460 ^MANUAL OF ZOOLOGY. 



they are distinct bones, and are not anchylosed with the scap- 

 ula. The coracoid bone on each side is always the strongest 

 of the bones forming the scapular arch. Superiorly it articu- 

 lates with the clavicle and scapula, and forms part of the gle- 

 noid cavity for the humerus. Inferiorly each coracoid bone 

 articulates with the upper angle of the sternum. The position 

 of the coracoids is more or less nearly vertical, so that they 

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

 ward stroke. The clavicles (fig 180, 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" ("four- 

 chette " of the French). 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 ster- 

 num. 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. The furculum is rudimentary 

 in the Ostrich, Emeu, and Cassowary. 



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 peculiar life of 

 the bird, it is in it that we find some of the most characteristic 

 points of structure in the whole skeleton. Though consider- 

 ably modified to suit its function as an organ of aerial progres- 

 sion, 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. 180, 

 A, and fig. 181). The upper arm (brachium) is supported by 

 a single bone, the humerus, which is short and strong, and 

 articulates above with the articular cavity formed partly by 

 the scapula and partly by the coracoid (fig. 18 1, h). The 

 humerus is succeeded distally by the fore-arm {antibrachium) 

 constituted by the normal two bones, the radius and ulna 

 (fig. 181, r, u), of which the radius is much the smaller and 

 more slender, and the ulna much the larger and stronger. 

 The ulna and radius are followed inferiorly by the bones of 

 the wrist or carpus ; but these are reduced in number to two 

 small bones, "so wedged in between the antibrachium and 

 metacarpus as to limit the motions of the hand to those of 

 abduction and adduction necessary for the folding up and ex- 



