Meeting Abstract
The unique design of the avian shoulder girdle is generally attributed to flight adaptations: the large sternal keel for expanded flight muscle attachment and elongate strut-like coracoids for resisting compression of those muscles. X-ray video studies have shown that the furcula spreads laterally during flapping flight. The ends of the furcula are firmly attached to the anterolaterally projecting coracoids which imply that any spreading of the furcula is due to movement of the coracosternal joints. In this study we ask, what limits the lateral extent of coracosternal mobility? The furcula is linked to the sternum and coracoids by a membranous system called the sternocoracoclavicular membrane (SCCM), in which two sheets of the membrane connect the furcular arms to the coracoids along their length and then converge to form a single sheet running from the rostral sternum to the ventral tip of the furcula. The SCCM in pigeons has a thickened band running parallel to the coracoids. We initially hypothesized that SCCM-furcular complex were primarily responsible for limiting lateral excursion of the coracosternal joint. To test this, we applied a lateral torque to the coracosternal joint of dissection preparations of pigeon carcasses in which various parts of the SCCM complex were cut and the measured difference of lateral movement was compared to that of the intact specimens. Surprisingly neither the furcula nor membrane appears to limit lateral coracosternal movement, but instead collateral sternocoracoid ligaments arising from the internal surface of the coracoid and inserting onto the left and right pila coracoidea limit lateral spreading.
