474 VERTEBRATE LIFE AND ORGANIZATION 



of the tail, lollow the synsacrum. The terminal caudal vertebrae are 

 fused together as a pygostyle and support the large tail feathers. 



The last two cervical vertebrae of the pigeon and the thoracic 

 vertebrae bear distinct ribs. The thoracic basket is very firm, for most 

 of the ribs have posteriorly projecting processes that overlap the next 

 posterior rib, and the thoracic ribs articulate with the expanded breast- 

 bone, or sternum. The sternum has a large midventral keel which in- 

 creases the area available for the attachment of the flight muscles. 



llie bones of the wing are homologous to those of the pectoral ap- 

 pendage of the frog and other tetrapods. A humerus, radius and ulna 

 can be recognized easily, but the bones of the hand have been greatly 

 modified. Two free carpels are present and a carpometacarpus (a safety- 

 pin-shaped complex of bone representing the fused carpals and meta- 

 carpals of three fingers) lies distal to them. The end of the most anterior 

 finger is represented by a spur-shaped phalanx articulated to the 

 proximal end of the carpometacarpus. The main axis of the hand passes 

 through the next finger, and it has two distinct phalanges articulated 

 to the distal end of the carpometacarpus. Another small, spur-shaped 

 jjhalanx at the distal end of the carpometacarpus represents the end of 

 the last finger. There is some doubt whether the fingers are homologous 

 to the first three or to the second, third and fourth fingers. The pectoral 

 girdle, which supports the wing, consists of a narrow, dorsal scapula, a 

 stout coracoid extending as a prop from the shoulder joint to the 

 sternum, and a delicate clavicle, which unites distally with its mate of 

 the opposite side to form the wishbone. 



The legs of birds resemble the hind legs of bipedal archosaurs. The 

 femur articulates distally with a reduced fibula and a large tibiotarsus 

 (fusion of the tibia with certain tarsals). The remaining tarsals and the 

 elongated metatarsals have fused to form a tarsometatarsus. The fifth 

 toe has been lost in all birds and the fourth in some species. The first 

 toe is turned posteriorly in the pigeon and many other birds. It serves 

 as a prop and increases the grasping action of the foot when the bird 

 perches. The action of the leg as a lever in running on the ground and 

 jumping on the take-off is increased by the elongation of the metatarsals, 

 and by the elevation of the heel off the ground. The various fusions of 

 the limb bones reduce the chance of dislocation and injury, for birds' 

 legs must act as shock absorbers when they land. The pelvic girdle is 

 equally sturdy; the ilium, ischium and pubis of each side are firmly 

 united with each other and with the vertebral column. The pubes and 

 ischia of the two sides do not unite to form a midventral pelvic sym- 

 physis as they do in other tetrapods. This permits a more posterior 

 displacement of the viscera, which, together with the shortened trunk, 

 shifts the center of gravity of the body nearer to the hind legs. The ab- 

 sence of a symphysis also makes possible the laying of large eggs with 

 calcareous shells. 



Muscles. The intricate movements of the neck and the support of 

 the body by a single pair of legs entail numerous modifications of the 

 muscular system, but the muscles concerned with flight are of particular 

 interest. A large pectoralis, which originates on the sternum and inserts 



