first digit of the manus (hand) on the wing (sometimes labeled the second digit, as homologous to the human index finger); also called the pollex or thumb; 4 alular feathers attach to it by ligaments and tendons, and sometimes are referred to collectively as the alula; the alula bone moves to lift and lower the alular feathers to help control air flow across the top of the wing in flight, especially to prevent a stall; photo © Idaho Virtual Museum