On the archetype and homologies of the vertebrate skeleton / by Richard Owen.

  • Richard Owen
Date:
1848
Catalogue details

Licence: Public Domain Mark

Credit: On the archetype and homologies of the vertebrate skeleton / by Richard Owen. Source: Wellcome Collection.

Provider: This material has been provided by King’s College London. The original may be consulted at King’s College London.

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    chelonians, the ichthyic independence of the parapophyses (4, 4). In ba. chians the epencephalic arch is reduced to the two important elements, neurapophyses ; which meet and join each other below as well as above foramen magnum, and develope the exogenous zygapophyses, or two occi] condyles, for articulation with the corresponding processes of the neural i of the atlas. The basioccipital, if it exists in batrachians, is rudimental confluent with the basisphenoid, and the supraoccipital is in like mai recognisable only as the posterior border of the backwardly produced pari; The parapophyses are short exogenous processes of the neurapophyses ofi much simplified epencephalic arch in all batrachian reptiles. . The chief modification that distinguishes the above-described segmep the crocodile’s skull from its homologue in the fish, is the absence ot attached inverted or haemal arch. We recognise, indeed, the special ho logues of the piscine constituents of that arch in so, si and 52, fig. 22. upper suprascapular piece (so) is however free, disconnected from any , ment, and retains, in connection with the loss of its proximal or era articulations, its cartilaginous state : the scapula (si) is ossified, as is like the coracoid (32), the lower end of which is separated from its fellow by. interposition of a median, symmetrical, partially ossified piece called ‘epit it num ' (/ts). The power of recognising the special homologies of so, si, 32 in the crocodile, with the simil.arly numbered constituents of the archi in fishes (fig. 5), though masked not only by modifications of form and ] portion but even of very substance, as in the case of so, depends upon circumstance of these bones constituting the same essential element of archetypal skeleton : for although in the present instance there is superad to the adaptive modifications above-cited the rarer one of altered connect!' Cuvier does not hesitate to give the same names (suprascapulaire) tr and (scapulaire) to si, in both fish and crocodile : but he did notperceiv. admit that the narrower relations of special homology were a result of, necessarily Included in, the wider law of general homology. According the view of this law here taken, we discern in so and si, fig. 22, a teleologici compound pleurapophysis, in 52 a hcemapophysis, and in hs the hoei spine, completing the haemal arch. The general relations of the scapulo-coracoid arch to a haemal or cot one have been long recognised, but the vertebral segment to which it apj tains seems not hitherto to have been suspected, and has certainly not b satisfactorily determined. Oken, who had observed the free cervical ribi a specimen of the Lacerta apoda, Pallas (Pseudopus), deemed them rej sentatives of the scapula, and this bone to be, in other animals, the coales- homologues of the cervical pleurapophyses*. In no animal are the conditi for testing this question so favourable and obvious as in the crocodile: only do cervical ribs coexist with the scapulo-coracoid arch, but they arc unusual length and are developed from the atlas as well as from each s ceeding cervical vertebra: we can also trace them beyond the thorax to ' sacrum, and throughout a great part of the caudal region, as the sutures the apparently long transverse processes of the coccygeal vertebrae denn strate in the young animal; the lumbar pleurapophyses being manifes at the same period as cartilaginous appendages to the ends of the long c pophyses. * “ Auch die Scapula nicht ein Knochen, sondern wenigstens eine aus fiinf Halsrip zusammengeflossene Platte ist.”—Programm, &c., 4to, 1807, p. 16. He reproduces same idea of the general homology of the scapula in the ‘ Lehrhuch der Natur-philosoph 1843, p. 331, ^ 2381. Cams also regards the scapulo-coracoid arch as the reunion of sfi ral (at least three) protovertehral arches of the trunk-segments. ' Urtheilen des Kuoc und Schalen gerustes, fol. nxmi.