92 VERTEBRATED ANIMALS. 



what manner these changes with respect to the centre of 

 gravity are affected, but also how nearly the bones of 

 birds correspond with our own. The neck of this bird is 

 composed of twenty-three bones, most of them so articu- 

 lated together as to allow of free motion in all directions. 

 By extending this part in a straight line, the bird while 

 flying, moves the centre of gravity so as to bring it to 

 some point between the insertions of the wings, whereas, 

 while the swan is floating on the water, or resting on the 

 feet, the neck is thrown backward and curved into the 

 form of the letter S, by which the equilibrium of the whole 

 system is preserved by throwing the centre of gravity be- 

 tween the feet. On the same principle all other birds are 

 enabled to preserve their equilibria in any position they 

 choose to take. 



262. Comparison between the Bones of Men and Birds. 

 We have already shown that there is a general simili- 

 tude in the skeletons of all the vertebrated animals, and 

 especially in their spines. At first view it would hardly 

 be thought that there could be much similarity between 

 the bones of a bird, and those of a man, and yet on a closer 

 examination, we shall find that the general principles of 

 structure are the very same, and not only so, that some of 

 the individual bones approximate each other in form. 

 Thus the humerus, of which a, Fig. 70, is the head, has 

 a general form like 4hat of our own species. It is flat- 

 tened in the same manner at the lower extremity for 

 the articulation of the two bones, the radius and ulna. 

 The two latter bones, 6, with which the humerus forming 

 the elbow-joint, are also the same in number, and some- 

 what similar in shape to those forming the corresponding 

 part of the human skeleton. The carpus, or wrist, c, con- 

 sists of only two bones, the one articulated with the 

 radius, the other with the ulna. These move together as 

 one piece. The metacarpus, or hand, d, also consists of 

 two bones, but these are united so closely as to form 

 only one in effect and use. Below these, at e, there is a 

 little projecting bone which may be considered as a rudi- 

 mental thumb. 



263. The prehensile organ in birds being the bill, and 



