitudinal series of parts on each side together form the system of epaxial parts. The middle longitudinal series of parts on each side together form the system of paraxial parts. The inferior longitudinal series of parts on each side together form the system of hypaxial parts. Epaxial parts were defined as "parts embracing the cerebro-spinal axis, or parts serially homologous with parts which embrace that axis." Paraxial parts were defined as " parts external to and more or less tending to embrace the pleuro-peritoneal cavity, or parts serially homologous with parts which are so situated in the trunk." Hypaxial parts were defined as "parts placed between the skeletal axis and some part of the pleuro-peritoneal cavity, or serially homologous with parts so situated. By epaxial parts I denote the neural arches and lateral walls of the brain-case. By paraxial parts I denote transverse processes, ribs (both upper and lower of fishes), and sternal bones or cartilages. By hypaxial parts I denote both hypapophyses, whether exogenous or autogenous, and also visceral arches, however complex. By "visceral arches" I mean the system of arches forming the hyobranchial apparatus, and also the mandible, the palato-quadrate arch, and the trabecula-* cranii. In this latter respect I follow the happy and original suggestion of Professor Huxley, as far as regards the resemblance of the trabecule cranii to the other visceral arches; but I venture to differ from him as far as regards the general homology of these visceral arches, which I regard not as ribs (i. e. paraxial parts), but as hypaxial elements of the skeleton. The position of the heart and aortic roots, with regard to the branchial arches, is, 1 submit, fatal to their costal character. The external branchial cartilages of Sharks, and the branchial basket of the Lamprey, however, m a y really represent costal elements. In the paper referred to, I gave m y reasons for considering the subcaudal arches of fishes to be generally of more or less mixed paraxial and hypaxial nature, differentiation, in this respect, being, I believe, less complete in that class. This radiating system of skeletal parts of the trunk corresponds to the radiating system of spinal nerves, first pointed out by Professor Huxley in his last course of Hunterian Lectures, the epaxial parts corresponding with the ascending nerves, the hypaxial parts with the nerves of the inner ventral laminse of the embryo (i. e. with the sympathetic), and the paraxial parts corresponding with the nerves of the outer ventral lamina?. Moreover, as there are nerves passing directly outwards (above the abdominal nerves), so it was suggested that there may be an upper and lower series of paraxial parts, perhaps coalescing to form the ribs of the higher vertebrata. If this view of the visceral arches be accepted, then the nerves accompanving |
