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West Indian med. j ; 14(2): 129, June 1965.
Artigo em Inglês | MedCarib | ID: med-7337

RESUMO

Radii of the smaller toothed-whales are striking examples, developmentally and structurally, of accommodation to the requirements for secondary marine adaptation. This was studied, with cleared whole mounts and histological sections from the fetus and with radiographs of adult bones, in the beluga (Delphinapterus leucas) and pilot whales (Globicephala melaena). Radii are flattened, semistreamline in cross-section and, like the ulnae, articulate (synchondrosis) end-on with the humerus. Distal to the elbow, the flipper is amuscular. Radial development is characterized by a high ratio of transverse to longitudinal growth rates, by a slow, irregular process of endochondral ossification, by retention of endochondral spicules in lieu of a medullary cavity, and by a deep deposit of variably spongy periosteal bone. Ultimately, the radius has medial and lateral concentrations of bone a lesser central density mimetic of a mechanically advantageous box-beam. Throughout life, however, the history of the bone is revealed by proximal and distal endochondral cones and surrounding masses of periosteal bone, distinguishable in lateral radiographs. From this geometry, and from cross-sections, it is clear that the radius grows eccentrically, i.e., circumferential increments to epiphyseal plates, and to periosteal surface, are greatest on the posterior margin, intermediate along medial and lateral sides and least on the anterior margin. In the ulna, growth is in the opposite sense. Opposite margins of the elongating antibrachial bones thus move apart while adjacent margins hold relatively the same positions. Eccentric growth maintains the ratio of major to minor axes in the cross-section. The Arctic beluga radius is more dense, and radiographs reveal within it the outline of the neonatal radius. Beyond this outline, variable porosity suggests periodic growth (AU)


Assuntos
21003 , Rádio (Anatomia) , Desenvolvimento Ósseo , Baleias
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