442 THE MECHANICAL FUNCTIONS. 



The mode in which the vertebrae are articu- 

 lated together differs widely from what we have 

 seen in fishes, and approaches to the structure of 

 the higher classes of vertebrata. The body of 

 each vertebra, instead of having at its posterior 

 surface a cup-like cavity, terminates by a pro- 

 jecting ball, which is received into the cavity in 

 the anterior surface of the next vertebra, so as 

 to compose a true ball and socket joint, capable, 

 when other circumstances permit, of a rotatory 

 motion. But the vertebrae of the tadpole, as we 

 have seen, are constructed on the model of those 

 of a fish; that is, have cup-like cavities on both 

 their surfaces, which play on balls of soft elastic 

 matter interposed between them. We should 

 naturally be curious to learn the mode in which 

 the transition from this structure to that of the 

 frog is accomplished. By carefully watching 

 the progress of ossification, while this change is 

 taking place, Dutrochet found that the gelatinous 

 ball, on which both the adjacent vertebra play 

 in the tadpole, becomes gradually more solid, 

 and is converted into cartilage. This cartilage 

 afterwards becomes united by its anterior surface 

 to the vertebra which is in front of it; and the 

 whole then becomes ossified, so as to compose 

 only one bone, its posterior surface remaining 

 distinct, and continuing to play within the cup- 

 like hollow of the vertebra which is behind it. 

 The cartilaginous coccygeal vertebrae of the tad- 



