390 DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 



formed in the air and part in the water. Those movements, so numerous, varied, 

 and beautiful, are all the result of volition. It is impossible to resist this con- 

 clusion after deliberate and careful watching. 



A Regulating Power necessary in Flight. — That the wing is propelled for the 

 most part by voluntary movements, may be ascertained in the following manner. 



If the sentient nerve of a pigeon's wing be divided (the motor nerve being 

 left intact) the bird nutters most energetically, but altogether fails to fly.* In 

 this experiment neither the flexibility, elasticity, nor the power which the wing 

 possesses of moving in all its parts, are tampered with. The guiding or con- 

 trolling power alone is impaired. 



That the wing is vibrated intelligently admits of direct proof. Thus if we 

 hold a captured bird in the hand, we feel that it directs and controls the action 

 of its wings in such a manner that a tractile force is produced, now in one 

 direction now in another, in its efforts to escape ; nay more, that the force after 

 a brief fluttering is concentrated at that point where it is most loosely held, and 

 which offers the greatest chance of escape. 



Second, The wings of birds, as any one may readily ascertain by watching 

 the flight of rooks, are visibly under control both during the down and up 

 strokes. They are, moreover, deliberate leisurely movements. By leisurely 

 movements, I mean such as are the result of design, and not such as would 

 be produced by the sudden recoil of a merely elastic apparatus. Those who have 

 watched, as I have frequently done, the rapid vibrations of natural and artificial 

 wings, will readily understand the difference here indicated. In the living wing 

 we have a smooth soft fanning continuous movement, quite devoid of dead 

 points ; whereas in artificial elastic wings, especially if worked vertically and 

 without elastic bands at their roots, we have a wavering, jerking, irregular 

 motion, particularly at the beginning of the up stroke. 



Third, The blow-fly, as stated (p. 326), can fly with only one-third of its 

 original wing area, the two-thirds which represent the more highly elastic 

 portions of the wing being removed. In this case the wing is wielded intelli- 

 gently figure of 8 fashion, the mutilation not interfering either with the freedom 

 of motion enjoyed by the pinion at its root, or the power the insect possesses 

 of directing and controlling the wing throughout its entire vibration. 



There are therefore at least five separate items to be considered in flight, 

 viz., intelligence and voluntary movements ; secondly, mobility or the power 

 which the wing possesses of moving its several parts ; thirdly, the flexibility and 

 elasticity of the wing ; fourthly, the resistance and resiliency of the air upon 

 which the wing operates ; fifthly, the weight of the body of the flying animal, 

 which may be regarded as an independent moving power. 



* " Experiments practically demonstrating the laws by which birds fly," by DrW. Smyth. Second 

 Annual Report of the Aeronautical Society of Great Britain for 1867. 



