MUSCULAR SYSTEM OF AVES. 97 



the clavicle, then turns downward and outward, and is inserted, 

 fleshy, above and in front of the great pectoral, into the upper ex- 

 tremity of the humeral crest. 



The interspace between the clavicle, coracoid, and scapula, 

 through which its tendon passes, serves as a pulley, by means of 

 which the direction of the force of the carneous fibres is changed, 

 and although these fibres ascend from below toward their inser- 

 tion, yet they forcibly raise the humerus, and thus a levator of the 

 wing is placed without inconvenience on the lower part of the 

 trunk, and the centre of gravity proportionally depressed. 



In the Penguins, Guillemots, and Gulls, this muscle is almost 

 the largest of the three, occupying the whole length of the ster- 

 num. It is remarkable for the length and strength of its tendon, 

 which is inserted so as to draw forwards the humerus with great 

 force. It is proportionally the smallest in the Raptores ; and is 

 very small and slender in the Struthious birds. 



We have already alluded to the use which the Penguin makes 

 of its diminutive anterior extremities as water-wings, or fins ; to 

 raise these after making the down-stroke obviously requires a 

 greater effort in water than a bird of flight makes in raising its 

 wings in air : hence the necessity for a stronger developement of 

 the second pectoral muscle in this and other diving birds, in all 

 of which the wings are the chief organs of locomotion, in that 

 action, and consequently require as powerful a developement of 

 the pectoral muscles as the generality of birds of flight. 



The third pectoral 7?iitscle, ^vhicli is in general the smallest of 

 the three, arises from the anterior part of the sternum at the angle 

 between the body and keel, and also by a more extended origin, 

 from the posterior moiety of the inferior surface of the coracoid 

 and the coraco-clavicular membrane ; it is directed forward, 

 rising, to pass through the scapulo-coracoid trochlea ; its tendon 

 glides through a sheath, attached to the capsule of the shoulder- 

 joint, and in some birds to the os humero-scapulare ; and is inserted 

 into the radial tuberosity of the humerus which it helps to raise. 



It is proportionally large in the Penguins and Gulls, but attains 

 its greatest developement in the Gallinaceous order. 



Above the preceding muscle there is another longer and more 

 slender one, analogous to the coraco-hrachialis, which arises 

 from the middle of the posterior surface of the coracoid ; its direc- 

 tion upward is less vertical than that of the third pectoral, along 

 the outer side of which it is attached to the anterior tuberosity 

 of the humerus. This muscle is wanting in the Struthw?iidce, is 

 of small size in the Heron and Goose, is much more developed m 



VOL. IT. PI 



