DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 323 



the one hand, and the resistance and resiliency of the air on the other. . . 

 The neurse or nervures in the insect's wing are arranged at the axis or root of 

 the pinion, after the manner of a fan or spiral stair ; the anterior one occupy- 

 ing a higher position than that farther back, and so of the others. As this 

 arrangement extends also to the margins, the wings are more or less twisted 

 upon themselves, and present a certain degree of convexity on their superior 

 or upper surface, and a corresponding concavity on their inferior or under 

 surface ; their free edges supplying those fine curves which act with such 

 efficacy upon the air, in obtaining the maximum of resistance and the minimum 

 of displacement ; or what is the same thing, the maximum of support with the 

 minimum of slip. ..... All wings obtain their leverage by presenting 



oblique surfaces to the air, the degree of obliquity gradually increasing in a 

 direction from behind forwards and downwards during extension, when the 

 sudden or effective stroke is being given, and gradually decreasing in an oppo- 

 site direction during flexion, or when the wing is being more slowly recovered 

 preparatory to making a second stroke. The effective stroke in insects, and 

 this holds true also of birds, is therefore delivered downwards and forwards, 

 and not as the majority of writers believe, vertically, or even slightly backwards. 

 . . . . To confer on the wing the multiplicity of movement which it re- 

 quires, it is supplied at its root with a double hinge or compound joint, which 

 enables it to move not only in an upward, downward, forward, and backward 

 direction, but also at various intermediate degrees of obliquity. . . . The wing 



of the bird, like that of the insect, is concavo-convex, and more or less twisted 

 upon itself. The twisting is in a great measure owing to the manner in which the 

 bones of the wing are twisted upon themselves, and the spiral nature of their 

 articular surfaces, the long axes of the joints always intersecting each other 

 at nearly right angles. As a result of this disposition of the articular surfaces, 

 the wing may be shot out or extended, and retracted or flexed in nearly 

 the same plane, the bones of the wing rotating in the direction of their 

 length during either movement. This secondary action, or the revolving of the 

 component bones upon their own axes, is of the greatest importance in the 

 movements of the wing, as it communicates to the hand and forearm, and con- 

 sequently to the primary and secondary feathers which they bear, the precise 

 angles necessary for flight. It, in fact, insures that the wing, and the curtain 

 or fringe of the wing, which the primary and secondary feathers form, shall be 

 screwed into and down upon the wind in extension, and unscrewed or with- 

 drawn from the wind during flexion. The wing of the bird may therefore be 

 compared to a huge gimlet or auger, the axis of the gimlet representing the 

 bones of the wing ; the flanges or spiral thread of the gimlet the primary and 

 secondary feathers." 



The lecture referred to formed part of a memoir which was communi- 



