282 



VERTEBRATE ANIMALS. 



shaped bone, which is technically called the " furoulum," but is 

 familiarly called the "'merry thought." The function of this cla- 

 vicular arch is to keep the wings asunder during their downward 

 stroke, and the strength of the furculum varies, therefore, with the 

 powers of flight enjoyed by each bii-d. The bones which form the 

 limb proper, or " wing," are considerably modified to suit the 

 special function of flight, but essentially the same parts are present 

 as in the fore-limb of the Mammals. The upper arm (fig. 203) is 

 constituted by a single bone, the humerus (h), which is generally 



short and stout. The fore-arm 

 is composed of two bones, the 

 radius (r) and the ulna (jt), of 

 which the ulna is the bigger. 

 These are followed by the 

 small bones, which form the 

 wrist or carpus (j), but these 

 are reduced to two in number. 

 The carpus is followed by the 

 bones which constitute the root 

 of the hand or metacarpal,'! (mi), 

 but these are also reduced to 

 two, instead of being five in 

 number, as they are in most 

 Mammals. The two metacar- 

 pal bones are also amalgamated 

 together at both ends, so as to 

 form a single piece, at the base 

 of which, on its outer side, 

 is a rudimentary digit, the 

 "thumb" (t), which carries a 

 tuft of feathers, known as the 

 " bastard wing." The meta- 

 carpal bones, finally, support 

 each a single finger (p), of 

 which one is never composed 

 of more than one bone or phalanx, whilst the other is composed of 

 two or three phalanges. (To understand thoroughly the leading 

 modifications of the limbs of Birds, the student will do well to refer 

 to the general description of the limbs of Vertebrates, pp. 222, 223, 

 figs. 160, 161.) 



As regards the composition of the hind-limb in Birds, the two 

 halves of the pelvic arch (i.e., the innominate bones) always form a 

 single piece each, and they are always firmly united with the sacral 

 region of the spine. With the single exception, however, of the 



Fig. 203.— Fore-limb of the Jer-faloon. k 

 Humerus ; r Radiu.s : u Ulna ; t " Thumb ; " 

 7)1 Mc1;n-arpals, anchyloseil at their extrem- 

 itie.s ; p p Phalanges of fingers. 



