386 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



developed, as we know, in certain Birds the skeleton 

 of the fore-arm is specially modified (see page 352), and 

 forms for the wing a firm anterior bar, comparable to 

 the anterior nervure of the insect's wing ; this bar is 



moved by special mus- 

 cles, which are at- 

 tached near its base ; 

 but all of which lie 

 on the ventral or 

 lower surface of the 

 body, and thereby 

 enable the centre of 

 gravity of the bird 

 to be lower than it 

 would be were the 

 extensor muscles of 

 the arm placed, as in 

 other Vertebrates, on 

 the dorsal surface. A 

 large surface of at- 

 tachment for the pec- 

 toral muscles is ob- 

 tained by the great 

 development in flying 

 birds of the keel of 

 the sternum (see page 

 347), and the extensor 

 muscle works on a 

 pulley. The greater 

 portion of the wing is 



not formed by membrane or integument, but by the 

 development of those integumentary structures which 

 we call feathers. These feathers overlap one another 

 in such a way that the wing is convex above and 

 concave below, and that pressure from below forces 

 the feathers more closely together. From this arrange- 

 ment it is clear that in the up and down movement of 

 the wing in the air, much greater effect is gained by 



Fig. 164. The Common Swift. 



