252 JIATURAL THEOLOGY. 



We must observe the very peculiar mechanism 

 of the bird's respiration. The breast-bone or ster- 

 nuniy A, runs the v^hole length of the animal's body, 

 and the great central spine of that bone called the 

 keel, rises from it, so as to give lodgement and at- 

 tachment to the great muscles of the wings. It 

 will be easily understood how naturalists distin- 

 guish birds of passage by the size of the keel, 

 (crista,) since its greater prominence implies 

 strength of wing for long-continued flight. Under 

 the breast-bone, and between it and the back-bone, 

 we perceive the space occupied by air-cells. The 

 lungs, B, lie behind ; and by the motion of the bone 

 A, like a great bellows, the air is drawn through 

 the lungs, and through the windpipe, c, into the 

 cells, D, E ; at once eflfectually oxygenating the 

 blood in the lungs, and renewing the air within 

 the recesses of the body. 



The next thing remarkable is in the vertebrae 

 of birds — for the back-bone is in its constitution 

 unlike that of man or quadrupeds. The back is 

 firm, and the caudal extremity loose and moveable. 

 The first is obviously intended to give a fixed ori- 

 gin to the muscles of the wing, and the second to 

 afford motion to the tail. It is by the wings they 

 raise or propel themselves, and by the tail they di- 

 rect their flight. We need hardly add that there 

 is a change in the centre of gravity compared with 

 that of animals ; in birds the centre is between 

 the wings. 



If our reader has followed us in these details, 



