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Furcula
The ラテン語:furcula ("''little fork''" in Latin) or wishbone is a forked bone found in birds and some other animals, and is formed by the fusion of the two clavicles. In birds, its primary function is in the strengthening of the thoracic skeleton to withstand the rigors of flight.
==In birds==
The furcula works as a strut between a bird's shoulders, and articulates to each of the bird's scapulae. In conjunction with the coracoid and the scapula, it forms a unique structure called the triosseal canal, which houses a strong tendon that connects the supracoracoideus muscles to the humerus. This system is responsible for lifting the wings during the recovery stroke.〔
As the thorax is compressed by the flight muscles during downstroke, the upper ends of the furcula spread apart, expanding by as much as 50% of its resting width, and then contracts. X-ray films of starlings in flight have shown that in addition to strengthening the thorax, the furcula acts like a spring in the pectoral girdle during flight. It expands when the wings are pulled downward and snaps back as they are raised. Acting like a spring, the furcula is able to store some of the energy generated by contraction in the breast muscles, expanding the shoulders laterally, and then releasing the energy during upstroke as the furcula snaps back to the normal position. This, in turn, draws the shoulders toward the midline of the body. While the starling has a moderately large and strong furcula for a bird of its size, there are many species where the furcula is completely absent, for instance Australian scrubbirds and bush birds, some toucans and South American barbets, some owls, some parrots, turacos from Africa and mesites of Madagascar. These birds are still fully capable of flying. They also have close relatives where the furcula is vestigal, reduced to a thin strap of ossified ligament, seemingly purposeless. Other species have evolved the furcula in the opposite direction, where it has increased in size and become too stiff or massive to act as a spring. In strong flyers like cranes and falcones, the arms of the furcula are large, hollow and quite rigid.〔(The Inner Bird: Anatomy and Evolution, by Gary W. Kaiser )〕
In birds, the furcula also may aid in respiration by helping to pump air through the air sacs.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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