96 VERTEBRATED ANIMALS. 



manner these changes with respect to the centre of 

 gravity are effected, 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 

 articulated together as to allow of free motion in all di- 

 rections. 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 backwards and 

 curved into the form of the letter S, by which the equi- 

 librium of the whole system is preserved by throwing 

 the centre of gravity between the feet. On the same 

 principle all other Birds are enabled to preserve their 

 equilibria in any position they choose to take. 



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 that of our own species. 

 It is flattened in the same manner at the lower extremity 

 for the articulation of the two tones, the radius and ulna. 

 The two latter bones, b, with which the humerus form- 

 ing the elbow joint, are also the same in number, and 

 somewhat similar in shape to those forming the corres- 

 ponding part of the human skeleton. The carpus, or 

 wrist, c, consists 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 rudimental thumb. 



The prehensive organ in Birds being the bill, and as 



