BIRDS. 227 
of the muscles, a number extending down the leg from the 
pelvis to the outside of the knee, then winding around and 
blending with the principal muscles of the toes. When 
the bird settles in roosting, the leg is bent upon the thigh, 
the weight of the bird acting as a lever that tautens the 
muscles and draws the toes and claws tightly about the 
limb or roost ; thus the bird can sleep while standing upon 
one leg without fear of falling, being held fast by the 
weight of its body. The tail or caudal vertebre are mov- 
able to some extent and end in the plowshare-bone, 4, 
that supports the large quill-feathers of the tail. On the 
under portion of the skeleton we note the sternum, 4, or 
breast-bone, to which the wing-moving muscles are at- 
tached. In the flying birds, as Fig. 268, it is keeled and 
has a sharp edge, thus offering a greater surface of attach- 
ment, but in birds like the ostrich the keel is absent. 
Above the sternum are the ribs, that vary in number from 
seven toeleven pairs. In front of the sternum the clavicles 
join and form a V-shaped bone called the lucky or collar 
bone (Fig. 268, m). The coracoid bone over which the 
tendon works to pull up the wing, is seen at ¢, and this 
and the scapula and clavicle constitute the “ pectoral arch” 
to which the humerus, a, or first bone of the wing, is at- 
tached. The position of the elbow is seen at ¢ ; then fol- 
lows the fore-arm (ulna and radius), fa. The position of 
the wrist is shown at w ; then follow the carpal and meta- 
carpal bones, which are joined at the extremities. At the 
upper end of the metacarpal bones there is a rudimentary 
thumb (Fig. 268, 4). It has a single joint and supports 
what is called the false wing. The end of the wing (Fig. 
268, ha) corresponds to the hand, and three rudimentary 
fingers are generally observed, that correspond to the first 
and third fingers of the human hand. The fingers only in 
rare cases are clawed, and the arm, that in other animals is 
used for digging, clinging, or crawling, is now only used to 
propel the bird through the air or water as the case may be. 
