BIRDS. 247 



At its anterior angles the sternum exhibits two pits for the 

 attachment of the coracoid bones. 



The scapular or pectoral arch consists of the shoulder-blade 

 or scapula, the collar-bone or clavicle, and the coracoid bone, 

 on each side. The scapula, as a rule (fig. 582, A, s, s) is a 

 simple elongated bone, not flattened out into a broad plate, 

 and carrying no transverse ridge, or spinous process. Only 

 a portion of the glenoid cavity for 

 the articulation with the head of 

 the humerus is formed by the scap- 

 ula, the remainder being formed by 

 the coracoid. The coracoid bones 

 (fig. 582, A, Jc, k) correspond with 

 the coracoid processes of man, but in 

 birds they are distinct bones, and 

 are not anchylosed with the scap- 

 ula. The coracoid bone on each 

 side is always the strongest of the , .^js- sss -sternum of the os- 



/ " tnch. s, Scapula ; c, Coracoid. 



bones forming the scapular arch. 



Superiorly it articulates with the clavicle and scapula, and 

 forms part of the glenoid 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 downward stroke. The clavicles (fig. 582, A, c) 

 are rarely rudimentary or absent, and are in some few cases 

 separate bones. In the great majority, however, of birds, the 

 clavicles are anchylosed together at their anterior extremi- 

 ties, so as to form a single bone, somewhat V-shaped, popu- 

 larly 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 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. 



