. The biology of birds. Birds. ADAPTATIONS OF THE SKELETON 59 The salient features of the skeleton of the bird's leg are the elongation of the different regions, the more or less vertical position of the instep region, and the reduction of the number of separate bones. The thigh-bone has a conspicuously defined round head, working deeply and freely in the acetabulum ; the fibula is reduced to an incomplete splint, which may be partially fused to the tibia ; the upper or proximal row of ankle-bones or tarsals has coalesced in embryonic life with the lower end of the tibia, forming a tibio-tarsu
. The biology of birds. Birds. ADAPTATIONS OF THE SKELETON 59 The salient features of the skeleton of the bird's leg are the elongation of the different regions, the more or less vertical position of the instep region, and the reduction of the number of separate bones. The thigh-bone has a conspicuously defined round head, working deeply and freely in the acetabulum ; the fibula is reduced to an incomplete splint, which may be partially fused to the tibia ; the upper or proximal row of ankle-bones or tarsals has coalesced in embryonic life with the lower end of the tibia, forming a tibio-tarsus, which is peculiar to birds ; the lower or distal row has coalesced with the upper end of three (in the ostrich, two) fused metatarsals, forming a tarso - metatarsus, which is also peculiar to birds. Thus the adult shows no free tarsal bones, but they are clearly seen in the unhatched bird ; and in a true, not merely courtesy, chicken on the table it is easy to separate an im- perfectly ossified piece from the lower end of the tibio- tarsus and the upper end of the tarso-metatarsus. These separable pieces correspond to the two rows of tarsal bones. When a bird moves its ankle- joint it is really moving the upper tarsals against the lower tarsals, the interest of which is that it corresponds to the reptilian inter-tarsal movement. This inter-tarsal ankle-joint of reptiles and birds is in. Fig. II.—Fowl's hind-leg. From a specimen. F., femur ; p., pa- tella or knee-pan ; , cnemial crest on the tibio-tarsus () ; the head of the fibula ; , lower end of the tapering incomplete fibula; , the ankle joint; , tarsometa- tarsus ; , the first meta- tarsal, quite free; , the Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Thomson, J. Arthur (John Arthur), 1861-1933. New Yo
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectbirds, bookyear1923
