CLASS AVES 



579 



color; this is caused probably either by an actual chemical 

 change in the pigment, or by the breaking off of the tips of the 

 feathers. 



The Skeleton. The principal differences between the skele- 

 ton of a pigeon and that of a reptile are those that are made 

 necessary by the methods of locomotion of the former. The 

 hind limbs and pelvic girdle are modified for bipedal locomotion; 

 the fore limbs and pectoral girdle are modified for flight; the 

 skeleton of the trunk is rigid; the sternum has a distinct crest 

 for the attachment of the large muscles that move the wings ; 

 short projections, called uncinate processes, which extend back- 

 ward from some of the ribs, make the thoracic framework more 

 firm; and the bones are very light, many of them containing air- 

 cavities. The skeleton of the common fowl (Fig. 474) is larger 

 and more easily studied than that of the pigeon, and is similar 

 to the latter in most respects. 



The skull (Fig. 474, 7-7) is very light, and most of the bones 

 in it 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 beak ; the 

 quadrate is movable and connects the lower jaw with the 

 squamosal of the cranium ; there is but a single occipital 

 condyle for articulation with the first vertebra; and no teeth 

 are present. 



The cervical vertebra (Fig. 474, 8) are long and move freely 

 upon one another by saddle-shaped articular surfaces, making 

 the neck very flexible. This enables the bird to use its bill for 

 feeding, for nest building, and for 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 four or five free caudal vertebrae followed 

 by a terminal pygostyle (Fig. 474, 18) consisting of five or six 

 fused vertebrae. The pygostyle (Fig. 471, Q) supports the large 

 tail feathers (rectrices, Fig. 471, RX), and the free caudal ver- 

 tebrae allow the movements of the tail which enable the bird to 



