THE AMERICAN SWAN. 299 



a hand, the former is very long, and flexible in various direc- 

 tions, reaching its greatest length in the Cygnus, or Swan 

 genus, and the latter is variously and most skillfully mod- 

 eled according to the habit of the bird, but always having 

 cutting edges of a horny substance. Flight, as the principal 

 characteristic of the class, is well anticipated by the great 

 extent and peculiar form of the breast-bone or sternum, to 

 which so many of the muscles of flight are attached, which 

 has its surface augmented by a broad keel, and of which the 

 ossification is more or less complete, according to the pow- 

 ers of flight possessed by the bird. Notwithstanding the 

 great pressure of the wings of the flying bird upon the 

 shoulders, these last are kept a proper distance apart by a 

 system of bones formed into a sort of double arch, well 

 braced forward and backward. The ribs again are remark- 

 ably strengthened by a line of flat, bony processes, extend- 

 ing from one to the other, like purlines joined into the raft- 

 ers of a building. All the bones are especially laminated 

 and firm, and at the same time contain, for the most part, 

 air cavities, to secure their greater levity. 



Nor is the muscular system any less remarkable in its 

 adaptation to the peculiar functions of the bird; some of the 

 muscles, extending from the trunk of the body to the tips 

 of the toes, being so arranged that the bird clings to its 

 perch without any voluntary effort during its unconscious 

 hours of sleep, and may thus support itself even on one 

 foot. " In birds of flight the mechanical disposition of the 

 muscular system is admirably adapted to the aerial locomo- 

 tion of this class; the principal masses being collected below 

 the center of gravity, beneath the sternum, beneath the pelvis, 

 and upon the thighs, they act like the ballast of a vessel, and 

 assist in maintaining the steadiness of the body during 

 flight; while at the same time the extremities require only 



