37S THE MECHANICAL FUNCTIONS. 



and it can never rise to more than five or six feet above the 

 surface of the water. 



A species of lizard, called the Draco Fb/a7Z5, has a singu- 

 larly constructed apparatus, which appears like two wings, 

 affixed to the sides of the back, and quite independent of 

 either the fore or the hind extremities. By the aid of these 

 moveable flaps, the animal is able to descend from the tops 

 of trees, or flutter lightly from branch to branch; but this is 

 the utmost that it can accomplish by means of these imper- 

 fect organs. The construction of these anomalous mem- 

 bers is highly curious in a physiological point of view; as 

 showing how Nature, in effecting a new purpose, is in- 

 clined to resort to the modification of structures already 

 established as constituent parts of the frame, in preference 

 to creating new organs, or such as have no prototype in the 

 model of its formation. Frequent proofs of this law, indeed, 

 are afforded by the comparative examination of the anatomy 

 of the organs of progressive motion. The ribs, in particular, 

 are often the subject of these conversions to uses very dif- 

 ferent from their ordinary function, which is that of assist- 

 ing in respiration. Thus, we have seen that in the Tortoise 

 they are expanded to form the carapace, uniting with corre- 

 sponding dilatations of the sternum, and sterno-costal appen- 

 dages, in composing a general osseous ineasement 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 Cobra de Capello, or 

 hooded snake of the East Indies, are employed for the me- 

 chanical purpose 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 respiration of the ser- 

 pent. 



In the Draco volans, which was to be furnished with rii- 



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



