430 



MANUAL OF ZOOLOGY. 



form fixed points for the action of the wings in their down- 

 ward stroke. The clavicles (fig. 168, A, c) are rarely rudimen- 

 tary or absent, and are in some few cases separate bones. In 

 the great majority, however, of birds, the clavicles are anchy- 

 losed together at their anterior extremities, so as to form a 

 single bone, somewhat V-shaped, popularly known as the 

 "merry-thought," and technically called the "furculum." The 

 outer extremities of the furculum articulate with the scapula 

 and coracoid ; and the anchylosed angle is commonly united 

 by ligament to the top of the sternum. The function of the 

 clavicular or furcular arch is " to oppose the forces which tend 

 to press the humeri inwards towards the mesial plane, during 

 the downward stroke of the wing" (Owen). Consequently 

 the clavicles are stronger, and their angle of union is more 

 open, in proportion to the powers of flight possessed by each 

 bird. 



We have next to consider the structure of the bones which 



compose the fore -limb or 

 " wing " of the bird ; and as 

 this organ is the one which 

 chiefly conditions the pecu- 

 liar life of the bird, it is in it 

 that we find some of the most 

 characteristic points of struc- 

 ture in the whole skeleton. 

 Though considerably modi- 

 fied to suit its function as an 

 organ of aerial progression, 

 the wing of the bird is readily 

 seen to be homologous with 

 the arm of a man or the fore- 

 limb of a Mammal (fig. 168, 

 A, and fig. 169). The upper 

 arm (brachium) is supported 

 by a single bone, the humerus, 

 which is short and strong, and 

 articulates above with the arti- 

 cular cavity formed partly by 

 the scapula and partly by the 

 coracoid (fig. 169, h). The 

 humerus is succeeded distally 

 by the fore-arm (antibrachiwn) 

 constituted by the normal two 

 bones, the radius and ulna (fig. 169, r, u\ of which the radius 

 is much the smaller and more slender, and the ulna much 



P 



Fig. 169. Fore-limb of the Jer-falcon. 

 k Humerus; r Radius; u Ulna; t 

 "Thumb;" -m Metacarpals, anchylosed 

 at their extremities; pp Phalanges of 

 fingers. 



