45© THE BIRDS xvi. 8-9 



bundle attached near to the last and producing the opposite effect of 

 abduction and medial rotation, lowering the hinder aspect of the wing. 



The deltoid muscle is divided into several parts and besides its main 

 abductor action on the wing also has slips inserted into the skin of the 

 anterior patagium, muscles known as the long and short tensors of 

 that membrane. There is also a tensor accessorius, running from the 

 surface of the biceps to the skin of the leading edge of the wing. 



The muscles within the arm itself serve to extend or fold the whole 

 wing and to alter the positions of the parts, especially by pronation and 

 supination during flight. Large triceps and smaller biceps muscles act 

 at the elbow. In the forearm there is a large extensor carpi radialis and 

 an extensor carpi ulnaris, serving to keep the wing extended at the 

 wrist. Flexor carpi ulnaris folds the wing. There are also two large 

 pronators, brevis and longus (brachio-cradialis), rotating the radius 

 medially and lifting the back of the wing. A system of digital flexors 

 and extensors, inserted into the distal phalanx of the main digit, keeps 

 the wing tip spread out or folds it. The position of individual feathers 

 is controlled by an elaborate system of tendons and muscles along the 

 back of the hand. The first digit is moved independently by abductor 

 and adductor pollicis muscles, controlling the position of the bastard 

 wing, which increases the angle of stall and thus allows slow flying 

 speeds in take-off and landing. 



9. Principles of bird flight 



A plane surface moved through the air in a direction inclined at an 

 angle to this plane is known as an aerofoil. The forces generated can 

 be resolved into a lift force acting upwards and a drag force tending to 

 stop the motion. On this fact depends the power of supporting weight 

 in the air that is possessed by birds and human heavier-than-air 

 machines. Both lift and drag forces are proportional to the square of 

 the speed, and the requirement for sustained flight in still air is that 

 the object shall have sufficient speed to generate a lift force equal 

 to its weight. 



The flow of air over the upper surface of the wing reduces the 

 pressure there and provides the main portion of the lift (Figs. 261-2). 

 By tilting the wing (increasing the 'angle of attack') the pressure on the 

 underside can be increased, but the air flow now tends not to follow 

 the upper surface but to become turbulent, especially at the hind edge, 

 destroying the lift (Figs. 261-2). When an aerofoil falls below this 

 critical speed it stalls; that is to say, drops suddenly, being no longer 

 supported. The smooth flow of air over the wing tends to be especially 



