xvi. s SKELETON 439 



between the vertebral and sternal portions of the ribs. The latter are 

 bony, not cartilaginous as in mammals, and are jointed to the sternum, 

 which is a very large keeled structure in all flying birds, serving to 



Sunsacrum 

 (1 thoracic, 5 lumbar 

 Z sacral, S caudal ) 



vac. 



Fig. 253. Skeleton of the pigeon (Columba). 



at. atlas; fl.v. axis; C12, 12th cervical vertebra; car. keel of sternum; carp, carpus; cor. cora- 

 coid; c.r. cervical rib; delt.r. deltoid ridge of humerus ; fern, femur ;fib. fibula ;fur. furcula; 

 hyp. hypapophysis; il. ilium; isch. ischium; m. auditory meatus; mc. 2 and 3, metacarpals; 

 pub. pubis; pyg. pygostyle; rad. radius; sc. scapula; St. body of sternum; tar. met. tarso- 

 metatarsus; tib.tar. tibio-tarsus; uln. ulna; up. uncinate process; vac. vacuity in side of skull. 



carry the weight of the body to the wings by the attachment of the 

 main wing muscles (Fig. 254). The pectoralis major, which depresses 

 the wing in flight, is attached to the edge of the sternum and the great 

 depth of the keel serves to increase the length and mechanical advan- 

 tage of the fibres of the muscle and also, by its shape, to strengthen 

 the sternum. When the bird is in the air the sternum is carrying a large 

 part of the weight. By this arrangement the centre of gravity is kept 

 well below the centre of pressure, giving great stability. 



