414 THE MECHANICAL FUNCTIONS. 



bility of the trunk, is to unite in one rigid bony 

 column all its vertebrae, and to allow of motion only 

 in those of the neck, and of the tail. The former, 

 accordingly, are all anchylosed together, 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 pre- 

 served, as usual, above the bodies of these coalesced 

 vertebrae, and is formed by their united leaves ; 

 the arches being completed by the spinous pro- 

 cesses. But these processes do not terminate in a 

 crest as usual ; they are farther 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 cara- 

 pace. 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 

 another along their whole length, are inserted 

 below, between the bodies of every two adjoin- 

 ing vertebrae ; 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 

 designed to be very moveable, we find them at- 

 tached either to the extremities of the transverse 

 processes, or to the articular surfaces of a single 

 vertebra ; but where solidity and security are to be 

 provided, they are always inserted between the 

 bodies of two vertebrae. This we shall find to be 

 the case also in birds, where the bones of the thorax 

 are required to be immoveable. It is remarkable, 



