322 STERNAL MUSCLES. 



which is formed by the uniting of the coracoid, the 

 clavicle, and the scapular, and returning outwards, is 

 attached, bv an extended termination, on the upper 

 side of the crest of the humerus, rather inward of 

 the centre of motion of the shoulder-joint. This is 

 the principal muscle employed in raising the wing ; 

 and the mode of its action is a little curious. Its 

 insertion is nearly the same as that of its antagonist, 

 the great pectoral muscle ; and when it acts, it tends 

 to raise and keep steady the centre of motion in the 

 shoulder-joint, just as that does ; but as its tendon 

 passes through the pulley, or foramen, above-men- 

 tioned, its power to produce motion is reversed, and 

 though similarly inserted, and as a muscle acting in 

 the very same way, it produces exactly the opposite 

 effect upon the humerus. The tendon which ]>,:--'- 

 through the pulley is such that, besides having 

 additional firmness from its structure, the upper 

 part of it acts as a more powerful lever in the raising 

 of the wing. 



The action of these two muscles, which are the 

 most important ones in the operation of flying, is one 

 of the most beautiful in the whole range of animal 

 mechanics, wide and varied as it is. The great pec- 

 toral which, by depressing the wing, gives the stroke, 

 and therefore performs the most essential operation 

 in flying, is the most free in its action, as it is pressed 

 by the integuments only. It also pulls the wing 

 downwards with the greatest advantage, as its tendon 

 goes directly to the lower part of the humerus with- 

 out the joint. Its antagonist, which raises the wing, 

 does it by the reversed action of the tendon passing 

 through the pulley ; but it acts more rapidly, and it 

 has the same tendency to support the centre of mo- 

 tion as the other : and though there must be some 



