324 STERNAL MUSCLES. 



diate between the two, and partaking of the properties 

 of both. The average line, or axis, of its motion is 

 oblique, elevated to the front, and depressed to the 

 rear, but both the principal head of the humerus and 

 the socket have their outlines curved, so that the 

 wing may be advanced or drawn backward at the 

 same time that it is elevated or depressed, and by 

 some of the muscles acting more and some less, all 

 the motions of the shoulder-joint, which are requisite 

 both for the action of flight and the direction of that 

 action, are performed. The double head of the 

 humerus, which may be understood by looking back 

 to the wing of the jer-falcon, or forward to the sternal 

 portion of the skeleton of the golden eagle, assists 

 very materially in both the direct and the oblique 

 motions oi this joint. In proportion as the action of 

 the wing is more powerful, the crest of the humerus, 

 that is, the process or portion of the head which is 

 toward the convex side of the bone, or that which is 

 most in advance when the bone is in its natural 

 position, is more produced. It is more so in the 

 humerus of the jer-falcon than perhaps in that of any 

 other bird, and more so in the falcons which prey on 

 the wing than in any other genus of birds. 



The second set of muscles in the wing are those 

 which are inserted on the coracoid, and attached to 

 the humerus and the radius. There are two of them 

 attached by a common tendon to the external part of 

 the extremity of the coracoid, near the glenoid cavity, 

 but below it. The coraco-brachial is attached to 

 the crest of the humerus, and acts as the antagonist 

 of the third pectoral, and extends the hinuerus from 

 the body. The biceps is attached to the radius, and 

 extends the fore-arm or great medial portion of the 

 wing. 



