VERTEBRATA: AVES. 581 



state of pronation ; all power of flexion, extension, or of rota- 

 tion, is removed from the wrist-joint, so that the wing strikes 

 firmly, and with the full force of the contraction of the de- 

 pressor 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. (According to Morse, there is a fourth 

 carpal, which early anchyloses with the base of the meta- 

 carpal of the middle finger.) 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 meta- 

 carpals are absent, and the remaining three are anchylosed 

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

 (fig. 324, 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. 324, p}. At 

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

 generally attached a single phalanx, constituting the so-called 

 "thumb" (fig. 324, /), which carries the " bastard-wing," and 

 is sometimes furnished with a claw. The digit which is 

 attached to the ulnar metacarpal corresponds to the middle 

 finger, and never consists of more than a single phalanx (fig. 

 324). In the Apteryx and the Cassowary there is only one 

 complete digit to the hand. 



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 with one 

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

 chylosed, by the medium of the greatly-elongated ilia, with 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. 325, A) 

 consists of a femur, a tibia and fibula, a tarsus, metatarsus, and 

 phalanges ; but some of these parts are considerably obscured 



