,jj^^ THE PIGEON. PHYLUM CHORDATA 



which the pigeon uses only when about to land and in sexual 

 display, the wings are held motionless, and the air pressure pro- 

 duced b\^ the horizontal velocity previously obtained by flapping, 

 produces, because of the shape of the wing, a hft which reduces 

 the rate of fall. In soaring flight, which the pigeon does not use, 

 birds are carried upwards on outspread wings in vertical air 

 currents. Many of the special features of aeronautics can be seen 

 in bird wings and flight. Thus the wings are cambered from fore 

 to aft to increase hft, and for the same purpose a slotted wing is 

 often produced by the separation of the primaries in flight 

 (Fig. 313, 2). In many birds this effect is increased by the 

 emargination of the feathers ; that is, the front part of the vane 

 is narrowed lor part of its length. The tail feathers assist in 

 braking and steering. 



SKELETON 



The pigeon has a chest or thorax, walled by ribs and a broad 

 breastbone, but lacks the midriff or diaphragm of mammals. 

 The bones are very light and spongy in texture, and most of them, 

 except those of the tail, forearm, hand, and hind-limb, contain 

 air spaces. A tendency to the fusion of bones is seen in various 

 regions, and the proportion of cartilage is very small. The centra 

 of the free vertebrae (Fig. 314) are distinguished from those of 

 all other vertebrates in being heterocoelous, that is, their surface 

 is like that of a saddle, or col of a hill, convex in one direction 

 and concave in the other ; the front surface has the concavity 

 running from side to side. The backbone is divided into five 

 regions : (i) The neck contains thirteen to fifteen cervical 

 vertebrae, the commonest number being fourteen. The atlas is 

 procoelous. The third to the eleventh or twelfth cervical vertebrae 

 bear short ribs fused to the centra and transverse processes. The 

 ribs of the last two are free, but do not reach the breastbone. 

 (2) Behind these come five thoracic vertebrae, whose ribs reach 

 the breastbone. Of these the first three are fused together, the 

 fourth is free, and the fifth is fused with those behind it. (3) The 

 next half-dozen vertebrae are called lumbar and are fused in 

 front with the last thoracic and behind with (4) the two sacral 

 and (5) the first five caudal. Thus there is a long group of fused 

 vertebrae, known as the sacrum, to which the pelvic girdle is 

 attached. Then follow six free caudal vertebrae and the plough- 



