436 



COLLEGE ZOOLOGY 



Feather tracts 

 Capita 



Ventra 



Featherless areas 



Cervical 



Latera 

 Ventra 



Feather tracts 

 Capital 



Humeral 



.;:::. j)i^.. V^A Alar 



Ventral ' '^ Dorsal 



Figure 308. Feather tracts of the pigeon, showing that feathers do not develop equally on all 

 parts of the body as they do in some primitive birds such as the penguin. (After Nitzsch.) 



The skull is very light, and most of the 

 bones are so fused together that they can be 

 distinguished only in the young bird. The 

 cranium is rounded; the orbits are large; the 

 facial bones extend forward into a bill; there 

 is but a single occipital condyle for articu- 

 lation with the first vertebra; and no teeth 

 are present. 



The cervical vertebrae are long and move 

 freely upon one another by saddle-shaped 

 articular surfaces, making the neck very flex- 

 ible. This enables the bird to look in all 

 directions and to use its bill for feeding, nest 

 building, and many other purposes. The 

 vertebrae of the trunk are almost completely 

 fused together into a rigid skeletal axis which 

 is necessary to support the body while in 

 flight. There are 4 to 5 free caudal vertebrae, 

 followed by a terminal pygostyle consisting 

 of 5 to 6 fused vertebrae. The pygostyle sup- 

 ports the large tail feathers, and the free 

 caudal vertebrae allow the movements of the 

 tail for use as a rudder while flying and as 

 a balancer while perching. 



There are two cervical ribs and 4 to 5 

 thoracic ribs on each side. The second cer- 

 vical and first 4 thoracic ribs each bear a 

 uncinate process which arises from the pos- 

 terior margin and overlaps the succeeding 



rib, thus making a firmer framework. The 

 thoracic ribs are connected with the breast- 

 bone (sternum). The sternum is united in 

 front with the coracoid of the pectoral 

 girdle and bears on its ventral surface a 

 median ridge, the keel, to which are at- 

 tached the large muscles that move the 

 wings. 



The pectoral girdle consists of a pair of 

 long, narrow, bladelike scapulae, the shoul- 

 der blades, which lie above the ribs, one on 

 each side of the vertebral column, in the 

 thorax. The coracoids connect the sternum 

 with the anterior ends of the scapulae at the 

 shoulders. A concavity in these bones at 

 their junction furnishes the articular surface 

 for the long wing bone (humerus) and is 

 called the glenoid fossa. The two clavicles 

 connect proximally with the shoulder and 

 fuse distally to form a V-shaped furcula or 

 "wishbone." The clavicles are homologous 

 to the collarbones of man and serve to 

 brace the shoulders. 



Figure 309. Facing page, skeleton of a bird 

 (domestic fowl). Structurally the bird is adapted to 

 flight and bipedal locomotion. The large keel pro- 

 vides a place for the attachment of the powerful 

 wing muscles. 



