406 BIRDS. 



bones of the back (3), which sustain the ribs, are 

 immoveably fixed to each other ; the ribs (5), of the 

 lightest structure, are locked together by overlapping 

 plates, while the breast-bone (6) not only constitutes 

 a broad shield or breast-plate, but is furnished with 

 a deep crest or keel, from whence the muscles em- 

 ployed in flight take their origin. Birds having 

 this shield largest and most complete are those 

 that fly the best. The bones of the shoulder (7, 8) 

 are disposed in a manner most favourable to the 

 support of the wings, and the two collar-bones (7) 

 are joined together so as to form but one piece, 

 having the shape of the letter Y, an arrangement 

 well calculated to hold the shoulders apart, in spite 

 of the violent force applied in the contrary direction 

 by the exercise of the wings, a force that increases 

 with the energy of the muscles employed in flight. 

 The wing of a bird corresponds to the human arm ; 

 and, like it, is composed of three principal parts, 

 the arm, the fore-arm, and the hand. The bone of 

 the arm is of great strength and lightness, being 

 constructed upon the principle of a hollow cane. 

 The fore-arm consists of two bones, the length of 

 which is in relation wdth the power of flight, while 

 the hand is reduced to a single piece, that serves 

 for the support of the large feathers of the wing ; 

 there are also a rudimentary thumb and the vestiges 

 of a third finger. 



The legs of a bird are designed for support and 

 for progression ; sometimes they are constructed 

 for swimming, and are frequently employed fpr the 

 purpose of seizing or holding food. The bones of 

 the haunches (14) are of great strengtii, and solidly 

 fixed to the vertebral column. The thigh-bone (15) 

 is short and directed forward ; the leg (16) is strong, 

 and composed of but one bone, while the bones of 

 the foot and instep (17) are represented by a single 

 piece, the length of which determines the height of 

 the bird on its legs. The number of toes (18) varies 

 from five to two ; generally, however, there are three 



