404 THE MECHANICAL FUNCTIONS, 



subject of this conversion to uses very different 

 from their ordinary function, whicli is that of as- 

 sisting in respiration. Tims we have seen that in 

 the Tortoise they are expanded to form the cara- 

 pace ; uniting with corresponding dilatations of the 

 sternum, and sterno-costal appendages, in com- 

 posing a general osseous encasement to the body. 

 In Serpents, again, the ribs are employed as organs 

 of progressive motion ; performing the functions of 

 legs, and having, affixed to their extremities, the 

 abdominal scuta, by way of feet. The cervical ribs 

 of the Cobi:a de Capello, or hooded snake of the 

 East Indies, are employed for the mechanical pur- 

 pose of supporting an expansion of the skin of the 

 neck, which forms a kind of hood, capable of being 

 raised or depressed at the pleasure of the animal.'^ 

 These ribs are entirely unconnected with the respi- 

 ration of the serpent. 



In the Draco volans, which was to be furnished 

 with instruments for assisting it in its distant leaps 

 through the air, it is again the ribs which are re- 

 sorted to for furnishing the basis of such an appa- 

 ratus. On each side of the dorsal vertebra?, as is 

 seen in the skeleton of this animal (Fig. 222), the 

 eight posterior ribs on each side, instead of having 

 the usual curvature inwards, and instead of being 

 continued round to encircle the body, are extended 

 outwards and elongated, and are covered with a 

 thin cuticle, derived from the common integuments. 

 The ordinary muscles which move the ribs still 

 remain, but with greatly increased power, and 

 serve to flap these strangely formed wings at the 



* Phil. Trans, for 1804, p. 346. 



