CHARACTERS OF AVES. 431 



the larger and stronger. The ulna and radius are followed 

 inferiorly by the bones of the wrist or carpus; but these 

 are reduced in number to two small bones, "so wedged in 

 between the antibrachium and metacarpus as to limit the mo- 

 tions of the hand to those of abduction and adduction neces- 

 sary for the folding up and expansion of the wing ; the hand is 

 thus fixed in a state of pronation ; all power of flexion, exten- 

 sion, or of rotation, is removed from the wrist-joint, so that 

 the wing strikes firmly, and with the full force of the con- 

 traction of the depressor muscles, upon the resisting air" 

 (Owen). One other bone of the normal carpus (namely, 

 the " os magnum") is present, but this is anchylosed with one 

 of the metacarpals. There are thus really three carpal bones, 

 though only two appear to be present. The carpus is followed 

 by the metacarpus, the condition of which agrees with that of 

 the carpal bones. The two outermost of the normal five 

 metacarpals are absent, and the remaining three are anchy- 

 losed together with the os magnum so as to form a single 

 bone (fig. 169, m). This bone, however, appears externally as 

 if formed of two metacarpals united to one another at their 

 extremities, but free in their median portion. The metacarpal 

 bone which corresponds to the radius is always the larger 

 of the two (as being really composed of two metacarpals), and 

 it carries the digit which has the greatest number of phalanges. 

 This digit corresponds with the " index" finger, and it is com- 

 posed of two, or sometimes three, phalanges (fig. 169, /). At 

 the proximal end of this metacarpal, at its outer side, there is 

 generally attached a single phalanx, constituting the so-called 

 " thumb " (fig. 169, /), which carries the " bastard-wing." The 

 digit which is attached to the ulnar metacarpal corresponds to 

 the " ring finger," and never consists of more than a single 

 phalanx (fig. 169). 



As regards the structure of the posterior extremity or hind- 

 limb, the pieces which compose the innominate bones (namely, 

 the ilium, ischium, and pubes) are always anchylosed to one 

 another ; and the two innominate bones are also always an- 

 chylosed, by the medium of the greatly-elongated ilia, to the 

 sacral region of the spine. In no living bird, however, with 

 the single exception of the Ostrich, are the innominate bones 

 united in the middle line in front by a symphysis pubis. The 

 stability of the pelvic arch, necessary in animals which sup- 

 port the weight of the body on the hind-limbs alone, is amply 

 secured in all ordinary cases by the anchylosis of the ilia with 

 the sacrum. 



As in the higher Vertebrates, the lower limb (fig. 170, A) 



