298 



ANIMAL MOVEMENT 



The Skeletons of Bij'ds in relation to Flight. — As in the case 

 of the feathers, unusual Hghtness and great firmness are here com- 

 bined. The joints of the backbone in the regions of the chest and 

 loins are closely fused together so as to constitute a firm central 

 support, to the front part of which the large breastbone is hung 

 by means of the ribs. And the last few joints of the tail-region 

 are fused into a ploughshare bone, from which the steering 

 feathers radiate like a fan. This is a marked specialization as 

 compared with the extinct toothed bird i^Archcpopteryx) already 

 mentioned (see p. 296) as the oldest known member of its class. 

 For this creature possessed a long tail like that of a lizard, with a 

 series of quill-feathers along either side. Possibly this remark- 

 able organ was used para- 

 chute fashion. It would, at 



SCAPULA 



RADIALE 



ULNARE 



RADIUS 



HEAD 



FURCULA 



STERNUM 



KEEL OF STERNUM 



Fig. 832.— Shoulder-girdles and Breastbone of Pigeon 



HUMERUS 



CARPO METACARPUS ' 



Fig. 833. — Wing-bones of Pigeon. I-III, dif 



any rate, be a hindrance rather than a help to active flight, and 

 by gradual shortening became converted long ages since into the 

 stumpy form with which we are familiar in recent birds. 



The skeleton of the wing naturally exhibits many modifications 

 in relation to flight. The shoulder-girdle (fig. 832) consists of 

 three bones. One is the sabre-shaped shoulder-blade (scapula), 

 which is firmly bound by muscles to the skeleton of the trunk. 

 From it a stout bone (the coracoid) runs down to the breast- 

 bone (sternum), with which it is closely united, thus acting as a 

 prop. The third element of the girdle is the collar-bone (clavicle), 

 which unites with its fellow to make up the " merry-thought " 

 (furcula). This helps to keep the two shoulder-girdles apart, 

 but, being elastic, also permits a certain amount of play. Pass- 

 ing now to the bones of the free part of the wing (fig. 833), we 

 find that the head of the upper-arm bone (humerus) fits into a 

 cup (glenoid cavity) where shoulder-blade and coracoid meet, and 



