466 THE- MECHANICAL FUNCTIONS. 



tures, than those afforded by the skeleton of the 

 tortoise. 



The first step taken to secure the relative 

 immobility of the trunk, is to unite in one rigid 

 bony column all its vertebra, and to allow of 

 motion only in those of the neck, and of the tail. 

 The former, accordingly, are all anchylosed to- 

 gether, leaving, indeed, traces of their original 

 forms as separate vertebrae, but exhibiting no 

 sutures at the place of junction. The canal for 

 the spinal marrow is preserved, as usual, above 

 the bodies of these coalesced vertebras, and is 

 formed by their united leaves ; the arches being 

 completed by the spinous processes. But these 

 processes do not terminate in a crest as usual ; 

 they are further expanded in a lateral direction, 

 forming flat pieces along the back, which are 

 united to one another by sutures, and which are 

 also joined to the expanded ribs, so as to form the 

 continuous plane surface of the carapace. The 

 transverse processes of the vertebrae are well 

 marked, but, though firmly united to the ribs, 

 do not give rise to them ; for the ribs, which are 

 flattened and expanded, so as to touch one ano- 

 ther along their whole length, are inserted below, 

 between the bodies of every two adjoining ver- 

 tebrae ; while above, they are united by suture 

 with the plates of the spinous processes. This 

 change in the situation of the ribs is the con- 

 sequence of the change in their office. When 



