386 BIRDS. 



two bones which in all existing birds are anchylosed at both 

 ends, but which are free in the remarkable extinct Archaop- 

 teryx. The metacarpal, which corresponds to the radius, is the 

 larger of the two, and it carries the digit which has the greatest 

 number of phalanges. At the proximal end of the radial 

 metacarpal is generally attached a single phalanx, constituting 

 the so-called " thumb." The digit which is attached to the 

 ulnar metacarpal never consists of more than a single phalanx. 



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. 330, A) 

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

 and phalanges ; but some of these parts are considerably ob- 

 scured by anchylosis. The femur or thigh-bone (fig. 330, A,/) 

 is generally very short, comparatively speaking. The chief 

 bone of the leg is the tibia (^), to which a thin and tapering 

 fibula (r) is anchylosed. The upper end of the fibula, how- 

 ever, articulates with the external condyle of the femur. The 

 ankle-joint is placed, as in Reptiles, between the proximal and 

 distal portions of the tarsus. The proximal portion of the 

 tarsus is undistinguishably amalgamated with the lower end of 

 the tibia. The distal portion of the tarsus is anchylosed with 

 the whole of the metatarsus to constitute the most character- 

 istic bone in the leg of the Bird the " tarso-metatarsus " (mi). 

 In most of the long-legged birds, such as the Waders, the dis- 

 proportionate length of the leg is given by an extraordinary 

 elongation of the tarso-metatarsus. 



The tarso-metatarsus is followed inferiorly by the digits of 

 the foot In most birds the foot consists of three toes directed 

 forwards and one backwards four toes in all. In no wild 

 bird are there more than four toes, but often there are only 

 three, and in the Ostrich the number is reduced to two. In 

 all birds which have three anterior and one posterior toe, it is 

 the posterior thumb or hallux (that is to say, the innermost 

 digit of the hind-limb) which is directed backwards ; and it 



