464 THE MECHANICAL FUNCTIONS. 



which forms these strong bucklers, both above 

 and below, is a real osseous structure, developed 

 in the same manner as" other bones, subject to 

 all the changes, and having all the properties 

 of these structures. The great purpose which 

 nature seems to have had in view in the forma- 

 tion of the Chelonia is security ; and for the 

 attainment of this object she has constructed a 

 vaulted and impenetrable roof, capable of re- 

 sisting enormous pressures from without, and 

 proof against any ordinary measures of assault. 

 It is to the animal a strong castle, into which 

 he can retire on the least alarm, and defy the 

 efforts of his enemies to dislodge or annoy him. 



These considerations supply us with a key to 

 many of those apparent anomalies, which cannot 

 fail to strike us in viewing the dispositions of 

 the parts of the skeleton (Fig. 213), and the re- 

 markable inversion they appear to have under- 

 gone, when compared with the usual arrange- 

 ment. We find, however, on a more attentive 

 examination, that all the bones composing the 

 skeleton in other vertebrated animals exist also 

 in the tortoise ; and that the bony case which 

 envelopes all the other parts is really formed by 

 an extension of the spinous processes of the ver- 

 tebrae and ribs on the one side, and of the usual 

 pieces which compose the sternum on the other. 

 The upper and lower plates thus formed are 

 united at their edges by expansions of the sterno- 



