CLASS AVES. 



125 



FIG. 217. 



Skeleton of a Bird.* 



and that these may be easily worked, large pectoral muscles 

 are developed, attached on each side of the strong keeled 

 breast-bone, where also they act as ballast. The other 

 bones of the shoulder arch are stout and compactly fast- 

 ened together; the clavicles 

 (kiav'i klz) joining anteriorly, 

 to form the "wish-bone." The 

 ribs and dorsal vertebrae are 

 also more firmly joined than 

 in other animals. In flying, 

 the wing has a motion some- 

 what like the stroke of an 

 oar; it strikes the air with 

 the broad side, but, in return- 

 ing, presents only the sharp 

 edge. The flight -offers a cu- 

 rious problem, to which as 

 yet no satisfactory solution has been given. It is sup- 

 posed that the upward pressure of the air caught in the 

 saucer-like hollow of the wing can not bend the inflexible 

 bone and muscles of the front margin, but curves the* 

 ends of the elastic quills upward, pushing them forward, 

 and so forcing the bird ahead. 



The Plumage is a model of adaptation. The delicate 

 filaments of a feather are covered with grasping barbs 

 of a microscopic fineness. The under side of the wing 



* The principal bones of the bird's skeleton are as follows : a, Cranium. 

 6, Cervical vertebrae, e, Humerus. d, Caudal vertebrae, e, Sternum. /, Over- 

 lapping processes of ribs. ;/, Keel of sternum. A, Scapula, i, Ilium, j, Clavicle. 

 A, Coracoid. (I, Elbow joint.) m, Radius, w, Ulna, o, Carpus, p, Single bone, 

 representing one finger. ?, Bones of a second finger, r. Bones of a third finger. 

 (s, Opening in pelvis.) ?', Femur, v, Fibula and tibia, below which is the upright 

 tarsometataraus, which is terminated by never more than four toes. The first toe 

 is generally twisted round posteriorly ; it has two joints. The second toe has 

 three joints; the third, four; and the fourth, five. 



