DRACO VOLANS. WINGS OP BIRDS. 505 



with a wing-like appendage on either side, formed by an 

 expansion of the skin over six lengthened ribs. These 

 appendages serve as a kind of parachute, on which this little 



Fig. 248. DRACO VOLANS. 



animal, not more than a few inches long, flutters from branch 

 to branch in search of its insect prey, or shoots, like the flying 

 squirrel, from tree to tree. They cannot be made to strike the 

 air, and therefore are not true wings ; but they can be folded 

 up and extended at the will of the animal. 



668. True wings, or instruments of propulsion as well 

 as of support in the air, are found in some members of all 

 classes of air-breathing Vertebrata; but they are especially 

 characteristic of the class of Birds, in which the absence of 

 them is the exception to the general rule, whilst in Mam- 

 mals and Reptiles it is their presence which constitutes the 

 exception. These wings are universally formed by some 

 modification of the anterior extremities, which renders them 

 unfit to be used as instruments of progression on the ground ; 

 but the nature of this modification varies considerably. In 

 the Bird, the required extent of surface is chiefly given by 

 the feathers ; these are supported upon an anterior member, of 

 which the arm and fore-arm (especially the latter) constitute 

 the largest part, the hand being contracted and consolidated. 

 The general structure of the Bird's skeleton, the whole of 

 which is modified with special reference to the actions of 

 flight, is shown in fig. 249, which represents that of the 

 Vulture. The head is supported upon a very flexible neck, of 

 which the vertebrae vc are often very numerous. The ver- 

 tebrae of the back and loins, however, are usually few in 



