Humeral elevator
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Suboptimal design > Humeral elevator
Essentially, the avian pectoral apparatus is wired backwards from that seen in the only other flying vertebrates, bats. Logically, the dorsal elevators such as the deltoids ought to be the principal muscles affecting elevation of the humerus, and indeed this is what we see in bats. In birds, however, a ventral muscle, M. supracoracoideus, affects elevation of the wing via a deflected tendon of insertion. Thus a muscle which initially functioned as a humeral depressor, has undergone a complete reversal in function. Why didn't our alleged "master designer" just wire things right from the outset, with the dorsal musculature elevating the wing? Why the convoluted (literally) system whereby a ventral muscle elevates the wing through a looped tendon of insertion?
