DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 217 



reverse occurring during flexion. Both bones, moreover, while those movements are 

 taking place, revolve to a greater or less extent upon their own axes. The bones of the 

 forearm articulate at the wrist with the carpal bones, which being spirally arranged, and 

 placed obliquely between them and the metacarpal bones, transmit the motions to the 

 latter in a curved direction. The long axis of the wrist-joint is, as nearly as may be, at 

 right angles to that of the elbow-joint, and more or less parallel with that of t he shoulder. 

 The metacarpal or hand-bones, and the phalanges or finger-bones, are more or less fused 

 together, the better to support the great primary feathers, on the eiiieiencj of which 

 flight mainly depends. They are articulated to each other by double hinge-joints, the 

 long axes of which are nearly at right angles to each other. 



As a result of this disposition of the articular surfaces, the winu r may be ^hot out or 

 extended and retracted or flexed in nearly the same plane, the bones composing the 

 wing, particularly those of the forearm, rotating on their axes during either movement. 



This secondary notion, or the revolving of the component bones upon their o\\ n axes, 

 is of the greatest importance in the movements of the winurs, as it communicates to 

 the hand and forearm, and consequently to the primary and secondary leathers which 

 they bear, the precise angles necessary for flight ; it in fact ensures that the wing, and the 

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

 screwed into and down upon the wind in extension, and unscrewed or withdrawn from 

 it 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. As the degree of rotation 

 made by the bones of the forearm and hand during extension amounts, as nearly as pos- 

 sible, to a quarter of a turn of a spiral, it follows that in flexion t he wing presents a kni f< - 

 like edge to the wind (Plate XIV. fig. 29*/) ; whereas in extension {d'e'f) the curtain 

 of the wing (c b) is rotated in a downward direction, until its concave or under surfnee 

 makes an angle of 30°, or thereabouts, with the horizon*. The angle in question is given at 



fig. 32, Plate XIV., if a line be drawn from a toe and another from a to d, the line" <> 



being made to represent the horizon, and the line ad the angle which the wing makes 

 with it when fully extended. From this description it will be evident that by the mer< 

 rotation of the bones of the forearm and hand the maximum and minimum of resi st- 

 ance is infallibly secured f. 



* This is the reverse of what happens in the wing adapted for flying under water (see p. 212). 

 t The suhjoined notes of observations and experiments made by the author on the wing of the Crested Crane, with 

 a view to determine its movements, may prove interesting : 



The Movements peculiar to the Elbow-, Wrist-, and other Joints in the Wing of the Bird. 

 oulder.joint.~1he head of the humerus is convex and somewhat oval (not round), the long axis of the oval 



Sk 



being directed from above downwards, i. e. from the dorsal towards the ventral aspect of the bird. The humerus ran 

 therefore glide up and down in the facettes occurring on the articular ends of the coracoid and scapular bone, „th 



* * _.-■.. t i . il.. /i:~A„1 ^ n A ^f *kr» hiimoniu Rut tlw» 



But the 



great facility, nrneh in the same way that the head of the radius glides upon the distal end of the humerus 

 humerus has another motion ; it moves like a hinge from before backwards, and the reverse. The axis of the latter 

 movement is almost at right angles to that of the former. As, however, the .houlder-jmnt „ connected by long 

 ligaments to the body, and can be drawn away from it to the extent of one-eighth of an inch Of more, it follows that a 

 third and twisting movement can be performed, the twisting admitting of rotation to the extent of something like a 



2l2 



