DRACO VOLANS.-—WINGS OF 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 Beptiles 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 
